Pests, Diseases, and Disorders
Elepidote (Hymenanthes) Rhododendrons
Azalea Hybrids – Deciduous
Knap Hill Azaleas
Azalea Hybrids – Evergreen
Standish and Noble
Introducing the first part of his son's work The Rhododendrons of the Sikkim Himalaya (1849), Sir William Hooker remarked: 'Perhaps, with the exception of the Rose, the Queen of Flowers, no plants have excited a more lively interest throughout Europe than the several species of the genus Rhododendron, whether the fine evergreen foliage be considered, or the beauty and profusion of the blossoms. . . .' At the time the elder Hooker wrote those words the number of species known (including azaleas) was around 50, to which his son, in the three parts of his famous work, added a further 19, all of which he had himself discovered in one small segment of the Himalaya. The number of species at present recognised is in the region of 800, of which over 500 are cultivated in Britain in the open ground. The size of the genus is impossible to estimate with any precision, since many species at present recognised will certainly be relegated to synonymy in the near future. No doubt others will be described, but it is probable that the trend will be downwards. Further botanical exploration, while it may result in new discoveries, is just as likely to bring to light links between species at present considered to be distinct.
The name Rhododendron, used originally by Greek authors for the oleander, derives from rhodon, rose, and dendron, tree.
The majority of the species are erect shrubs 3 to 30 ft high, a few are intricately branched shrublets or prostrate, very few are trees with a definite trunk; some are epiphytes. Leaves persistent, more rarely deciduous, alternate, with a few exceptions, entire. A character separating Rhododendron from all but a few other ericaceous genera is that the early development of the inflorescence (and mostly of the shoots also) takes place within a bud made up of numerous scales (perulae) the innermost of which subtend the flowers and usually fall away as the inflorescence expands; but the inflorescence-bud in the subgenus Tsutsia (q.v.) is exceptional. In most species the inflorescence-buds are terminal only, but a few species bear supplementary inflorescences in the uppermost leaf-axils and in others all the inflorescences are axillary and the terminal bud is then either vegetative or is not formed. The inflorescence varies from umbellate to corymbose or racemose, depending on the length of the rachis and the length and posture of the pedicels, which have a pair of deciduous bracteoles at the base. Calyx varying from well developed and leafy or fleshy to a mere rim. Corolla deciduous, five- to eight-lobed, funnel-shaped to campanulate, sometimes tubular or with a slender basal part more or less abruptly widening into a spreading limb, which is sometimes zygomorphic; the corolla is never urn-shaped (urceolate) as in many ericaceous genera. Stamens usually twice as many as the corolla-lobes (sometimes more numerous, occasionally fewer); filaments and anthers without appendages; anthers dehiscing by apical pores. Ovary superior, with five or often more chambers; style terminal, with a capitate or (more rarely) discoid stigma. Fruit a septicidal capsule, containing numerous small often winged or tailed seeds.
The genus is distributed through the more humid and cooler parts of the northern hemisphere, and extends across the Equator to New Guinea and north-east Queensland. Yet over much of this area it is poorly represented. The genus is at its most abundant and most varied in the Sino-Himalayan region – the great ranges that border the Tibetan plateau on the east and south, from western Szechwan southward to N.W. Yunnan and then westward across the upper Irrawaddy and the eastern feeders of the Brahmaputra to the Himalaya. Yet even in this region, though rhododendrons are almost everywhere abundant above 8,000 ft, they are still unequally distributed in terms of variety. The heartland of the genus is a region of small extent, which cannot be precisely defined, but could be said to have its eastern limit along a line that stretches from the Tali range in Yunnan north-west to the great gorges of the Mekong and Salween, and its western limit in Sikkim and bordering Nepal. In India its southern limit is of course the plains of Assam; in Burma and Yunnan the southern border is less definite, since mountains high enough to support a temperate flora extend far to the south, and many harbour rhododendrons, but the boundary of 'Rhodoland', as Kingdon Ward nicknamed it, could be taken as a line drawn from Tali in Yunnan to Myitkyina in Burma, near the confluence between the two upper branches of the Irrawaddy. This in fact corresponds well with the southern limit of the Sino-Himalayan floristic region in these longitudes. The boundary on the inner, Tibetan side, is determined by the limit of penetration of the Indian monsoon. In Sikkim the transition from the rhododendron world to the arid wastes of Tibet is very abrupt, but in the Himalaya east of 92° E. the range is comparatively low and allows the monsoon, and with it both forest and rhododendrons, to penetrate to the ranges beyond the Tsangpo. Farther east the boundaries of Rhodoland are the great peaks that stand above the feeders of the Irrawaddy and the gorges of the Salween and Mekong around 28° N. (See map pp. 544-5.)
The climate of this area is as favourable to plants as it is abominable for humans, wrapped in mist and rain-cloud during the period of the monsoon, when the rhododendrons flower and make their growth, and with heavy winter snow, which may fall as early as October and blocks the higher passes from midwinter until April or even June. But the snow, though it makes human traffic difficult or impossible, is as beneficial to plants as the summer rain, protecting or mulching them in winter and supplying them with water as it melts. Fortunately, rhododendrons do not need in cultivation the precise conditions that they enjoy in the wild, for if they did it would be impossible to grow them. But all who have grown the monsoonal rhododendrons will have witnessed the almost ceremonial lifting of the leaves that takes place sometimes in spring, or early summer on warm muggy days, as if in exile they were remembering their homeland.
To enumerate the types of rhododendron found in the heartland region would be pointless, since the majority of the series and subseries into which the genus is divided are either endemic to it or at least have the majority of their species within it. Yet the absences are significant. Apart from a few species of the Obtusum subseries (subgen. Tsutsia) the azaleas are not native to this region, and the large section Vireya, which has its main development in Indonesia, New Guinea, etc., is very poorly represented in Sino-Himalaya. It is also of interest, for the possible light it throws on the history of the genus, that groups widely distributed in China, such as the Argyrophyllum and Fortunei subseries, are either absent from the heartland region or poorly represented there, and that the Ponticum series, which extends to North America and Western Europe, is also absent. So the heartland of the genus may not have been its original homeland. Certainly the genus cannot have originated in the Himalaya, as was once thought even by some botanists, for these mountains are young and had probably not even emerged from the sea when the first rhododendrons appeared on the earth. There is in fact a marked falling off in variety westward along the Himalaya, so much so that Sikkim, despite its wealth of species, lacks many of the types characteristic of the heartland area, a notable absentee being the Neriiflorum series. On the other hand, the Himalaya have a few specialities of their own, notably R. griffithianum and the Cinnabarinum group.
No other woody genus has produced such a diversity of species and life-forms in such a small area as Rhododendron in the Sino-Himalaya west of the Mekong. It is almost beyond belief how many perfectly distinct species are to be found in a single square mile, and within a cubic mile the variety is even greater, for the deep valleys and gorges cut by the rivers allow the more warmth-loving species to penetrate to within sight of the eternal snow. It is appropriate to take Sikkim as an example, even though, as already mentioned, it is relatively poor in species. In this tiny segment of the Himalaya, barely 50 miles long from north to south and 40 miles wide, there are about thirty species, representing twenty-three series or subseries. The genus here is just as abundant as it is farther east. 'It is the traveller's constant companion throughout every day's march; on the right hand and on the left of the devious paths, the old bushes are seen breast high or branching over head, whilst the seedlings cover every mossy bank' (Hooker). But on the Doshong La, a pass at the eastern end of the Himalaya, there are, by Kingdon Ward's reckoning, twenty-five species. Below, in the vicinity of the Pemakochung monastery at the head of the Tsangpo Gorges, Frank Ludlow and his companions collected twenty species during their expedition of 1946-7. 'It seemed as though Pemakochung was the birth-place of the genus, the very epicentre from which it had sprung' (Gard. Chron., Vol. 143 (1958), p. 103).
It is the diminished variety of the rhododendron populations rather than any falling off in abundance that characterises the northern and more eastern parts of Sino-Himalaya. East of the Mekong the climate becomes less humid as the influence of the Indian monsoon weakens, though the Tali range is still very rainy on its western side and represents an eastern outpost of the rhododendron heartland. North-eastward from Tali the types of rhododendron so characteristic of the Irrawaddy-Salween divide gradually disappear and we enter the region of the rhododendron moorlands in and around the double bend of the Yangtse, where the Lapponicums and Anthopogons come into their own and the elepidote (Hymenanthes) rhododendrons are of the tougher sort. The climate here is more agreeable and it is no wonder that Forrest preferred the Lichiang range to the richer areas farther west. It reminded him of his native Scotland.
Farther north again we enter the Wilson territory of western Szechwan, which so far as its rhododendron flora is concerned has very little in common with the heartland region. The Hymenanthes (elepidote) rhododendrons here are predominantly of the Maculiferum, Argyrophyllum, and Fortunei type, and red-flowered rhododendrons, already on the wane east of the Mekong, are represented by a single, anomalous species – R. strigillosum. The Triflorum type of rhododendron is much better represented here than it is in the heartland region. The distinctness of the rhododendrons of northern Sino-Himalaya is emphasised by the absence of any heartland types from Mount Omei, and neighbouring mountains, which are exceptionally humid; the epiphytic rhododendrons here belong to the endemic Moupinense series. The Grande and Falconeri series, both so characteristic of the heartland area, are both represented in W. Szechwan, but by very anomalous members.
In central and eastern China the rhododendrons, in the narrow sense, are represented almost wholly by members of the Fortunei, Argyrophyllum, and Maculiferum subseries and the lepidote Triflorum series, but Hupeh has one notable endemic, R. auriculatum. The azaleas and azaleastrums now become an important feature of the rhododendron flora; Hong Kong alone has five of these (and one member of the Argyrophyllum subseries). In S.E. China and mainland S.E. Asia generally, south of the Rhododendron heartland, the genus is comparatively poorly represented, mainly by members of the Ciliicalyx subseries and Azaleastrums of the Stamineum series. The Hymenanthes (elepidote) rhododendrons, so richly represented in the heartland region, are scarce here, but one member of the Irroratum series occurs in Malaya and two in Sumatra. These are the only Hymenanthes rhododendrons in the Malesian region, which is the home of an extraordinary development of the lepidote rhododendrons – the section or subgenus Vireya – which represents in terms of species perhaps a quarter of the whole genus (see further on page 575).
The rhododendron flora of Japan has virtually nothing in common with the Sino-Himalayan region – an interesting fact, considering that so many Japanese trees and shrubs have close relatives in the Himalaya. No other country is so rich in azaleas, which here meet rhododendrons whose main home lies in continental north-east Asia (s. Dauricum, R. parvifolium and R. aureum).
North America and western Eurasia are the main home of the Ponticum series and the Luteum azaleas, two groups which are poorly represented in eastern Asia, and three small endemic series (s. Carolinianum and s. Albiflorum in North America and s. Ferrugineum in Europe). R. lapponicum is common to Europe (Scandinavia) and North America.
