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Eucalyptus includes over 900 species and is largely endemic to Australia, though 16 species occur naturally in New Guinea, Indonesia and the Philippines (Hill 2002b). It is perhaps the most significant tree genus in Australia, occurring in almost every habitat type. Eucalypts are also widely grown outside Australia, to stabilise soil and as a source of timber, firewood, pulpwood and essential oils. Their use in such plantations is controversial but some of the supposedly deleterious effects have been exaggerated, and they seldom regenerate naturally (although where they do they can be invasive weeds). No genus of this size and importance is without taxonomic controversy, and in Eucalyptus the opposing taxonomic viewpoints are vigorously defended. The account of Eucalyptus in Flora of Australia (Chippendale 1988) recognises two genera: Angophora and Eucalyptus. However, a phylogenetic study (Johnson & Briggs 1984) led Johnson to propose that Eucalyptus should be split into 10 or 11 segregate genera. This proposal proved controversial (see ‘Letters to the Editor’ in the Australian Systematic Botany Society Newsletter, nos. 38/39) and was never implemented, though Hill & Johnson (1995) did later split the bloodwoods, spotted gums and ghost gums into a separate genus Corymbia K.D. Hill & L.A.S. Johnson (see p. 275). The recognition of three eucalypt genera in Australia (Angophora, Corymbia and Eucalyptus) is broadly in agreement with recent phylogenetic studies and has been accepted by most authors (for example, Ladiges & Udovicic 2000, Hill 2002a, 2002b, 2002c), though Brooker (2000) disputes this and recognises only Eucalyptus.
The descriptions below are based on the Eucalyptus account in Flora of Australia (Chippendale 1988). Since 1988, however, numerous new species have been described, and taxonomic study of Eucalyptus has been intensive (see, for example, Johnson & Hill 1990a, 1990b, Hill & Johnson 1991a, 1991b, 1992, 1994, 1995, 1998, Johnson & Hill 1999, Hill & Johnson 2000, Hill et al. 2001). A comprehensive review of 894 Australian Eucalyptus species – EUCLID (third edition, usually known as EUCLID3) – was published in October 2006 by the Centre for Plant Biodiversity Research, Australian National Herbarium, Canberra, as a DVD. It is a guide to identification and a source of information, and should be used by anyone with a deep interest in this fascinating genus.
Eucalyptus species are trees or large shrubs, or form dwarf shrubs with woody tubers producing numerous coppice shoots (mallees). They are often fast-growing (some of the hardy species are by far the fastest trees in temperate climates), and one species, the Australian Mountain Ash (E. regnans), is the tallest hardwood tree in the world. Eucalypt bark is extremely variable and often diagnostic. It may be persistent (the so-called rough barks) or partly or wholly deciduous (smooth barks). The leaves are evergreen and often have numerous oil glands. They are typically dimorphic: the shape and size of adult and juvenile leaves are different, and juvenile leaves are strictly opposite while adult leaves often appear alternate. Some species, however, retain juvenile-type leaves throughout their lives. Inflorescences are solitary and axillary or rarely clustered in terminal or axillary structures; they are composed of umbellasters with 1–30 flowers. The individual flowers are hermaphrodite (rarely male only) and have a cup-shaped hypanthium containing numerous stamens and a nectar-producing disc. The stamens are curled tightly inwards in bud and are released at anthesis, when the cap-like calyptra is shed. Eucalyptus flowers may have a single calyptra (formed from the fused calyx and corolla) or an inner (corolla) and an outer (calyx) calyptra, and these leave distinct scars when they are shed. After pollination the stamens are shed, leaving a scar (staminal ring), and the hypanthium becomes a woody capsule. Within the capsule the ovary breaks down via a number of valves, and is enclosed within the capsule (included) or may protrude from the top (exserted). The wingless seeds drop out of the capsules, though some species require that the capsules are burnt to enable seed release (Kelly et al. 1983, Chippendale 1988, Hill 2002b).
