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According to the most recent taxonomic study (Eckenwalder 1996), the genus Populus has 29 species, though Flora of China (Fang et al. 1999) suggests that there may be as many as 100. This discrepancy is in large part due to misidentification of hybrids as true species; hybridisation is rife in the poplars (as in Salix), and a vast number of selections have been made in the search for the ideal tree for forestry purposes. The DNA-based study of Hamzeh & Dayanandan (2004) even suggested that some well-known species such as P. nigra are of hybrid origin. Poplars occur across temperate and subtropical Eurasia, North Africa and North America, with one species in tropical East Africa (P. ilicifolia (Engl.) Rouleau). They are fast-growing deciduous (or semi-evergreen) trees that sometimes produce prolific sucker shoots. Winter buds have several irregular scales; terminal buds are present. The bark is smooth or furrowed. The leaves are typically ovate to deltoid, though narrow-lanceolate (willow-like) leaves occur on P. angustifolia and may occur on immature growth in some species. Stipules are small and deciduous. Poplars are typically dioecious, though hermaphrodite flowers can occur (common in P. lasiocarpa). Inflorescences (catkins) are pendulous and are produced before the leaves emerge; inflorescence bracts are lobed or toothed, and caducous. The flowers have a cup-like floral disk, nectaries absent; staminate flowers have 5–60 stamens, the anthers dull red; pistillate flowers have a green stigma. The fruit is a capsule with two to four (to five) valves; seeds are few to numerous, each with a tuft of silky hair (Eckenwalder 1996, Fang et al. 1999).
Several genera of trees have a specialist society devoted to them, but only Populus has a statutory body within the United Nations Food and Agriculture Organization – the International Poplar Commission – reflecting the immense value of poplars (and willows) to humans as producers of timber and biofuel, and in environmental enhancement. The International Poplar Commission maintains an extensive website with numerous links to further sources of information (International Poplar Commission 2009). Most interestingly, it promises (as of July 2008) a comprehensive new book, Poplars and Willows in the World, that will review all aspects of their biology and uses. It is planned that chapters will be available electronically online, as well as in a printed book. Although there had been suggestions that text would be completed by December 2006, at the time of writing no chapters have been posted.* It is not unknown, however, for book projects to run late. When the work does appear it will evidently be required reading for any with an interest in these two genera. Until then a useful reference to the diversity and origins of cultivated poplars is to be found in Jobling (1990). Many unfamiliar names appear in Flora of China (Fang et al. 1999) and it is conceivable that some of these will find their way into cultivation (although the species concept employed by Fang and his co-authors does not always equate with modern views in the West).
In horticulture, poplars are mostly trees for the grand landscape rather than for small sites, but when appropriately placed they have great presence and character, and their shimmering foliage can be a delight. The different shades of the emerging leaves and their often brilliant yellow autumn colour give them two very ornamental seasons each year. In consequence of their importance as forestry trees, innumerable, often hybrid, cultivars have been selected for various qualities, and these may be found in arboreta. Such clones are usually very difficult to identify, and this issue is not addressed here. Jacobson’s terse comment (1996), ‘Identification is difficult, experts disagree and confusion reigns’, sums up the position very well. Nomenclature also varies greatly – some authors, for example, using P. ×euramericana Guinier for the apparently more correct P. ×canadensis (P. deltoides × P. nigra). It is to be hoped that the situation will be clarified by Poplars and Willows in the World, but even with an authoritative text physical identification is always likely to be challenging.
The problems caused, especially in plantation conditions, by bacterial canker in Europe (unknown in North America), canker from the Septoria fungus, and foliar fungal disease and insect damage, have driven much of the breeding and selection work in Populus, in the search for resistant cultivars. Propagation is usually by cuttings, as seed retains viability for a very short time. Most taxa propagate easily from winter hardwood cuttings but this does not work with some species (notoriously P. lasiocarpa). For these (and other Populus), summer cuttings rooted under mist are usually successful.