When the first two volumes of this work were published in 1914 the species of Rhododendron introduced in the 18th and early 19th centuries had mostly been displaced by their hybrids, but Hooker's Sikkim rhododendrons and other Himalayan species were grown in some western gardens and were beginning to move eastward to the gardens of Sussex. The latest novelties in 1914 were the Wilson rhododendrons from Szechwan and Hupeh, none then more than about twelve years old and some still to flower. Among these were some of the finest species in the genus – R. auriculatum, R. calophytum, R. discolor, R. sutchuenense, and R. augustinii. Yet Wilson's work was no more than a prologue to the great rhododendron epic. It is mainly due to the wonderful work and amazing industry of George Forrest, also to his organisation and training of native collectors, that rhododendrons are so prominent in horticulture today. He worked chiefly in N.W. Yunnan and bordering parts of Tibet and Burma, in latitudes considerably to the south of Wilson's area and with a more humid climate, and his introductions on the whole are not so easy to cultivate successfully in our average climate. Forrest died at Tengyueh on 5 January 1932, just as he was on the point of returning home from what he had intended should be his last collecting expedition.
Frank Kingdon Ward started his career as a plant collector in China. But his efforts there largely duplicated those of Forrest and it is fortunate for us that from 1924 onwards he devoted his energies to parts of Burma that were beyond the reach of Forrest's collectors, and to the easternmost Himalaya, which is quite as rich in species as the Forrestian territory farther east, as was proved by Kingdon Ward's pioneer journey of 1924 and the later explorations by Ludlow and Sherriff and their companions. Unlike Forrest, who wrote well but reluctantly, Kingdon Ward was a prolific writer and it is to him that we owe much of our knowledge of the landscapes, peoples, and plant associations of the rhododendron heardand.
The American Collector J. F. Rock sent large quantities of rhododendron seed from N.W. Yunnan, among which were fine forms of species already introduced by Forrest; his 1923-4 collections, the so-called 'Rock 59s', are particularly well represented in gardens, Reginald Farrer discovered and introduced several species during his expedition to the Burma-Yunnan border and we are indebted to his companion Euan Cox for a fine account of this area (Farrer's Last Journey). But Forrest's achievement must still be considered the greatest, and the discovery and first introduction of the newer Asiatic rhododendrons now in gardens are directly due to him and to those who organised and financed his expeditions. He was the pioneer and was working for enthusiasts with the land and means to raise and cultivate his rhododendrons in quantity. By the time of his death appetites were becoming jaded and species were beginning to be thrust aside by the new hybrids. Rhododendrons from seed sent by other collectors after his death and since the second world war are consequently not so well represented in gardens as those introduced in Forrest's lifetime. This is true even of Kingdon Ward's rhododendrons, most of those grown being from his expeditions in the 1920s.
Soil. – For all practical purposes it is true to say that rhododendrons and azaleas, like most ericaceous plants, are lime-hating and need a soil well on the acid side of neutral if they are to thrive. The acidity or alkalinity of a soil is expressed by the logarithmic pH scale. A neutral soil has a pH value of 7 (10-7 being the concentration of hydrogen ions in neutral water). A pH value below 7 indicates that the soil is acid, a reduction by one integer signifying a tenfold increase in the concentration of free hydrogen ions. A 'limy' soil may have a pH value of 8 or slightly more. Rhododendrons prefer a soil with a pH value of around 5. All plants need a certain quantity of calcium in their tissues, and rhododendrons, like many other ericaceous plants, have the ability to extract it even from soils where it is present in very small quantities – hence their ability to thrive on acid soils. But they lack the ability to control the amount absorbed by their roots, and on calcareous soils will take up excessive calcium at the expense of other nutrients such as iron and magnesium. It appears to be this factor, rather than alkalinity per se, that is important. Dr Henry Tod has shown that rhododendrons can be grown in quite strongly alkaline soils provided the base is magnesium and not calcium (R.C.Y.B. 1959 pp. 19-24). In California, R. occidentale has been found growing in a soil with a pH value at the roots of 7.6 to 8.6, but here the rock was serpentine and the source of alkalinity magnesium (R.C.Y.B. 1957, pp. 47-51).
In parts of N.W. Yunnan and bordering Szechwan there are limestone ranges that carry an abundance of rhododendrons and here too the limestone is said to be of the magnesian type, but nothing seems to be known about the chemical and physical qualities of the soil in which the rhododendrons actually grow. A few species are said to be moderately lime-tolerant in cultivation and of course it is possible with ingenuity to cultivate rhododendrons in chalky districts by growing them in raised beds, and to acidify slightly alkaline soils by the use of various chemicals, but such matters are beyond the scope of a general work such as this. Rhododendrons look most in place on the soils that are naturally suited to them and elsewhere they are an incongruous intrusion.
Most naturally acid soils can be made to suit rhododendrons. The easiest to convert are the 'heath-soils' that develop on sandstone formations such as the Lower Greensand or Bagshot Sands or on the older non-calcareous rocks of the north and west. But light fibrous loams are just as good or even better, though rarer. The most difficult soil to adapt to rhododendron culture is one of a heavy clayey nature – one that gets cold and wet in winter, hard and liable to crack in summer droughts. The method of improving such heavy soils is really the same whatever is to be grown in them, except, of course, that lime cannot be used when rhododendrons are the object. The addition of well-decayed leaves is always an advantage, whatever the soil. Peat is also invaluable, for although it contains little in the way of nutrients it increases the acidity of the soil, makes it more retentive of moisture and improves the texture of heavy soils. Well-decayed manure is useful, and so too is garden compost, provided the activator used to make it does not contain lime. But raw or half-decayed leaves and other vegetable matter such as sawdust should never be used. Roots cannot assimilate material in that state and its gradual decay and consequent shrinkage produces a looseness or 'puffiness' of the soil, besides having a denitrifying effect.
It is often said that rhododendrons are shallow rooting, and this is true in the sense that they produce feeding roots near the surface and that they can exist if need be in thin, humous soils, even on mossy rocks. But, provided the soil beneath them has been well prepared or is naturally porous and well aerated, their roots will go deep and it is obviously to the plant's advantage that part of its root system should lie where the soil is permanently moist or at least slow to dry out, and below the level to which frost normally penetrates in a hard winter. So cultivation to a depth of 2 ft or so is always advisable, especially in dry districts.
A mature garden soil is not so suitable for rhododendrons as a virgin one. The plants, especially if they are small, seem reluctant to push their roots into it, perhaps because the sort of soil that the gardener normally aims to produce for his herbaceous plants or kitchen garden crops is rather too crumby for rhododendrons. But in time they will adapt to it, especially if a peaty mixture is put round their roots as a starter. Old garden soils should, incidentally, always be regarded with suspicion, as they may have been heavily limed in the past.
Siting and Planting. – All the rhododendrons and azaleas are light-loving plants, but none is averse to protection from the hottest sun and some need it, especially when making their new growth. The most tolerant of full exposure to sun are the Hardy Hybrids, the azaleas and the smaller-leaved lepidotes. The species most in need of cool, half-shaded positions are those that have their homeland in the rainiest parts of Sino-Himalaya and in general all those with large leaves (most of which come from this region anyway). The provision of shade presumes as a rule the existence of at least moderately sized trees, and to plant rhododendrons near enough to benefit from their shade involves, or may involve, competition at the root. Rhododendrons are all moisture lovers, both at the root and in the atmosphere, and although in places where the rainfall is fifty or more inches per annum the soil may keep moist enough for both trees and rhododendrons in association, in the drier and especially in the eastern counties, the close proximity of such greedy trees as elms, beeches, limes, birches, or sycamores makes the cultivation of rhododendrons very unsatisfactory unless artificial watering can be done, and trenches periodically dug deep enough to keep the tree roots at bay. There is the further consideration that if rhododendrons are planted under the spread of trees they will be deprived of the benefit of all but the heaviest summer rain. For these reasons, no doubt, rhododendrons situated fully in the open may suffer less in a droughty summer than those grown under a dense canopy. Also, rhododendrons planted under low branches, but with abundant side-light, are inclined to grow outwards towards the light, almost horizontally if need be, and so quickly lose their shape.
The best of all places that can be provided for rhododendrons is oak woodland, in which the stand has been thinned to admit abundant light, but is still dense enough to cover the ground with light shadows. In the wild, rhododendrons are often associated with conifers and in this country are often grown successfully in woodland of Scots pine. Birch woodland is less suitable; in mountainous districts it may be that the silver birch is able to send its roots deep into fissures, but on shallow soils it is a greedy tree, and the downy birch even more so. However, few planters nowadays have woodland at their disposal or the means to prepare it. By exploiting the shade cast by ornamental garden trees or buildings it is possible to grow even those species and hybrids that need cooler conditions, and the higher the average rainfall the less need there is for shade. On the whole, the necessity for shade has been too much stressed, and as a result rhododendrons have been condemned to live in dark woodland where they flower sparsely and become ugly, gangling objects and often suffer from permanent drought and starvation near the trunks of greedy trees. It should also be borne in mind that the sun tolerance of rhododendrons increases dramatically once the plant has grown wide enough to shade its own roots, though this applies, of course, only to those that are of naturally bushy habit.
In choosing rhododendrons the garden climate must be borne in mind. The flowers of most rhododendrons will be killed by a few degrees of frost, so it is hopeless for the owner of a frosty garden to expect much from plants whose flowering time is before mid-April. The incidence of fine-weather frost is governed to a large extent by local topography. In one garden, situated towards the top of a slope, the early-flowering hybrid Praecox will give its wonderful display four years out of five, while in another, less than a mile away but in a cold-air sump, the owner will be lucky if he sees it in flower one year in three. A tree canopy may help to ward off fine-weather frost but it cannot be relied on. The best policy in the less favoured gardens is to avoid the earliest flowerers altogether or to choose those that take up little room but pay for their keep by their good foliage and habit, e.g., Elizabeth, Cilpinense, or R. leucaspis. The Lapponicums mostly have flowers that will withstand a few degrees of frost and very rarely cause disappointment. Some early flowerers, such as R. mucronulatum and R. Nobleanum, open their trusses over a period of several weeks in most seasons, so some at least of the flower-buds may escape damage. There is always one comfort to be taken when a promising display has been ruined by frost: the plants have been prematurely deadheaded and will grow all the better in the following summer. Loss of young growths through frost is a different matter, but is less frequent, since the majority of species and hybrids do not flush until May or later. If it is the growths from terminal buds that are killed, no great harm is done, since there is a reserve of axillary buds to produce a second growth later, though next season's flower may be lost if the plant is one that normally produces its flower-buds on growths from terminal buds. The loss of axillary growths by frost can be damaging, especially if it is repeated year after year, but very few rhododendrons produce axillary growths early enough to be killed by frost.
Most of the rhododendrons treated in this work are hardy in the sense that they will withstand low winter temperatures in still air. But continuous biting, dry winds can be very damaging, and the more so if the ground is deeply frozen, since the plant is then unable to replace the moisture lost by the leaves. So protection from the north and east is very important if the more demanding rhododendrons are grown. Mulching, always beneficial, also helps here, by keeping frost out of the ground.
The planting of rhododendrons and other evergreens has been dealt with in the introductory chapters (see Vol. I, p. 63) and there is little that need be added here. The plant should always be placed in the ground with the top of its root ball level with the surrounding soil or at the centre of a shallow depression, which can be filled with a light mulch. The advantage of planting in a saucer is that it makes watering easier in the early stages and helps to keep the mulch in place. It is important to achieve as perfect a union as possible between the root ball and the planting soil by filling in every nook and cranny and carefully firming the mixture with a rammer. It is not enough merely to spade back the soil and tread it in. It is always advisable to add extra peat to the planting soil, and perhaps sand also, especially if it is heavier than the soil in the ball, as is often the case when the plant has come from one of the specialist nurseries, most of which are situated on light sandy soils.