Identification of Eucalyptus is not easy and requires both practice and the presence of fertile material. Knowledge of the provenance is often useful, as similar species often occupy different localities or ecological niches. There are numerous guides to Eucalyptus as wild trees in Australia and these should be consulted. The multi-volume works of Kelly et al. (1983) and Brooker & Kleinig (1999–2002) are very useful, but EUCLID3 is now the dominant source of information. There are also some excellent websites. In consequence of this abundance of information and its associated illustration by drawings or photographs, we have not indicated other sources of illustrations in our usual way.
Eucalyptus is a genus that arouses strong emotions. Its members evoke the sundrenched Australian landscape, more perfectly perhaps than any other combination of tree and location, and they carry the redolence of the outback with them – even if many hardy species come from rain-soaked parts of Tasmania! They can be productive timber and firewood trees, yet are reviled by environmentalists for the damage that plantations of them can sometimes cause to water catchments and native vegetation. In horticulture they are often regarded with suspicion as being ‘not suitable for the English landscape’ (frequently by those who will happily plant a Sequoiadendron to punctuate the sky for the next millennium), or avoided ‘because they are not hardy’. To exclude Eucalyptus from a collection on any of these grounds is not sensible. Many of these species are among the most beautiful of trees, with grace and poise, and the shimmering of their foliage in a breeze is always a pleasure. Their bark, too, often rivals or exceeds anything offered by the birches or Pinus bungeana. All in all, they offer a great deal in the way of interest and beauty, and have the advantages of being often fast-growing and sometimes short-lived – useful if a quick but temporary screen is needed.
The first horticultural consideration in Eucalyptus is hardiness. It is a recurrent theme in the dendrological literature, and enthusiasts are constantly scouring Australia for the coldest or highest provenances of each species. The importance of selecting seed from such locations cannot be overemphasised, as it can make a significant difference to the winter temperatures a tree can withstand, and where their survival in cultivation is marginal this is very important. In the past this effect was not so widely appreciated as it is now, and seed collections were often made at the first point of contact with a species – usually therefore at the lower end of its altitudinal range (a problem by no means confined to Eucalyptus). Fortunately seed collectors now frequently offer seed from stated provenances, enabling an informed choice to be made.
This account of the genus describes 54 species currently established in cultivation, adding to the 17 described in full by Bean (1981a), and mention is made of several more that have been attempted but are not yet established in our area. This increase can be attributed to two factors. First, the quest for the hardiest provenances has enabled several species to become established where previous introductions had failed, and secondly, the steadily rising temperatures being experienced worldwide have given just sufficient leeway and encouragement for devotees to experiment ever more widely. It remains the case, however, that most Eucalyptus cannot be grown outside in most of the area covered by New Trees, and many of the species described below are distinctly marginal denizens here. It is probable that a severe winter would wipe out most of them. Gum trees have always been regarded as potentially ephemeral in northern Europe, and the words of H.J. Elwes from a century ago are relevant today: ‘[They] are so easy to raise from seed, that the certainty of their death after a few years will not deter gardeners from planting them’ (Elwes & Henry 1912).
The point is that in ‘a few years’ most Eucalyptus will make a substantial tree of great beauty. Its loss would be regretted, but not nearly so much as if, like an oak, it had taken decades to reach that stature. The growth rate of some species, even in apparently less than ideal situations, can be staggering. The 16 m achieved in four years by E. nitens in Oxfordshire – by no means the warmest area of southern England – is just an extreme demonstration of this. Growth rates slow as the tree branches and rounds out, but can still be rapid even then. There is good reason to believe that many species increase in hardiness as they mature, the younger plants being more vulnerable to frost by virtue of their lesser bulk, so a few mild winters before anything testing is a bonus. It is difficult to specify precise temperatures at which any given species will succumb because there are so many variables, including the effects of the previous summer and autumn, the wetness or dryness of the ground, and the duration of the frost. In the following species accounts, all-too-frequent mention has to be made of a devastating frost in November 2005 that wiped out a large number of Eucalyptus trees at Lullingstone Castle in Kent, home of Tom Hart Dyke, who had personally collected many as seed from the best possible provenances. Several factors were causative in this devastation. The site is a frost pocket on rather damp ground, and neither of these are ideal conditions for growing eucalypts – but the real damage was caused because up until the frost (20 November) the autumn had been mild and moist, and many plants, including the Eucalyptus, were still growing vigorously. There had been no prior hardening off with a gradual decline in temperatures, so the trees were unprepared for a first frost of –9 ºC, that many might have withstood if it had come after a suitable hardening period. On the other hand, John Purse (pers. comm. 2007) reports that many of his trees, of the same species, growing in an exposed site in north Kent, were unaffected by the same frost. He suggests that this is because in such a site they grow more slowly and more hardily (although the temperatures in the Lullingstone frost pocket were lower). His view is that very mild, apparently favourable sites can actually be less suitable for Eucalyptus than slightly harsher ones. Dry cold winds in a dry spring can kill eucalypts very effectively as well, and there is a good case to be made for wrapping young eucalypts in their first winter, or providing a shelter of horticultural fleece to moderate the wind.
Even if the top growth of a Eucalyptus is cut by frost there is a strong chance that it will resprout from the roots and form an often attractive shrubby plant. From the resurgent shoots one can then be selected as a replacement main stem.
Almost entirely excluded from northern gardens are those species with large red, pink or yellow flowers, which are simply insufficiently hardy to survive even light frosts. This explains why there is such an interest in finding a eucalypt with ‘coloured’ flowers hardy enough to grow outside in the north temperate region. With two exceptions the flowers of all those described below, or covered by Bean (1981a), are white or lightly cream-coloured. A few red-flowered gums can be cultivated in western North America as far north as the San Francisco Bay Area, and indeed Eucalyptus sideroxylon is used as a street tree in that city, along with the spectacular Corymbia ficifolia.
Eucalyptus are in general very easy to raise from seed. This should always be sourced from specific provenances in Australia, or from proven trees in the northern hemisphere. Some seed merchants offer seed from orchards in South Island, New Zealand, and such sources may also be useful in the quest for hardiness. It should be noted that Eucalyptus are quite promiscuous, and even wild-origin seed may throw hybrid surprises. Seed should be surface-sown in gentle warmth in late winter or early spring. Some species require stratification, full details of which will be found in the book Eucalyptus Seed (Boland et al. 1980). Transplant the seedlings at the earliest opportunity and keep them growing vigorously, with regular repotting as necessary, at all times ensuring that the roots do not spiral the pot. From an early sowing young plants can be set out in their final sites in early summer, when no more than 30–45 cm tall. They should then grow away rapidly and produce good root systems to assure stability. Another option is to sow a little later and grow the plants to overwinter in a pot in a frame or polytunnel, to be planted out promptly in spring. The importance of keeping the roots active and unspiralled in the pot cannot be overemphasised, if good establishment and long-term stability are required. The system adopted by Cistus Nursery (and others in the United States) of growing Eucalyptus seedlings in batches, offering them to customers while still very young, and destroying them if they remain unsold after six weeks or two months in two-litre pots is greatly to be commended, and is an example that should be followed.