At what spacing to plant rhododendrons is a question difficult to answer. Obviously it is quite impracticable to give each plant from the start the space it will need a quarter-century on and, rhododendrons being sociable plants, it is neither necessary nor desirable to do so. The dwarfer species and hybrids are so easily shifted around when young, or even middle-aged, that they can be planted closely to begin with and respaced after a few years, by which time their relative vigour and habit will be evident and the final arrangement can be made with confidence. Eventually they will grow into each other, but there is no harm in that provided each has adequate living-space. The larger rhododendrons cannot easily be moved around like so much furniture and should be well spaced from the start. Provided the plants are of similar vigour and habit a spacing of around 8 ft should be sufficient, for most of the larger elepidote rhododendrons, less if they are all of the same clone or species, and 5 to 6 ft for plantings of azaleas and Triflorum rhododendrons.
Transplanting. – The moving of large rhododendrons has already been dealt with in the Introductory Chapters (see Vol. I, pp. 60-6). Rhododendrons that have been growing in well-prepared soil can be moved at any age; the limit is set entirely by the labour and tackle available. But rhododendrons that were planted when young in inadequately prepared positions are a different matter. If the soil is suitable for them, and if they are mulched deliberately or by natural leaf-fall, they may thrive. But when they come to be lifted it may be found that they have subsisted off the leafy top layer of the soil and developed a root system like a door mat. Such plants do not transplant well. Young plants, when shifted from one part of the garden to another, should be lifted with a fork, not chopped out with a spade. Evergreen sorts, when young, can safely be moved at any time, except when making their growths or in hot, dry weather. But deciduous azaleas should always be moved at the normal time, the earlier the better.
Mulching, Feeding, and Watering. – Rhododendrons are all moisture-loving, and delight in a continuously cool damp condition at the root. For plants whose roots are not shaded by their own lower branches, a surface mulching of 4 to 6 in. of leaves is very beneficial. If the soil is poor, a proportion of one-third well-rotted manure may be added to the mulch. Bracken is useful as a mulch, and has considerable nutritional value if cut when the fronds are just fully expanded. Peat is perhaps for most gardeners the most readily available material and is invaluable for protecting the roots of young plants.
Contrary to what was once supposed, rhododendrons respond well to artificial fertilisers, but they should be used in moderation and there is no need to use them at all if the plant is healthy and vigorous. Obviously the fertiliser should be a balanced one formulated for general garden use. It has been suggested that fertilisers that supply phosphorus in the form of calcium superphosphate leave an alkaline residue, but research has shown that is has no such effect.
Except in the driest parts, or where rhododendrons are growing in a rooty soil, artificial watering is not absolutely essential and many large collections in this country have grown up without it. But there is no doubt that rhododendrons benefit greatly from the generous use of the hose or sprinklers, especially when they are making their young growths. Once their growths have hardened, the drought tolerance of rhododendrons increases remarkably.
Dead-heading. – Whenever time and opportunity allow, the flowers of all rhododendrons and azaleas should be removed as soon as they have faded, at least from young plants. The truss will usually break off in one piece between thumb and forefinger, but care should be taken not to remove the upper buds or young growths at the same time, as can easily happen. Curiously enough it has not, so far as is known, even been scientifically proved that this practice benefits the plant or causes it to flower more freely, but if it does not do so much money and labour has been expended in vain over the years, both by nurserymen and garden owners. Generally azaleas and the dwarfer lepidotes flower freely every year even if they set seed; these are not as a rule dead-headed, though they might well live longer if they were. At the other extreme, some rhododendrons with heavy trusses, or which flower late, are inclined to make weak shoots from below the inflorescence whether or not this is removed. These shoots terminate in a growth-bud which in the following year produces a stronger growth ending in a flower-bud, so that what appears to be a single shoot is really the product of two seasons' growth. The individual branch is therefore biennial in its flowering, but so long as all the branches are not in step some flowers are produced every year, but always on shoots that have sprung from a terminal bud. In some rhododendrons the flower-buds terminate a growth of three or more segments, or no growth at all may be made from below an inflorescence until the following season.
Pruning and Disbudding. – Rhododendrons require no regular pruning and, provided they are grown in adequate light, will never need pruning of any sort. Most will respond to hard cutting if for any reason (usually overcrowding or too much shade) they have grown out of shape or become drawn up. Rhododendrons with a smooth, peeling bark are an exception, since they owe this attractive feature to the sloughing off of the outer bark, which takes the dormant growth-buds with it when it is shed. Hard pruning of grafted plants is also unlikely to be a success, for the removal of the top growth will almost certainly result in a forest of suckers from the stock.
Plants that flower when young will develop a bushy habit naturally, since the flowers automatically 'stop' the shoot on which they are borne. However, in the absence of flower-bud the same effect can be achieved simply by cutting off the terminal growth-bud as it expands; the axillary buds will then grow out just as they would have done if the shoot had ended in flowers. If this disbudding is not done, or if the plant is already leggy when received, the longer branches should be shortened in spring to just above a cluster of dormant side-buds. However, many large-leaved, tall-growing rhododendrons are best not treated in this way, since they are not by nature bushy and anyway will normally produce growths both from terminal and axillary buds even before they reach the flowering stage.
Some large-flowered rhododendrons flower too freely for their health when they are young, and should be relieved of some of their flower-buds, especially if more than one is produced at the end of each shoot, as may happen on young and vigorous plants.
Here it should be mentioned that Hardy Hybrids and some other elepidote hybrids are often propagated by grafting onto Ponticum stocks, which may give trouble by sending up suckers. These should be watched for and chopped off with a spade as soon as noticed, but often they are not noticed if the scion has leaves resembling those of R. ponticum or if, as is sometimes the case, the seedling used as the stock is itself a hybrid. Deciduous azaleas are now rarely propagated by grafting and plants raised this way should be avoided, as they always sucker abominably.
Rhododendrons are normally propagated from seeds, layers, cuttings and grafting scions on to suitable root-stocks.
The following account is mainly for the amateur gardener concerned with raising a few plants. The professional propagator will know how to cope with producing the quantities he requires.
The introduction of species from the wild has hitherto been almost entirely from collected seed, but the day may come when outstanding superior forms found by botanical collectors can be quickly established in cultivation by making use of modern methods of packaging and transport to send back cuttings and scions by air. Expeditions could be served by helicopters to carry the vegetative material to the nearest airport from which it could be quickly brought home and handled by skilled propagators. This method would short-circuit by many years the distribution to gardens by the present practice of waiting for plants to flower from seed and then to select the good forms and propagate these from cuttings or grafts.
Seeds. – Seeds from plant collectors in the field do not always arrive at what is generally taken as the optimum time for sowing, and the recipient has to adapt the basic principles to suit the particular circumstances. The successful raising of rhododendrons from seeds saved from cultivated plants really begins with the careful harvesting of the seeds, and the correct storage of these until sowing-time, which will depend on the facilities available.
The flowering period of different rhododendrons extends over several months and seeds are rather slow in developing and ripening. First of all it is important to remember that the species do not come true from seeds where these are collected from plants growing in a mixed collection, where the flowers have become 'open-pollinated'. If it is intended to collect seeds from individual species it is necessary to prevent cross pollination by covering the flowers by 'bagging' the truss with some suitable artificial device.
According to the kinds, seeds will ripen from late summer to the depth of winter, and a watchful eye must be kept so that they are not lost through the seed vessels splitting and the seeds escaping. It will be noticed that gradually the colour of the capsules will change from green to brown and that they will become hard to the touch. The time to collect is when these changes are completed, and the capsules should then be placed in transparent seed packets so that the final ripening and splitting open can be observed. The packets are best stored in vermin- and insect-proof tin boxes and placed in a cool dry place. Cleaning the seeds from the capsules requires meticulous care, particularly when different kinds are being handled in rotation. The straying of a few seeds of one into those of another can in due course cause a lot of trouble.
It will be found in practice that most of the seeds will have become dispersed from the capsules and will be found loose in the packets. Any which remain can be ejected by the blade of a knife. It is most important to separate the broken-up capsules from the seeds, as the debris from these, if sown with the seeds, can be responsible for promoting 'damping- off'. The storage period will depend on the time for sowing. If a warm greenhouse or frame is available a most successful time for sowing is from late January to the end of February. If only cold conditions can be provided sowing must wait until mid-April.
The ideal is to sow earlier in warm conditions so that the maximum growing season lies ahead to allow for the development of sturdy young plants which have become hardened to withstand the first winter. To raise small quantities of plants a good method to follow is to sow in a clean flower pot about 31⁄2 in. in diameter. This should be drained in the accepted fashion by placing broken crocks in the bottom and covering these with coarse peat to prevent the fine sowing mixture from filtering through. A suitable soil mixture is made up from seven parts by bulk of fine moss peat and three parts of silver sand which must not be alkaline.
The pots should be filled practically level with the rim but the mixture should neither be rendered hard nor soft and spongy. The pots are then placed in a shallow container of rain-water so that this percolates to the surface, and then left to drain. The seeds are then sown thinly on the surface and sprinkled over with a very light 'sugaring' of fine dry silver sand. The pots are then placed on a moisture-conserving base of clean weathered ashes from which the dust has been sifted, or on a bed of peat on a suitable warm greenhouse stage or in a frame where the temperature is maintained at from 55° to 6o° F.
A sheet of newspaper should be laid over the pots but prevented from touching the soil surface by some artificial means.
From this point the daily care consists of changing the paper each morning, germination normally takes place in about two to four weeks from the date of sowing and it is necessary to watch out for any pot which shows signs of becoming dry, and immersing this in rain-water, which should be of the same temperature as the house or frame. It is during the early days of the life of the tender new seedlings that 'damping-off' can occur and a close daily scrutiny of each pot should be made so that at the very beginning any tiny areas affected can be removed by simply taking these out with a pair of tweezers. The little depressions thus caused on the surface should be filled with dry, clean silver sand.
Germination will not occur in every pot at the same time, and before the seedlings have time to become white and drawn the pots must be placed on an open stage or shelf near the glass and carefully shaded. It is now simply a question of balancing the daily treatment required against the weather conditions, for example shading should not be continuous during a succession of very dull days.
By the end of April the most forward seedlings will be ready for pricking off into boxes, using the same basic mixture as that in which the seeds were sown, except that it need not be of such fine texture, while some propagators at this stage like to mix in a small proportion of fine acidic sandy loam, which helps the young plants over the intermediate stage between the seed pots and eventually planting out in prepared nursery beds.
The boxes at first are kept in the warm greenhouse or frame, kept shaded and watered as necessary and then transferred to a cold frame during the summer months and the plants gradually hardened off. Some will make sufficient growth to enable them to be planted out in late September or October, whereas the more backward ones will have to be over-wintered in the frame until the spring. It could be said that at this stage the very specialised work of the propagator gives way to established methods of cultivation.
Where no heated facilities can be provided sowing must be delayed until April, when the actual technique of sowing will be the same, but results of course will be slower. The light intensity will be greater and shading must be provided. Damping-off is a greater risk, and growth by the autumn will generally not be sufficient to allow for planting into nursery beds.
Layering. – This method of propagation comes naturally next to seeds, and many examples can be seen where branches at ground level have rooted into the surface soil or mulch. Layering provides the amateur gardener with an ideal trouble-free method of raising a few plants from the specimens he cultivates without detriment to these. A good time to 'put-down' the layers is in October but this can be extended into the early winter, and early spring.