This is a far cry from the usual pot-bound victims seen throughout the UK nursery trade – caveat emptor – although some specialist suppliers do promote the desirability of planting young plants. Some, it should also be noted, believe that planting at a larger size has advantages that can outweigh the problems caused by confined roots (G. Cooper, pers. comm. 2007). Modern potting concepts (see pp. 30–31) may be particularly advantageous for eucalypts. The ideal Eucalyptus planting site in our area would be in full sun, in good soil with perfect drainage, on a slope where cold air can flow off, and protected from cold winds from the east and north. In western locations some shelter from strong westerly winds is also advantageous. Some species are tolerant of moister soil, and a few will accept light shade, but these are exceptions. Most prefer acidic to neutral soil, but some tolerate higher alkalinity. In the British Isles the best specimens are almost all in southern or western locations, although the ameliorated climate of the west coast of Scotland and around the Irish coast makes it possible to grow many species into fine specimens at surprisingly northerly latitudes. The fine collection at Logan Botanical Garden on the Mull of Galloway is an outstanding testament to this, and the beautiful groves of Eucalyptus at Mount Usher in Co. Wicklow and the magnificent trees at the John F. Kennedy Arboretum, Co. Wexford, and many other locations along the eastern side of Ireland, are not very much further south. There are good collections at Kew and the RHS Garden at Wisley. The World Garden and grounds at Lullingstone Castle have an interesting range of survivors, and there are other collections in private hands in southern England, including a National Plant Collection owned by Mr & Mrs D.J. Smith at Wickham, Hampshire. One problem in maintaining collections of Eucalyptus, that became apparent while examining specimens in arboreta in the research for New Trees, is that the regularly sloughed bark makes it exceptionally difficult to attach a label to the trunk in the usual way. A regularly updated plan is particularly important for this genus.
Most of continental Europe and North America within our area is off-limits to Eucalyptus, being too cold in winter for them to succeed. That said, the fascinating accounts of survival in Cincinnati from Frank Callia, passed on by Sean Hogan, suggest that more experimentation may yield surprising results. Hogan (2008) considers that about 30 species are hardy in Zone 8 conditions in the United States. Eucalyptus are hardly grown in northern continental Europe, but most of the species described here would probably survive well in coastal France and northern Spain. In North America the hot humidity of the East Coast summer is not appreciated by most gum trees, even were they to survive the winters. A few are more tolerant, however, and some (unspecified) species are said to grow on the coast as far north as Delaware (L. Eirhart, pers. comm. 2007). Tony Avent conducted trials of Eucalyptus in North Carolina in the late 1980s and early 1990s (pers. comm. 2007), ‘before global warming’, and found that the hardiest tested was E. neglecta, which survived –18ºC without any loss of the trunk. The other two species to survive this milestone temperature (0 ºF), but with at least 50 per cent trunk loss, were E. nova-anglica and E. parviflora.
On the West Coast, as will be apparent from the accounts that follow, many species will survive and flourish along the Pacific Northwest seaboard and in milder areas inland, as far north as British Columbia. Good collections and plantings may be seen in the Portland area, at the Washington Park Arboretum and in Vancouver. The further south one goes in California the more eucalypts will be found, and the Bay Area botanical gardens have a very fine range of species (although in the interests of discussing true hardiness, specimens growing in California have not been cited here).
These accounts of the species in cultivation were prepared with very considerable input from the ‘Gum Group’, a panel of Eucalyptus enthusiasts (Graham Blunt, Geoff Cooper, Tom Hart Dyke, Sean Hogan, Terry & Maria Milton, Steve Verge), who are duly most sincerely and gratefully acknowledged. General views expressed at the Eucalyptus Workshop held at Colesbourne, Gloucestershire in January 2007 are referenced ‘Gum Group 2007’; attributable individual comments are recorded in the usual way. Flowering and fruiting times are not given, as these vary widely between hemispheres and latitudes.