It is usual to select the tips of branches of the larger-leaved plants, including deciduous azaleas, which are nearest to the ground. The length required is ideally about 12-15 in., which means going back from the growing point to two- or even three-year-old wood. The objective should be to produce young new plants of good shape, and this is accomplished by removing any leaves which are growing on the portion of stem to be buried and simply causing a wound on the underside of the stems by scraping away the bark or skin with a knife. It is not necessary to make any incision in the wood. The branch should then be buried firmly in the soil which should of course have been prepared by digging and improved if necessary by the addition of peat or leaf-mould with a little sand mixed in. Suitable wooden pegs can be driven into the ground to keep the branches firm; the growing tip should be bent abruptly upwards so that the layered portion resembles a fully bent elbow-joint. The tip can be kept upright by tying this to a small stake. It is better to complete the work with the disturbed soil matching the level of that which surrounds the parent plant rather than build up an artificial mound which will become very dry.
Sufficient roots will normally have developed to enable the layers to be cut away in about eighteen months to form independent new young plants.
Maintenance throughout this period consists of making certain that the layered branches are not dislodged in any way, such as being loosened by sharp frost, or by the tunnelling action of moles.
For small rhododendrons such as those in the Lapponicum series small rooted portions can be obtained by either pegging down suitable twigs, or by planting the parent plant deeper than usual and working in among the branches a generous quantity of peat and sand; this must be made as firm as possible and 'topped up' when necessary.
Cuttings. – In recent years because of improvements in propagating equipment there has been more progress in raising a wide range of rhododendrons from cuttings than by any other method. The propagating house in an up-to-date establishment is akin to the production line in a factory. The main change is that of raising from cuttings large quantities of hardy hybrids and deciduous azaleas which were previously increased by grafting. The amateur concerned only with raising small quantities has not been overlooked by the designers of equipment for he can purchase well-designed portable electrically heated propagating units.
It is convenient to broadly classify rhododendrons into groups according to the leaf-sizes in order to prescribe the treatment now being given.
The very small-leaved kinds such as those in the Lapponicum series with their hybrids respond well to cold treatment and there is no valid reason to change the established method of taking cuttings, about 11⁄2 in. long in the late summer months as soon as the current season's growth has become firm. The cuttings are made at a leaf joint and dibbled into a mixture of equal parts of fine peat and silver sand, either in boxes or pots which are placed in a draught-proof cold frame and kept shaded and moist. Here they will root slowly and be ready to prick out into a prepared nursery bed in the following spring. There is much to be said for using bell-glasses where these are still to be had and inserting the cuttings in September or October. For the larger-leaved kinds, particularly the hardy hybrids, the amateur can watch with interest the specialist nurserymen turning out large quantities of plants by using mist units, soil-heating cables and specialised lighting. He will not generally be able to emulate these but can raise plants from cuttings by taking these from late August to October. These should be made from the current year's growths, selecting the side shoots about 3 in. long, avoiding the strong central terminal shoots. The cutting is made at a leaf joint, and any leaves growing on the portion to be inserted should be cut off. It is usual to reduce the leaves which remain on the cutting, by cutting these in half. The terminal buds, whether flowering or growth buds, are removed. Some propagators wound the cutting at the base by removing with a sharp knife a thin shaving of the green skin without cutting deeply into the wood, and then dipping the exposed surface into a proprietary rooting powder according to the manufacturer's instructions. The wounded area provides a larger rooting surface than that available from the small basal cut surface.
The cuttings are then inserted in the sand and peat mixture in pots or boxes and these placed in the electrically heated propagating unit. The time taken for rooting varies with the kinds being increased, but the new young plants should be ready after hardening off for planting into prepared beds the following summer. In the absence of special propagating units the containers should be placed in a frame with a temperature of 65° F. and shaded when necessary.
Grafting. – Grafting is still undertaken by nurserymen, and despite existing prejudice about grafted plants and their attendant suckering nuisance, it must be clear to all that when a single rhododendron has received the R.H.S. Award of Merit or First Class Certificate the quickest way to increase and distribute this is by grafting every available scion on to a root-stock. Once this initial propagation is over then future stocks can be raised from cuttings or layers.
A simple method is to select young root-stocks of Rhododendron ponticum with a single clean stem of about 'pencil' thickness and on to this in January or February graft a scion, making the union just above the point where the stem and root join. It is now usual to use a side-graft made simply by taking a shoot of the previous summer's growth, about 3 in. long and cutting away a longitudinal section of this into the wood, and making a corresponding matching cut on the stem of the root-stock. The two cut surfaces are fitted together and tied with raffia or grafting string. The root-stock with its scion is then buried in a bed of peat in a heated frame which is covered by a glass light or well-fitting polythene sheeting. The scion and stock will knit together and be ready for removing into cold frames by April or May.
Propagators differ about leaving the top growth on the root-stock until the union is completed, or cutting this off at the time of grafting. But then, professional propagators differ about many methods and this comes out very clearly at a plant propagators' conference.
There is still much to learn, and experimental work should be continuous.
PESTS, DISEASES, AND DISORDERS
Rhododendrons and azaleas are remarkably free from pests and diseases when grown in the open ground and rarely is any so serious as to call for regular spraying. The most lethal enemy of rhododendrons, as of many other trees and shrubs, is honey fungus (Armillaria mellea), on which a note will be included at the end of Volume IV. The more important pests and diseases specific to Rhododendron are mentioned here; for further details see the Dictionary of Gardening published by the Royal Horticultural Society, and the papers listed in the Bibliography.
Rhododendron Bug or Rhododendron Fly (Stephanitis rhododendri). – This pest, believed to be a native of north-eastern North America, made its appearance in Britain at the beginning of this century. It is a sucking insect, whose presence can be detected in May and June by a mottling of the upper leaf surface and the presence of rusty or chocolate spots beneath. Eventually the leaf turns brown, and if the attack is severe and the summer dry, the plant may be severely weakened. The adult bug has a black body, almost transparent, lacy wings, and long antennae. It is about 1⁄/6 in. long. The eggs are laid in late summer and autumn beneath the new leaves under the epidermis alongside the midrib, and the young start to hatch out towards the end of May. The young bug is wingless, with antennae about as long as the body. It gradually develops into the adult state and in late summer the adults move to the young leaves, where the eggs are deposited. Despite their large wings the bugs do not fly, though when conditions are right they may be carried passively by the wind to infect new plants. Mild attacks can be checked by picking off the affected leaves and burning them. In the case of severe attack the undersides of the leaves should be sprayed two or three times with BHC or malathion at three-week intervals. The rhododendron bug seems to have a particular liking for hybrids deriving from R. catawbiense and R. maximum and thrives best on plants growing in full sun. So it is the older hybrids, which are normally planted in sunny positions, that are most prone to attack, but the most susceptible, such as 'Boule de Neige' and 'Francis B. Hayes' are no longer in commerce.
Rhododendron Leafhopper (Graphocephala coccinea). – This is not in itself a serious pest, but is now thought to be an indirect cause of attacks by the fungal disease known as bud blast (q.v.). The young emerge in spring and feed on the leaf-sap, without causing any noticeable harm. The adult, which is about 1⁄2 in. long, with green and yellow wings, appears in July and the eggs are laid in the bud-scales during late summer and autumn. The recommended treatment is to spray with malathion two or three times during August and September, but this is only necessary in the event of severe attacks of bud blast.
Rhododendron White Fly (Dialeurodes chittendeni). – This member of the white fly family was first noticed in Britain in 1926 and is named after a former Director of the R.H.S. Garden at Wisley, where the pest was first studied. The scale-like larvae feed on the undersides of the leaves from the time they hatch in August until the following April, when they pupate; the winged adults emerge in June. They fly up in swarms when an infected bush is shaken, but do not travel far. The sucking of the sap by the larvae weakens the plant, and the honeydew they secrete may cause the development of sooty moulds on the leaves beneath them. But the pest has proved to be less damaging than was at first thought. Curiously enough, this white fly, although thought to be a native of the Himalaya, shares with the pests already mentioned a preference for the Hardy Hybrids and their parents in the Ponticum subseries.
Various weevils scallop the edges of rhododendron leaves, but the damage they do in this country scarcely justifies the use of chemicals against them.
Bud Blast (Pycnostysanus azaleae). – This disease attacks hybrids with members of the Ponticum series in their ancestry and is indigenous to eastern North America. It was first recorded in Britain in the 1940s and has since become fairly widespread. The first sign of the disease is the appearance in late autumn or early winter of brown patches on the outer scales of the flower-buds. Eventually the bud is killed and becomes covered with pin-headed bristles; these are the spore-bearing organs that give rise to the next year's infection. There is good evidence that the fungus invades the bud through punctures made by the female rhododendron leafhopper when depositing eggs, though it has never been suggested that the fungus is solely dependent on this means of entry. In some gardens only a few buds are killed by the disease and it suffices to pick these off and burn them. Spraying against fungus itself is not effective and the best means of control, in the event of serious infestations, is to spray against the rhododendron leafhopper, as recommended above. Before the appearance of the characteristic bristles it is not at once obvious whether the bud has been killed by bud blast or by frost. But bud-tender rhododendrons are for the most part not subject to bud blast, and frost-damaged buds break off easily, while those killed by bud blast remain firmly attached to the stem.
Azalea Gall (Exobastdium vaccinii). – In Britain this disease seems to be commonest on evergreen azaleas. On plants grown out-of-doors it appears in May on the new leaves, which become blistered and thickened and eventually turn white when the fungus reaches the spore-bearing stage, by which time the leaf itself has become almost wholly replaced by the fungus. The disease is much less serious than it looks and it usually suffices to pick off the affected leaves and burn them.
Petal Blight (Ovulinia azaleae). – This disease attacks and destroys the flowers of evergreen azaleas, without harming the plant itself, and is most likely to occur in warm, damp weather. It has been found on deciduous azaleas and on some hardy hybrid rhododendrons. Of Asiatic origin, it reached the USA in the 1920s and often ruins the display in the south-eastern states, where the evergreen azaleas are much grown. The first sign of attack is the appearance of small specks on the corollas, white on coloured flowers, light brown on white flowers. Within a matter of three days or so the flowers dissolve into a slimy mass and remain for a long time on the bush in a withered state. If only a few plants are attacked the dead flowers should be picked off at once and burned, to prevent the release of spores, which spread the disease and also give rise to resting bodies (sclerotia) which drop to the ground and in the following season give off spores to start a new cycle of infection. The most promising means of control is to spray with the systemic fungicide benomyl when the buds show colour and then at seven-day intervals until just before they open.
Silver Leaf (Stereum purpureum). – It is not generally known that this disease, often called 'plum silver leaf', attacks many woody plants, including rhododendrons, causing die-back of the branches but not betraying its presence by a silvering of the leaves, as it does on plums and damsons. There is nothing that can be done about it, except to cut out all dead branches (which the wise grower does as a matter of routine). The cherry and Portugal laurels are both subject to the disease and could be a source of infection in some gardens.
Most rhododendron troubles are due not to pests or diseases, but to unsuitable conditions at the root, wrong siting or damage by frost. If, in a normal season, the foliage becomes yellow between the veins but these remain the normal green, the plants may be suffering from lime-induced chlorosis, either because the natural soil is too alkaline for them or because it has been heavily limed in the past. It is reported that the use of leaf-mould from trees growing in chalky soils may also contain enough lime to cause chlorosis. A more uniform yellowing of the foliage could indicate nitrogen deficiency (likely to occur if undecayed or half-rotted vegetable matter is added to the soil); or the plant may have been given a too sunny position.
Distortion of the foliage is the result either of dryness at the root at the time the young growths are unfolding, or of damage by spring frost, which often deforms or burns the leaves while leaving the new shoot unharmed. Spring frost may also cause bark-split. If this is noticed early enough the affected branch may survive if the split is bound with tape or treated with a proprietary sealing compound. Rhododendrons that flower before early May are most subject to bark-split, especially if their stems are exposed.
R.Y.B. Rhododendron Year Book
R.C.Y.B. Rhododendron and Camellia Year Book
Journ. R.H.S. Journal of Royal Horticultural Society
American Rhododendron Society. – Quarterly Bulletin, 1947-
Rhododendron Society. – This small and exclusive society published privately three volumes of notes by its members over the period 1916-31. Among the members were leading botanists, and the garden owners who financed the Forrest expeditions.
Royal Horticultural Society. – Rhododendron [and Camellia] Year Book, Nos 1-25 (1946-70). This valuable publication came to a close in 1970 with the Year Book for 1971. Camellias were first included in No. 8 (Year book for 1954).
– – Rhododendrons 1972, Rhododendrons 1973, Rhododendrons 1974 with Camellias and Magnolias. A continuation of the Year Book in more modest form.
Rhododendron Association. – This association, the forerunner of the present Rhododendron Group of the R.H.S., published a Year Book between 1929 and 1939.
Aberconway, Lord. – 'Rhododendrons in the Garden', R.Y.B. 1949, pp. 20-5.
André, E. – Plantes de Terre de Bruyères. Paris, 1864.
Basford, J. S. – 'Some Rhododendrons in the West of Scotland', R.C.Y.B. 1966, pp. 21-30.
Bond, John. – 'Shade Trees for Rhododendrons', R.C.Y.B. 1971, pp. 48-55.
Bowers, C. G. – Rhododendrons and Azaleas. Ed. 2, New York, 1960.
Campbell, Ilay. – 'Giants of the West', R.C.Y.B. 1964, pp. 9-16.
Cowan, J. M. – 'Rhododendrons in the Royal Botanic Garden, Edinburgh', R.Y.B. 1953, pp. 33-57.
Cox, E. H. M. and P. A. – Modern Rhododendrons. London, 1956.
Cox, P. A. – Dwarf Rhododendrons. London, 1973.
– – Rhododendrons (Wisley Handbook 2). R.H.S., London, 1972.
Davidian, H. H. – 'Rhododendrons in the Royal Botanic Garden, Edinburgh', R.C.Y.B. 1959, pp. 9-18; Part II, R.C.Y.B. 1960, pp. 42-53.
Findlay, T. H. – 'The Preparation and Planting of a Virgin Woodland for Rhododendrons', R.C.Y.B. 1955, pp. 45-7.
Fletcher, H. R. – The International Rhododendron Register. R.H.S., London, 1958. (Additions have been published annually in R.C.Y.B., starting in No. 16, and latterly in Rhododendrons.)
– – 'The Rating of Merit of Rhododendron Species', R.C.Y.B. 1962, pp. 30-47.
Gorer, G. – 'Some Notes on Rhododendrons of the Maddenii Series and their Hybrids', R.C.Y.B. 1968, pp. 17-26.
Grootendorst, H. J. – Rhododendrons en Azalea's. Vereniging voor Boskoopse Culturen, 1954.
Hanger, Francis. – 'The Cultivation of Rhododendrons', Journ. R.H.S., Vol. 70, pp. 355-62 (1945).
Kingdon-Ward, F. – Rhododendrons. London, 1949.
Hardy, A. – 'An Adventure with Rhododendrons', Part II, R.C.Y.B. 1971, pp. 34-41.
Leach, David G. – Rhododendrons of the World. London, 1962.
Lee, F. P. – The Azalea Book. Princeton, 1958.
Lucas Phillips, C. E. and Barber, Peter N. – The Rothschild Rhododendrons. London, 1967.
Mangles, J. H. – The numerous articles and notes contributed by Mangles to the Gardeners' Chronicle and other periodicals between 1879 and 1884 were republished by The Rhododendron Society in 1917 (Notes, Vol. I, pp. 41-116).
Millais, J. G. – Rhododendrons and the Various Hybrids. London, 1917.
– – Rhododendrons and the Various Hybrids. Second Series. London, 1924.
Rothschild, Lionel de. – 'Some Notes on Rhododendrons', R.Y.B. 1953, pp. 7-31; R.C.Y.B. 1954, pp. 33-56.
– – 'Notes on the Series of Rhododendrons', Year Book of the Rhododendron
– – 'Notes on Rhododendron Hybrids', ibid., 1933-9.
Royal Horticultural Society. – The Rhododendron Handbook; Part I, Rhododendron Species, 1967; Part II, Rhododendron Hybrids, 1969. This Handbook is periodically revised. The dates given are those of the editions current in 1975.
Russell, J. P. C. – 'Rhododendrons as Foliage Plants', R.Y.B. 1951-2, pp. 52-9.
Russell, James. – Rhododendrons at Sunningdale. Windlesham, 1960.
Street, F. – Rhododendrons. 1965.
Urquhart, Beryl L. – The Rhododendron. Vol. I, 1958; Vol. II, 1962. Sharp-thorne, Sussex.
Wallace, R. & Co. Ltd. – Rhododendron Species, n.d.
Watson, William. – Rhododendrons and Azaleas. London, 1911.
Wright, David. – 'The Grouping of Rhododendrons', R.C.Y.B. 1964, pp. 47-62; R.C.Y.B. 1965, pp. 105-18; R.C.Y.B. 1971, pp. 73-7.
Ammal, E. K. Janaki, et al. – 'Polyploidy in the Genus Rhododendron', R.Y.B. 1950, pp. 92-8.
Cowan, J. M. – The Rhododendron Leaf. Edinburgh, 1950.
– – 'A survey of the Genus Rhododendron', R.Y.B. 1949, pp. 29-56.
Hedegaard, J. – 'Beiträge zur Kenntnis der Morphologie von Rhododendron-Samen' (in the Year Book for 1968 of the German Rhododendron Society).
Hooker, J. D. – Rhododendrons of the Sikkim Himalaya. London, 1849-51.
Hutchinson, J. – 'Evolution and Classification of Rhododendrons', R.Y.B. 1946, pp. 42-7.
Kingdon Ward, F. – -'Observations on the Genus Rhododendron', R.Y.B. 1947, pp. 99-114.
Maximowicz, C. J. – Rhododendreae Asiae Orientalis. St Petersburg, 1870.
Philipson, Mlva N. and W. R. – 'The Classification of Rhododendron' R.C.Y.B. 1971, pp. 1-8.
– – 'The History of Rhododendron Classification', Notes Roy. Bot. Gard. Edin.,
Vol. 32 (1973), pp. 223-38.
Rehder, A. and Wilson, E. H. – A Monograph of Azaleas. Cambridge, Mass., 1921.
Seithe, A. – 'Die Haarformen der Gattung Rhododendron L.', Bot. Jahrb., Vol. 79(3) (1960).
Sleumer, H. – Ericaceae – Rhododendron, in Flora Malesiana, Ser. 1, Vol. 64 (1966), pp. 474-668.
– – 'The Genus Rhododendron L. in Indo-China and Siam', Blumea, Supplement IV (1958), pp. 39-59.
– – 'Ein System der Gattung Rhododendron L.', Bot. Jahrb., Vol. 74(4) (1949), pp. 511-53.
Stevens, P. P. – 'A Classification of the Ericaceae: Subfamilies and Tribes', Bot. Journ. Linn. Soc., Vol. 64 (1971), pp. 1-53.
Stevenson, J. B. (ed.). – The Species of Rhododendron. London, 1930. Ed. 2, 1947. The second edition is essentially the same as the first, but descriptions of an additional 31 species were bound in.
RHODODENDRONS IN THE WILD
Cowan, J. M. (ed.). – The Journeys and Plant Introductions of George Forrest. London, 1952.
Cox, E. H. M. – 'Are there more Rhododendron Species to be Found and Introduced?', R.C.Y.B. 1967, pp. 21-4.
– – Farrer's Last Journey. London, 1926.
– – Plant-hunting in China. London, 1945.
Cox, P. and P., and Hutchison, P. – 'An Expedition to North East India', R.C.Y.B. 1966, pp. 61-77.
Cox, P. and Hutchison, P. – 'Rhododendrons in North-Eastern Turkey', R.C.Y.B. 1963, pp. 64-7.
Fang, W. P. – 'Rhododendrons of Mount Omei', R.Y.B. 1947, pp. 115-23.
Forrest, G. – 'The Flora of North-Western Yunnan', Journ. R.H.S., Vol. 41 (1915), pp. 200-8.
– – 'Notes on the Flora of North-Western Yunnan', Journ. R.H.S., Vol. 42 (1916), pp. 39-46.
– – 'Exploration of N.W. Yunnan and Tibet 1921-1922', Journ. R.H.S., Vol. 49 (1924), pp. 25-36.
– – Lectures to Rhododendron Society, Rhod. Soc. Notes, Vol. II, pp. 3-23 (1920), 147-58 (1923).
Fletcher, H. R. – A. Quest of Flowers; The Plant Explorations of Frank Ludlow and George Sherriff. Edinburgh, 1975.
Hara, H. – 'Occurrence and Distribution of Rhododendrons in Japan', R.Y.B. 1948, pp. 112-27.
Herklots, G. A. C. – 'The Rhododendrons of Hong Kong', R.Y.B. 1949, pp. 183-6.
Hooker, J. D. – Himalayan Journals. London, 1854.
– – 'On the Climate and Vegetation . . . of East Nepal and the Sikkim Himalaya Mountains', Journ. Hort. Soc., Vol. 7 (1852), pp. 69-131.
Ingwersen, W. E. Th. – 'The Rhododendrons of Europe', R.Y.B. 1947, pp. 21-8.
Keenan, J. – 'George Forrest, 1873-1932', Journ. R.H.S., Vol. 98 (1973), pp. 112-18.
Kingdon Ward, F. – The Land of the Blue Poppy. Cambridge, 1913. 2nd Expedition (1911). N.W. Yunnan and bordering S.E. Tibet. No collections of rhododendron seed recorded.
– – In Farthest Burma. London, 1921. 5th Expedition (1914). Upper Burma, including Imaw Bum and Hpimaw, on the Irrawaddy-Salween divide. No rhododendron seed collected.
– – The Mystery Rivers of Tibet. London, 1923. 3rd Expedition (1913). N.W. Yunnan and bordering Tibet. Rhododendron seed-numbers, four only (406, 529, 768, 793).
– – The Romance of Plant Hunting. London, 1924. A confusing book. The early chapters are based on several of the author's expeditions before and immediately after the first world war. But Chapters VIII to XIII describe his visit to the Yungning-Muli area north of the Yangtse bend in 1921 (6th Expedition) and the first half of 1922 (7th Expedition, first part). Rhododendron seed-numbers for the 6th Expedition are 3776-5005, mostly collected in this area.