The following species of Eucalyptus are known to be, or to have recently been, in cultivation but have been excluded from this account – principally because they are insufficiently hardy to form a tree in our area, or are too little known: E. approximans Maiden subsp. approximans and E. approximans subsp. codonocarpa (Blakely & McKie) L.A.S. Johnson & Blaxell (short mallees to 6 m with smooth bark, not established in our area: the 12 m trees with flaky bark at Logan labelled subsp. codonocarpa are incorrectly identified); E. badjensis Beuzev. & M.B. Welch (potentially very fast-growing, but not hardy in southern England); E. barberi L.A.S. Johnson & Blaxell; E. blaxlandii Maiden & Cambage; E. cosmophylla F. Muell. (regarded as very tender by the Gum Group (2007), who report trees killed at –2 ºC, but there is a suppressed (5 m) specimen from 1995 at Logan, and it grows on Tresco: TROBI); E. deanei Maiden; E. fasciculosa F. Muell. (small seedlings are grown by Gum Group members, not tested); E. formanii C.A. Gardner; E. goniocalyx Miq.; E. kitsoniana Maiden; E. leucoxylon F. Muell. subsp. leucoxylon, E. leucoxylon subsp. pruinosa (F. Muell. ex Miq.) Boland; E. ligustrina DC.; E. nitida Hook. f. (hardy but straggly, resembling E. coccifera); E. olsenii L.A.S. Johnson & Blaxell; E. paliformis L.A.S. Johnson & Blaxell; E. stricta Sieber ex Spreng.; E. sturgissiana L.A.S. Johnson & Blaxell; E. willisii Ladiges, Humphries & Brooker; E. yarraensis Maiden & Cambage; and E. youmanii Blakely & McKie. Enthusiasts are constantly experimenting with this genus, and given its rapid growth rates an ‘unknown’ species may suddenly feature as a substantial tree, so the total diversity in temperate gardens is always increasing.
No other single genus of trees dominates so vast and climatically so diverse an area as do the eucalypts in Australia. By far the greater part of the natural vegetation of the sub-continent outside the deserts and semi-deserts consists of communities of woody plants in which the eucalypts play a predominant or leading role. Only in those parts of eastern Australia and Tasmania where the rainfall is high enough to support tropical to temperate rain-forest do they recede in importance.
Some 500 species are recognised, all in Australia and Tasmania save a few found in the larger islands of the S.W. Pacific. The genus is absent from New Zealand in the wild state. In life-form the eucalypts range from tall trees – in E. regnans attaining a height of over 300 ft – to shrubs and small, stunted trees. A peculiar form – the mallee – is assumed by many species in the dry regions of southern Australia; in these the most permanent part of the plant is a large, woody root-stock bearing numerous slender stems and containing a supply of dormant buds which quickly develop if the stems are killed by fire or drought.
The bark is very varied and serves as a useful character for identification in the field. In many species it is persistent and in mature trees becomes thick, hard and deeply furrowed (as in the Ironbarks) or fibrous (as in the Stringybarks and so-called Ashes). In the Gumbarks – and here belong nearly all the species cultivated outdoors in this country – the bark is smooth and deciduous, the outer (older) layers being regularly shed in flakes or ribbons. The freshly exposed bark is white or distinctly coloured, and contrasts with the darker colouring of the older layers.
The eucalypts have the peculiarity, not uncommon among Australasian plants, of having a juvenile phase during which the leaves produced are strikingly different from those of the adult plant. In Eucalyptus these are usually shorter and broader than the adult ones, sessile or short-stalked and, at least at the base of the shoot, opposite. The duration of this phase varies with the species and usually gives way, through an intermediate phase, to the production of adult leaves, which are alternate, stalked, mostly lanceolate or sickle-shaped, with entire margins. The blades are usually identical in appearance on the two surfaces (isobilateral). In some species (e.g. E. cordata) there is little difference between the juvenile and adult leaves. The juvenile phase can be prolonged by regular pruning and there is always a reversion to it on stump shoots.