– – From China to Hkamti Long. London, 1924. Chapters I & II describe the route followed from Burma to Yungning in China in 1921 (6th Expedition). The main part of the book, however, is devoted to the Tibetan Marches and a journey thence across the Salween and Upper Irrawaddy to Hkamti Long in Burma (7th Expedition). Rhododendron seed-numbers 5384-5602.
– – The Kiddle of the Tsangpo Gorges. London, 1926. 8th Expedition (1924). Describes what was, perhaps, the author's most fruitful journey, during which the extraordinary wealth of rhododendrons within and north of the region of the great Tsangpo bend was revealed for the first time.
– – Plant Hunting on the Edge of the World. London, 1930. Part I covers the author's 9th Expedition (1926) to the Seinghku and Di Chu valleys, on the borders between N.W. upper Burma, Tibet and Assam. Rhododendron seed-numbers 6735-7642. Part II is devoted to the 10th Expedition (1927-8), to the Mishmi Hills of Assam (Delei Valley). Rhododendron seed-numbers 7701-8592.
– – Plant Hunter in Tibet. London, 1934. 12th Expedition (1933). The area visited reaches from Assam northward through Rima across the snow-range to the Upper Salween. Rhododendron seed-numbers 10351-11060.
– – Plant Hunter's Paradise. London, 1937. 11th Expedition (1930-1). Upper Burma, mainly the Adung Valley, with a short incursion into Tibet. Rhododendron seed-numbers 9130-10231.
– – Assam Adventure. London, 1941. 13th Expedition (1935). Himalaya East of Bhutan (Balipara Frontier Tract and Mönyül and Tsari); mountains north of Tsangpo. Seed-numbers 11378-12414.
– – Burma's Icy Mountains. London, 1949. 14th Expedition (1937). After an unsuccessful attempt to resume plant exploration in China, the author paid a second visit to the Adung valley in the far north-west of Burma. Rhododendron seed-numbers 13006-13550. Chapters 17-24 describe the American Vernay-Cutting expedition to the Burma-China frontier (1938-9). Rhododendron seed-numbers 00005-00460.
– – Plant Hunter in Manipur. London, 1952. An expedition to Eastern Manipur for the New York Botanical Garden. Rhododendron seed-numbers 17044-17818.
– – Return to the Irrawaddy. London, 1956. Kingdon Ward's last expedition (1953), which took him to the Triangle, a mountainous region between the two main branches of the Upper Irrawaddy. Rhododendron seed-numbers 20601-22036.
– – Pilgrimage for Plants, with a Biographical Introduction and Bibliography by W. T. Stearn. London, 1960. A collection of essays, of which Chapter 7 is devoted to rhododendrons. It was Kingdon Ward's last work, and is particularly valuable for the summary of his plant-hunting expeditions and voluminous writings, contributed by Dr W. T. Stearn of the British Museum (Natural History).
– – 'Plant-hunting in the Triangle, North Burma', Journ. R.H.S., Vol. 80 (1955), pp. 174-90.
– – 'Botanical Explorations in North Burma', Journ. R.H.S., Vol. 71 (1946), pp. 318-25.
– – 'Six Months in Mönyul', Journ. R.H.S., Vol. 69 (1944), pp. 267-72, 305-7.
– – 'Rhododendrons in the Wild', R.Y.B. 1949, pp. 9-19.
– – 'Collectors' Numbers. Reasons for their Retention', R.C.Y.B. 1956, pp. 48-51.
– – 'Rhododendrons in Burma, Assam and Tibet', R.Y.B. 1947, pp. 13-20.
Lancaster, R. – 'An Account of the Species found by the University College Bangor Nepal Expedition 1971', Rhododendrons 1972, pp. 24-32.
– – 'Rhododendron Species encountered on the Hilliers Plant Trek to East Nepal 1973', Rhododendrons 1973, pp. 39-45.
Rock,Joseph F. – The Ancient Na-Khi Kingdom of Southwest China Cambridge, Mass. (1947). 2 vols. Although primarily an historical and ethnographic work, it contains superb collotype plates illustrating the landscapes of N.W. Yunnan and S.E. Szechwan, and four useful maps.
Schweinfurth, Ulrich. – Die horizontale und verticale Verbreitung der Vegetation im Himalaya (Bonner Geographische Abhandlungen Heft 20). Bonn, 1954.
Skinner, Henry T. – 'In Search of Native Azaleas', R.C.Y.B. 1957, pp. 9-28 (abridged from Morris Arboretum Bulletin, Vol. 6 (1955), Nos 1 and 2).
Stainton, J. D. A. – Forests of Nepal. London, 1972.
Sykes, W. R. – 'Rhododendrons in Western Nepal', R.C.Y.B. 1954, pp. 62-7.
Synge, P. M. – 'Sir Joseph Hooker and the Rhododendrons of the Sikkim Himalaya', R.Y.B. 1955, pp. 38-51.
Taylor, George. – 'Plant Collecting in South-eastern Tibet', Journ. R.H.S., Vol. 72 (1947), pp. 130-44, 166-77.
Ward, F. Kingdon. – See Kingdon Ward, F. Most of the author's works are signed F. Kingdon Ward and usually indexed under Ward, F. K. But from the late 1940s he hyphenated his name, and his later books and articles are therefore indexed under Kingdon-Ward, F. For convenience, all references to his writings in this Bibliography are listed under Kingdon Ward, F.
Wilson, E. H. – 'Rhododendrons of Hupeh Province', Rhod. Soc. Notes Vol. II (1923), pp. 160-74.
– – 'Rhododendrons of North-Eastern Asia', ibid. (1922), pp. 93-106.
– – 'Rhododendrons of the Bonin and Liukiu Islands and of Formosa', ibid. (1924), pp. 228-40.
– – 'Rhododendrons of Eastern China', op. cit., Vol. III (1925), pp. 18-28.
– – A Naturalist in Western China. London, 1913. 2 vols.
Aberconway, Lord, et al. – 'Aims in Breeding Rhododendrons . . .', R.C.Y.B. 1960, pp. 19-41.
Aberconway, Lord. – 'Rhododendron Hybrids', Journ. R.H.S., Vol. 76 (1951), pp. 257-64.
Barber, P. N. – 'Rhododendrons at Exbury', Journ. R.H.S., Vol. 94 (1969), pp. 379-86; R.C.Y.B. 1970, pp. 25-35.
Findlay, T. H. – 'Notes on Rhododendron Hybrids at the Great Park, Windsor', R.C.Y.B. 1967, pp. 73-4.
– – 'Some Dwarf Rhododendron Hybrids raised at Windsor', R.C.Y.B. 1971, pp. 58-9.
George, Arthur. – 'Modern Compact Rhododendron Hybrids', Journ. R.H.S., Vol. 96 (1971), pp. 449-55.
– – 'Choosing Rhododendrons for a Small Garden', Journ. R.H.S., Vol. 98 (1973), pp. 541-5.
Hillier, H. G., et al. – 'Symposium on the Most Popular Rhododendron Hybrids', R.C.Y.B. 1963, pp. 68-82.
Hobbie, Dietrich. – 'Rhododendrons at Linswege, Oldenburg, Germany', R.C.Y.B. 1959, pp. 43-8.
Puddle, C. E. – 'Dwarf Rhododendrons for the Small Garden', R.Y.B. 1950, pp. 27-37.
Puddle, F. C. – 'The Breeding of Pedigree Rhododendrons', R.Y.B. 1948, pp. 35-41.
Rose, F. J. – 'Hybrid Rhododendrons', R.Y.B. 1949, pp. 88-97.
Russell, J. P. C. – 'New Types of Hybrid Rhododendrons for the Small Garden, Journ. R.H.S., Vol. 70 (1945), pp. 225-32.
Slocock, O. C. A. – 'Garden Hybrid Rhododendrons', R.Y.B. 1949, pp. 101-13.
Street, F. – Hardy Rhododendrons. London, 1954.
PESTS, DISEASES, AND DISORDERS
Anon. – 'Bud Blast on Rhododendrons and Azaleas', Journ. R.H.S., Vol. 75 (1950), pp. 230-2.
Baillie, A. F. H. and Jepson, W. F. – 'Bud Blast Disease of Rhododendron . . .', Journ. R.H.S., Vol. 76 (1951), pp. 355-65.
– – 'Rhododendron Bud Blast', R.Y.B. 1951-52, pp. 72-8.
Brooks, Audrey V. – 'Disorders of Rhododendrons and Camellias', Rhododendrons 1974, pp. 57-62.
– – 'Diseases of Rhododendrons and Camellias', Rhododendrons 1975 (seen in manuscript by courtesy of Miss Brooks and the Royal Horticultural Society).
Harris, K. M. – 'Pests of Rhododendrons', R.C.Y.B. 1968, pp. 146-55.
Hoare, R. H. – 'The Rhododendron Bug', Journ. R.H.S., Vol. 48 (1923), p. 16-21.
Slocock, O. C. A. – 'The Lacewing Fly', Y.B. Rhod. Ass. 1934, pp. 89-92.
Street, F. – 'Some Observations and Notes on Bud Blast on Rhododendrons', R.Y.B. 1950, pp. 72-7.
Wilson, G. F. – 'Insect Pests of Rhododendrons', Journ. R.H.S., Vol. 50 (1925), pp. 46-54.
– – 'A Leaf-hopper (Jassid) on Rhododendron', Journ. R.H.S., Vol. 62 (1937), pp. 210-13.
– – 'The Rhododendron White Fly', Journ. R.H.S., Vol. 54 (1929), pp. 214-17 and (same title) Journ. R.H.S., Vol. 60 (1935), pp. 264-71.
Contributed by F. P. Knight
From the Supplement (Vol. V)
The following is a continuation of the note on rhododendron introductions (page 543). Thanks to improved relations with China, it was possible in 1981 for a British party, in the company of Chinese botanists, to visit the Cangshan (Tali) range of Yunnan, west of Lake Erhai. This mountain, and others a short way to the north-east, are of historical interest as the source of the many rhododendron specimens collected by the French Jesuit missionary Delavay between 1882 and 1888, when he was stationed in a village by the lake. It was these collections, described by Franchet soon after their arrival in Paris, which gave a first inkling of the immense wealth of rhododendrons in this part of China. The visit by the British party and their Chinese companions took place in May but, despite the season, a fair amount of unshed seed was found, especially of the larger species, and plants from the SBEC seed are now growing in several collections. See further the article by Peter Cox, a member of the expedition, in the article cited in the supplementary bibliography. How closely the Cangshan rhododendrons are associated with Père Delavay is shown by the following list of SBEC collections:
(a). Described by Franchet from specimens collected in the Cangshan itself: R. decorum, R. fastigiatum, R. haematodes, R. neriiflorum, R. sulfureum; also R. edgeworthii (syn. R. bullatum Franch.) and R. virgatum subsp. oleifolium (R. oleifolium Franch.).
(b). Described by Franchet from Delavay collections in the mountains near Tali: R. cephalanthum, R. heliolepis, R. irroratum, R. lacteum; also R. arboreum subsp. delavayi (R. delavayi Franch.) and R. fictolacteum, the type of which is R. lacteum var. macrophyllum Franch.
The opening up of some parts of China to tourism has enabled plantsmen to reintroduce many rhododendrons from Wilson's hunting grounds in western Szechwan. Those that have been most widely distributed were made by Roy Lancaster and Keith Rushforth, and most are mentioned below.
There is now what must be an almost complete set of Formosa (Taiwan) rhododendrons in cultivation here and in the USA, as a result of the 'Rhododendron Venture' of John Patrick and C. C. Hsu. See the R.H.S. Handbook of 1980, p. 326, for field-numbers.
In the following supplementary account, no mention is made of recent reintroductions from the Himalaya, unless there is some special reason for doing so. Most of these collections were made in eastern Nepal, where the rhododendron flora is similar to that of Sikkim, source of the 'Hooker rhododendrons' long established in cultivation.
Nepal is now well trodden ground. But there are still discoveries to be made in the less accessible parts of the eastern Himalaya and in the even more remote mountains between the Tsangpo (Brahmaputra) and Irrawaddy, not only of new species, but also of western extensions of those which on present knowledge have their western limit in Burma. Kingdon Ward made two incursions into the region east of Bhutan now known as Arunachal Pradesh (see his Assam Adventure and 'Six Months in Mönyul', both listed in the main bibliography). Incidentally, seed-numbers KW 13606-14342 belong to his second visit to this area (1938), but in the 1980 edition of the R.H.S. Species Handbook are misleadingly lumped with his 1937 collections from Burma (Adung valley). Mention was made in the main work of some Cox and Hutchison rhododendrons collected in the Apa Tani valley, Arunachal Pradesh, in 1965. The most distinct of their finds is the tender Vireya-type rhododendron R. santapaui; two others – R. coxianum and R. subsansiriense – appear in the present supplement.
Mention was made on page 543 of the 'Rock 59s'. As the R.H.S. Handbook of 1980 now makes clear, the numbers under which Rock's 1923-4 collections were distributed are those given them by the Plant Introduction Division of the US Department of Agriculture. The original field-numbers, which are of course the ones cited in botanical treatments, can be ascertained from the Handbook (or from Rock's published field-notes for this expedition) and are those that should always be used in future.
No mention is made on pages 557-8 of rhododendron powdery mildew. This was first noticed in Britain in 1980, though many growers did not make its acquaintance until the late summer and autumn of 1984, significantly after a rainy spell following a dryish summer. See the section on powdery mildews in the general account of the diseases and disorders of trees and shrubs by Audrey Brooks appearing at the end of this Supplement; also the note in The Garden (Journ. R.H.S.), Vol. 109, pp. 406-7 (1984).
(for main bibliography, see Vol. III, page 559)
Rhododendrons [year] Rhododendrons [year] with Magnolias and Camellias
Notes R.B.G. Edin. Notes from the Royal Botanic Garden, Edinburgh
Royal Horticultural Society – Rhododendrons [year] with Magnolias and Camellias, 1974-. The two previous issues in the same format were entitled Rhododendrons 1972 and Rhododendrons 1973.
Rhododendron Species Foundation – Rhododendron Notes and Records, 1984-.
Brickell, C. D. – 'Rhododendron Cultivars, their Nomenclature and International Registration', in Luteyn (op. cit.), pp. 63-71 (discussion pp. 71-3, 324-6). Reprinted in Rhododendrons 1978, pp. 41-51.
Chamberlain, David F. – 'A Revision of Rhododendron. II. Subgenus Hymenanthes', Notes R.B.G. Edin., Vol. 39 (2), whole issue, 486 pp. (1982).
Chamberlain, David F. – 'The Taxonomy of the Elepidote Rhododendrons, excluding Azalea', in Luteyn, (op. cit.), pp. 39-52.
Chamberlain, David F. – 'Species Collections and the Classification of the Genus Rhododendron', in Rhod. Notes & Records, Vol. 1, pp. 1-6 (1984).
Chamberlain, David F. and Ming, Tian-lu – 'The Distribution of Rhododendron subgenus Hymenanthes with special reference to Yunnan', Notes R.B.G. Edin., Vol. 43, pp. 7-13 (1985).
Chamberlain, David F. et al. (eds) – Proceedings of the Second International Rhododendron Conference, 11-13 May 1982, Notes R.B.G. Edin., Vol. 43, pp. 117-60 (1985). The contributions deal with rhododendron anatomy and morphology.
Ching, R. C. – Rhododendron, in Vol. III of Iconographia Cormophytorum Sinicorum, Beijing (Peking), 1974, translated from the Chinese by Judy Young and Dr Lusheng Chong, with the English title Rhododendrons of China. American Rhododendron Society and the Rhododendron Species Foundation, Oregon, 1980. For authorship, see Rhod. Notes & Records, Vol. 1, p. 166.
Cox, Peter A. – The Larger Species of Rhododendron. London, 1979.
Cox, Peter A. – The Smaller Rhododendrons. London, 1985. This work and the preceding, by a leading authority on the genus, complement admirably the Edinburgh revision by applying it to the cultivated plants.
Cox, Peter A. – 'Rhododendron Species and Forms introduced or reintroduced in recent years', Rhododendrons 1978, pp. 6-11.
Cox, Peter A. – 'Expedition to Cangshan in Yunnan', Rhododendrons 1981-2, pp. 1-11. An account of the SBEC expedition, 1981.
Cox, Peter A. – 'Natural Hybrids in Rhododendron', The Plantsman, Vol. 4(3), pp. 182-5 (1982).
Cullen, James – 'A Revision of Rhododendron. I. Subgenus Rhododendron suctions Rhododendron and Pogonanthum', Notes R.B.G. Edin., Vol. 39(1), whole issue, 270 pp. (1980).
Cullen, James – 'Rhododendron Taxonomy and Nomenclature', in Luteyn, op. cit., pp. 27-38.
Cullen, James and Chamberlain, D. F. – 'A Preliminary Synopsis of the Genus Rhododendron', Notes R.B.G. Edin., Vol. 36(1), pp. 105-26 (1978) and Vol. 37, pp. 327-38 (1979). These two papers are superseded by the complete revisions of subgenera Rhododendron and Hymenanthes referred to under the two authors separately.
Darden, Jim – Great American Azaleas. Clinton, North Carolina, 1985. Well illustrated in colour.
Davidian, H. H. – The Rhododendron Species, Vol. I. Lepidotes. London, 1982. Mr Davidian was the co-author with Dr Cowan, and later sole author, of the invaluable reviews of the Rhododendron series that appeared in the R.H.S. Rhododendron [and Camellia] Year Book between 1948 and 1964. As will be evident both from his remarks on pp. 18-19 and from the content of the work, the author is in strong disagreement with the new Edinburgh revision at many points. For all that, the work is of the greatest interest and usefulness.
Feng, Kuo-mei (ed.) – Unnan no Skakunage (Rhododendron of Yunnan). Tokyo, 1981. A translation into Japanese of a work prepared by the Kunming Botanical Research Station in Yunnan, splendidly illustrated in colour with photographs of rhododendrons in their native habitat.
Galle, Fred C. – Azaleas. Portland, Oregon, 1985. Illustrated with 366 colour photographs.
George, Arthur – 'The hybrids of Rhododendron yakushimanum', The Garden (Journ. R.H.S.), Vol. 106, pp. 262-4 (1981).
Hedegaard, Johannes – Morphological Studies in the Genus Rhododendron, two vols. Copenhagen, 1980. See also the lecture by the same author in Luteyn, op. cit., pp. 117-44.
Ito, Ihe – A Brocade Pillow. Azaleas of Old Japan. New York, 1984. A translation of a 17th-century work, with an introduction and commentary by John L. Creech.
Kessel, Mervyn – Rhododendrons and Azaleas. Poole, England, 1981.
Kraxberger, Meldon (ed.) – American Rhododendron Hybrids. The American Rhododendron Society, Tigard, Oregon, 1980.
Livingston, Philip A., and West, Franklin – Hybrids and Hybridizers. Rhodendrons and Azaleas for Eastern North America. Newtown Square, Pennsylvania, 1978.
Lucas Phillips, C.E., and Barber, Peter N. – The Rothschild Rhododendrons, second (revised) edition. London, 1979.
Luteyn, James L. (ed.) – Contributions Toward a Classification of Rhododendron. New York Botanic Garden, 1980. The Proceedings of the International Rhododendron Conference of 1978. Some individual contributions are listed separately in this bibliography.
Ota, Y., and Togashi, M. – Species of Rhododendron in Japan. Tokyo, 1981.
Philipson, W. R. – 'Problems in the Classification of the Azalea Complex', in Luteyn, op. cit., pp. 53-62.
Philipson, Melva N., and Philipson, W. R. – 'A Revision of Rhododendron section Lapponicum', Notes R.B.G. Edin., Vol. 34(1), pp. 1-72 (1975).
Philipson, Melva N., and Philipson, W. R. – 'A Preliminary Synopsis of the Genus Rhododendron Part III', Notes R.B.G. Edin., Vol. 40(1), pp. 225-7 (1982).
Royal Horticultural Society – An Alphabetical Checklist of Rhododendron Species. London, 1981.
Royal Horticultural Society – The Rhododendron Handbook 1980. Rhododendron Species in Cultivation. London, 1980. This supersedes the previous edition of 1967. The section devoted to collectors' numbers has been extended, but some of the determinations are now out of date, the handbook having been prepared before the publication of the new Edinburgh revisions.
Salley, Homer E., and Greer, Harold E. – Rhododendron Hybrids. London, 1986. A work originally published in the USA, illustrated with 600 colour photographs.
Schwartz, Diane – Rhododendrons. A Selected, Annotated Bibliography. New York Botanic Garden, 1975.
Schweinfurth, Ulrich, and Schweinfurth-Marby, Heidrun – Exploration in the Eastern Himalaya and the River Gorge Country of Southeastern Tibet. Wiesbaden, 1975. An annotated and very comprehensive bibliography of the writings of Frank Kingdon Ward, with two folded maps in an end-pocket.
Seithe, Almut – 'Rhododendron Hairs and Taxonomy', in Luteyn, op. cit., pp. 89-115.
Tam, Pui-cheng – A Survey of the Genus Rhododendron in South China. Hong Kong, 1983. A work devoted to the species of south-easternmost China, in Chinese, English and Latin, with six colour plates (four from paintings) and twenty-nine drawings. It contains a key in English and descriptions in Latin of about forty new species, mostly of the Azalea and Azaleastrum groups. Few of the previously known species from this area of China are in cultivation.
When the section on Rhododendron was being prepared for the present edition, work on a completely new taxonomic revision of the genus (excluding the 'Malesian' rhododendrons) was in progress in the herbarium of the Royal Botanic Garden, Edinburgh, which has long been the main centre of rhododendron studies and holds by far the largest collection of type-specimens. The first instalment, a revision of the Lapponicum group (subsect. Lapponica) by Melva and William Philipson, appeared shortly before Volume III went to press. Since then, this important enterprise has been brought nearer completion by the publication of revisions of the subgenera Rhododendron and Hymenanthes (the lepidote and elepidote rhododendrons). These form part I (by Dr James Cullen) and part II (by Dr David Chamberlain) of the overall revision and were published in volume 39 of Notes from the Royal Botanic Garden, Edinburgh. Each contains a preliminary discussion, descriptions, keys to the subsections, species and their subordinate taxa, distribution maps, an analysis of geographical distribution, and a lengthy list of specimens examined, with their determinations.
The basic unit in the Edinburgh classification, as in Dr Sleumer's, is the subsection, each of which broadly corresponds to a series or subseries of the old classification and usually takes its name, though with a termination of botanical form, e.g., subsection Auriculata for Auriculatum series. The subsections are grouped into sections and these into subgenera – subgen. Rhododendron with two sections apart from Vireya, and subgenus Hymenanthes with only one.