Flowers in most species are borne in axillary umbels (rarely solitary). The buds are formed in summer on the young shoots and open about a year later. The most conspicuous feature of the flower is the numerous stamens, which are creamy white in the species described here but in the beautiful but tender E. ficifolia (and other species from W. Australia) they are in some shade of red. They are inserted on the rim of the calyx-tube (receptacle) and in bud are enclosed in a cap known as the ‘operculum’, made up of the united sepals and petals, which fall off as the stamens develop. The ovary is embedded in the calyx-tube and develops into a capsule opening by valves to release the numerous small seeds; the calyx-tube itself becomes enlarged and woody, and is the most conspicuous feature of the fruit. The top of the capsule may be more or less level with the rim of the tube, with the valves exserted; or it may be sunk within the tube, with the valves wholly or partly concealed. The nectar-secreting disk of the flower may become woody and enlarged in fruit, partly covering the top of the capsule. The capsules need at least one year to ripen and may remain closed for several years. The seeds are small and usually, at least on cultivated trees, only a few are formed in each capsule, most of the ovules remaining unfertilised and turning into chaff.
Eucalyptus is, for the taxonomist, a genus full of complexities. A species of wide range may be subdivided into numerous geographical and ecological races which have in common the broader characters by which the species is defined but may differ quite considerably in minuter details. Intercrossing between species is also quite common in nature: this further increases the difficulty of identification and necessitates the proviso that some of the cultivated specimens mentioned in the following account might prove not to be true to type were they to be examined by an experienced eucalyptologist. It should also be added that there are few woody genera in which more complete material is needed if a specimen is to be accurately identified. In addition to adult foliage, flower and fruit, the sample should include juvenile foliage and a description of the bark.
These taxonomic complexities may not be of much concern to the grower, but the variability in hardiness shown by many species when brought into cultivation is of the greatest importance. All but the very hardiest species will have their more tender forms and equally the reputed tenderness of a certain species may be due simply to its having hitherto been represented in cultivation by a tender provenance. Seed from Australia, and plants raised from it, should therefore be treated with some circumspection, unless it is known that the seed was collected with frost-resistance in mind.
The eucalypts must be raised from seed, which should be sown thinly in deep containers and the seedlings potted-off, when the second pair of seed-leaves has developed, into paper, fibre, or polythene pots (or sleeves) 31⁄2 in. deep or longer. Ideally, the seed should be germinated in February-March with artificial heat, the plants grown on under cool-house conditions and put into their final positions during the summer of the same year. By autumn they will have become well established and can face the dangers of the ensuing winter with as good a chance of survival as potted plants overwintered in an unheated house or frame. Despite the obvious risks, this early planting out is to be recommended, as the eucalypts resent any restrictions at the roots and will develop quicker and need artificial support for a much shorter time if allowed to grow freely from an early age. If it is intended that the young plants should be planted out in their second year, the seed should be sown later, in late spring or early summer and potted-off by August. For further details, see the article by R. C. Barnard cited below.
Eucalypts have been so infrequently planted on the chalklands of south-eastern England that nothing can be said for certain about their suitability for such soils. But E. parvifolia lived for many years in chalk at Messrs Hilliers Winchester nursery and it has been reported that E. dalrympleana will tolerate a similar soil.
The most recent work on the taxonomy of the genus is: W. F. Blakely, A. Key to the Eucalypts, 2nd Ed., Canberra, 1955. Emendations to this, with references to recent literature, will be found in: R. D. Johnston and R. Marryatt, Taxonomy and Nomenclature of Eucalypts, Canberra, 1965. For British growers the most useful works are ‘An Introduction to some Garden Eucalypts’, by R. C. Barnard, published in Journ. R.H.S., Vol. 91 (1966), pp. 209-216, 250-261, 293-303; and the section on Eucalyptus in Dictionary of Gardening, Supplement 1969, pp. 282-288. Mr Barnard has kindly read through the following pages in proof and made some valuable suggestions.
Of particular interest to British growers is: M. I. H. Brooker and D. A. Kleinig, Field Guide to Eucalypts, Vol. 1 (1983). Devoted to the species of south-eastern Australia (including Tasmania), this consequently deals with virtually all the eucalypts grown outdoors in this country. There is a botanical key, colour photographs for each species showing buds, fruits, bark and habit, and a distribution map.