In the following summary of the new classification the corresponding series or subseries is mentioned under the subsection to which it wholly or mainly belongs, with a note on the transfers that have taken place. The abbreviations Rev. 1 and Rev. 2 indicate the first part (subgenus Rhododendron) and second part (subgen. Hymenanthes) of the Edinburgh revision (see above). The author of the subsectional name is Sleumer, unless otherwise stated.
For a general discussion, see pages 568-9 of the main work, under the heading Lepidote Rhododendrons. Under modern rules the subdivisons of a genus which contain its type-species – in this case R. ferrugineum – take the generic name. This rule was not operative when Dr Sleumer produced his system, and he used the name Lepidorhodium for this subgenus, which contains three sections:
See page 575. This section is not treated in the Edinburgh revisions, but a paper on the outlying subsection Pseudovireya (series Vaccinoides) will be the subject of a special paper.
References were given on page 576 to portraits of Malesian rhododendrons published in the new series of the now extinct Botanical Magazine. Later plates are: 726, 748, 766, 787, 808 and 816; and in its successor, the Kew Magazine: Vol. I (1), t.6, Vol. I (3), t.16 and Vol II (1), t.29.
sect. Rhododendron Rev. 1, pp. 23-5 (general and key to subsections).
To this section belong all the lepidote rhododendrons other than those placed in the other two sections, which thus contains subgenera Rhodorastrum (Maxim.) C.B. Cl., Pseudorhodorastrum Sleumer and Pseudoazalea Sleumer (see page 569, second full paragraph).
As in Sleumer's system, this contains numerous subsections, for whose names he is the author, unless otherwise stated.
subsect. Afghanica Cullen Rev. 1, p. 156.
Syn.: series Triflorum ss. Hanceanum, in part.
A new, monotypic subsection.
subsect. Baileya Rev. 1, 150-51.
Syn.: series Lepidotum ss. Baileyi.
subsect. Boothia Rev. 1, pp. 133-8.
Syn.: series Boothii ss. Boothii and ss. Megeratum. For ss. Tephropeplum, see subsect. Tephropepla.
subsect. Camelliiflora Rev. 1, pp. 138-9.
Syn.: series Camelliiflorum.
subsect. Campylogyna Rev. 1., pp. 145-6.
Syn.: series Campylogynum.
subsect. Caroliniana Rev. 1., pp. 91-2.
Syn.: series Carolinianum. The fact that R. carolinianum has been included in R. minus does not affect the validity of the subsectional name.
subsect. Cinnabarina Rev. 1, pp. 122-6.
Syn.: series Cinnabarinum.
subsect. Edgeworthia Rev. 1, pp. 25-9.
Syn.: series Edgeworthii.
subsect. Fragariflora Cullen Rev. 1, pp. 119-20.
This new subsection contains only R. fragariflorum, intermediate between subsections Saluenensia and Campylogyna.
subsect. Genesteriana Rev. 1, pp. 146, 148.
Syn.: series Glaucophyllum ss. Genesterianum.
subsect. Glauca Rev. 1, pp. 139-45.
Syn.: series Glaucophyllum ss. Glaucophyllum.
subsect. Heliolepida Rev. 1, pp. 87-91.
Syn.: series Heliolepsis.
subsect. Lapponica Rev. 1, pp. 92-109.
Syn.: section Lapponica Philipson & Philipson; section Setosa Philipson & Philipson; series Lapponicum.
Dr Cullen's account of this group is based on the revision by the Philipsons in Notes Roy. Bot. Gard. Edin., Vol. 34, pp. 1-71 (1975).
subsect. Lepidota Rev. 1, pp. 148-50.
Syn.: series Lepidotum ss. Lepidotum.
R. cowanianum is transferred to this subsection from the Trichocladum series.
subsect. Maddenia Rev. 1, pp. 29-57.
Syn.: series Maddenii (all subseries).
Dr Cullen divides this subsection informally into four groups. So far as the cultivated species are concerned, Group 1 = ss. Maddenii; Group 2 = ss. Megacalyx with the exclusion of R. megacalyx itself, which is the only species of Group 3; Group 4 = ss. Ciliicalyx.
subsect. Micrantha Rev. 1, p. 132.
Syn.: series Micranthum.
subsect. Monantha Cullen Rev. 1, pp. 59-61.
This new subsection contains four species, one from the Uniflorum series and three from the Triflorum series. Probably none is in cultivation.
subsect. Moupinensia Rev. 1, pp. 57-9.
Syn.: series Moupinense.
subsect. Rhododendron Rev. 1, pp. 110-11.
Syns.: subsect. Ferruginea Sleumer; series Ferrugineum.
subsect. Rhodorastra (Maxim.) Cullen Rev. 1, pp. 111-14.
Syns.: sect. Rhodorastrum Maxim.; series Dauricum.
subsect. Saluenensia Rev. 1, pp. 114-19.
Syn.: series Saluenense.
subsect. Scabrifolia Cullen Rev. 1, pp. 80-87.
Syns.: subgenus Pseudorhodorastrum sections Trachyrhodion and Rhodobotrys Sleumer; series Scabrifolium.
subsect. Tephropepla Rev. 1, pp. 126-9.
Syn: series Boothii subseries Tephropeplum (with the exclusion of the imperfectly known R. chrysolepis). In this subsection also come: R. longistylum (from series Yunnanense ss. Yunnanense) and R. hanceanum (from series Triflorum ss. Hanceanum).
subsect. Uniflora Rev. 1, pp. 120-22.
Syn.: series Uniflorum.
subsect. Virgata Rev. 1, pp. 129-32.
Syns.: subgenus Pseudorhodorastrum section Rhabdorhodion Sleumer; series Virgatum.
sect. Pogonanthum G. Don Rev. 1, pp. 156-69.
Syn.: series Anthopogon, page 569.
A very distinct group, not divided into subsections.
subgenus Hymenanthes (Bl.) K. Koch
Hymenanthes Bl.; subgen. Eurhododendron auct., not K. Koch
For a general discussion, see pages 576-7, under the heading Elepidote (Hymenanthes) Rhododendrons.
This subgenus is more uniform than subgenus Rhododendron, and contains only a single section, for which the correct name is sect. Ponticum G. Don. This is divided by Dr Chamberlain into twenty-four subsections, corresponding more or less to the old series and subseries, though the number of transfers is larger than in the typical (lepidote) subgenus.
The type of the subgenus is R. japonicum (Bl.) Schneider (Hymenanthes japonica Bl.; R. metternichii Sieb. & Zucc.). For this name, see further in this supplement.
subsect. Ponticum G. Don
subsect. Arborea Rev. 2, pp. 328-33.
Syn.: series Arboreum ss. Arboreum.
subsect. Argyrophylla Rev. 2, pp. 317-28.
Syns.: subsect. Floribunda Sleumer; series Arboreum ss. Argyrophyllum.
This subsection now also includes R. adenopodum from the Ponticum series. The former Argyrophyllum subseries was divided by Dr Sleumer into two subsections, typified respectively by R. argyrophyllum and R. floribundum, but this split is not recognised in the Edinburgh classification.
subsect. Auriculata Rev. 2, pp. 240-41.
Syn.: series Auriculatum.
subsect. Barbata Rev. 2, pp. 379-84.
Syn.: series Barbatum ss. Barbatum (excluding R. imberbe, which is probably a natural hybrid).
subsect. Campanulata Rev. 2., pp. 371-4.
Here belong R. campanulatum and R. wallichii. The other species of the highly artificial Campanulatum series have been distributed as follows: R. fulgens and R. sherriffii to subsect. Fulgensia; R. lanatum and R. tsariense to subsect. Lanata; and R. succothii to subsect. Barbata as an anomalous member.
subsect. Campylocarpa Rev. 2, pp. 262-7.
Syns.: series Thomsonii ss. Campylocarpum and ss. Souliei. It also includes Sleumer's subsect. Souliea.
subsect. Falconera Rev. 2, pp. 251-60.
Syn.: series Falconeri.
R. semnoides is transferred to this subsection, from the Grande series.
subsect. Fortunea Rev. 2, pp. 224-40.
Syn.: series Fortunei (all subseries).
subsect. Fulgensia Chamberlain Rev. 2, pp. 414-16.
Here are placed R. fulgens and R. sherriffii, formerly in series Campanulatum.
subsect. Fulva Rev. 2, pp. 366-8.
Syn.: series Fulvum.
subsect. Glischra Rev. 2, pp. 283-90.
Syn.: series Barbatum ss. Crinigerum and ss. Glischrum (but R. hirtipes and R. bainbridgeanum are transferred to subsect. Selensia). To this subsection also now belongs R. recurvoides, formerly in the Taliense series.
subsect. Grandia Rev. 2, pp. 241-51.
Syn.: series Grande (but R. semnoides goes to subsect. Falconera).
subsect. Griersonia Rev. 2, pp. 374-5.
Syn.: series Griersonianum.
subsect. Irrorata Rev. 2, pp. 290-305.
Syn.: series Irroratum ss. Irroratum.
subsect. Lanata Chamberlain Rev. 2, pp. 368-71.
To this new subsection are transferred R. lanatum and R. tsariense from the Campanulatum series.
subsect. Maculifera Rev. 2, pp. 267-74.
Syn.: series Barbatum ss. Maculiferum. The subsection also includes R. sikangense (cookeanum), from series Irroratum ss. Parishii.
subsect. Neriiflora Rev. 2, pp. 384-414.
Syn.: series Neriiflorum (all subseries).
subsect. Parishia Rev. 2, pp. 375-9.
Syn.: series Irroratum ss. Parishii (but R. cookeanum to subsect. Maculifera, as R. sikangense and R. venator to subsect. Venatora).
subsect. Pontica Rev. 2, pp. 305-17.
Syn.: series Ponticum (both subseries, but excluding R. adenopodum, which goes to subsect. Argyrophylla).
subsect. Selensia Rev. 2, pp. 274-83.
Syn.: subsect. Martiniana Sleumer; series Thomsonii ss. Selense.
This subsection also includes R. hirtipes and R. bainbridgeanum, from series Barbatum ss. Glischrum.
subsect. Taliensia Rev. 2, pp. 333-66.
Syns.: subsect. Lanata Sleum.; series Taliense (all subseries, but R. recurvoides transferred to subsect. Glischra); series Lanatum.
subsect. Thomsonia Rev. 2, pp. 416-28.
Syn.: series Thomsonii ss. Thomsonii and ss. Cerasinum.
subsect. Venatora Chamberlain Rev. 2, pp. 290.
subsect. Williamsiana Chamberlain Rev. 2, pp. 260-61.
Syn.: series Thomsonii ss. Williamsianum.
A fairly lengthy supplementary account is necessary, to bring the main work into line with the Edinburgh revision, and the opportunity has been taken to add more information concerning the species, where necessary. For the taxon subspecies; the problems presented by botanical 'sinkings'; and the new category of horticultural Group, see the introduction to this supplement.
For the references Rev. 1 and Rev. 2, see above under Classification. The abbreviation Rev. Lapp. indicates the separate revision of the Lapponicum group by the Philipsons, which is more detailed than the treatment in the main revison of the subgenus Rhododendron based on it.
In this supplement awards given to species at R.H.S. shows under cultivar-names are not mentioned unless the clone concerned is available in commerce.