A group of large, usually quick-growing, deciduous trees, with alternate leaves pinnately veined or three-nerved at the base, those on vigorous shoots usually larger, and often different in shape and character from those on lateral twigs. Flowers produced in catkins on the naked shoots in spring, the sexes nearly always on separate trees. Male catkins more densely flowered than the female, the flowers composed of usually numerous stamens attached to a disk, and springing from the axil of a toothed or fringed scale, which soon falls away. Anthers red or purple. Female catkins lengthening until mature, the egg-shaped or rounded ovary seated in a cuplike disk, and crowned by two to four stigmas. The seed is surrounded by a conspicuous tuft of white, cottony hairs which enables it to be carried long distances by wind. Poplars occur in most parts of the northern hemisphere, from subarctic regions to subtropical ones, some inhabiting arid places, others always found in association with moisture.
There are four well-marked groups of poplars cultivated in Britain:
I Populus (Leuce)
White and Grey Poplars, Aspens Younger trunks and main branches at first pale and smooth, then pitted with numerous diamond-shaped holes. Leaves toothed, often coarsely so, or lobed. Catkin-scales fringed with long hairs. This section can be subdivided as follows:
White and Grey Poplars. – Leaves on the long shoots woolly beneath, those of the short shoots less woolly or almost glabrous, and of different shape; petioles usually not much compressed laterally (except sometimes in P. canescens). Here belong P. alba, P. canescens and P. tomentosa, all species of the Old World. The first-named is propagated by hardwood cuttings, but P. canescens is not easy to root by this means.
Aspens. – Leaves glabrous or almost so beneath, more or less uniform in size and shape with laterally flattened stalks, and noted for their restless movement. This group includes P. grandidentata, P. tremula, and P. tremuloides, none of which roots readily from cuttings.
This section, which lacks a popular name, comprises the American P. heterophylla and the E. Asiatic P. lasiocarpa and P. wilsonii. The leaves in this section are large and leathery, usually tomentose when young, and are of more or less the same size and shape on long and short shoots. The floral disk surrounding the lower part of the stamens or ovary is deeply lobed, and the ovary is hairy. The bark is rough and scaly.
These burst into leaf the first, and are distinguished by very gummy winter leaf-buds and leaves, which emit a pleasant balsamic odour, especially when just expanding in spring. Leaves usually whitish, but not woolly beneath; leaf-stalk not compressed. This group includes P. angustifolia, P. balsamifera, P. candicans, P. laurifolia, P. maximowiczii, P. simonii, P. trichocarpa to name only the better known. Most of these can be increased by hardwood cuttings in the open ground, or by suckers.
Leaves green on both sides and with compressed, slender stalks, nearly always in motion; margins translucent and cartilaginous. Trunks with a corrugated bark. This group, which is confined to N. America and western Eurasia, is mainly represented in cultivation by hybrids (see P. × canadensis). All the black poplars except some forms of P. deltoides are easily propagated by hardwood cuttings about 8 in. long, inserted in late winter in the open ground (February or March). If inserted in early winter they may rot. Many of the male black poplars in spring are handsome on account of their richly coloured catkins.
The section Turanga, of which the best known species is P. euphratica, is not treated in this work, since neither this nor any other member of the group has ever become successfully established in this country.
Poplars do not produce good seed freely in cultivation, and when they do it will certainly produce hybrid plants unless male and female trees of the same species happen to grow near together, which is rarely the case. The polygamous clone of P. lasiocarpa, mentioned above, breeds true from seed, since male and female flowers are produced on the same plant and in the same catkin. The seed of poplars normally remains viable for a very short time after the capsules open, though it has recently been found that it will travel quite well in sealed packages, retaining its viability for a longer period than would have been thought possible a short while ago. The seed germinates rapidly in a moist soil – normally within two days. Hybrid poplars can be raised artificially by cutting branches from the two intended parents, placing them in water, and bringing them into flower under glass. Provided the flowering time can be made to coincide, cross- pollination is simple, and the seed will ripen on the female branch within a few weeks.
Uses, cultivation and diseases
On fertile and well-sheltered sites poplar grows faster than any other tree hardy in Britain. It reaches its greatest vigour in the milder parts of the country on deep, rich, and well-drained soils, and particularly in the southern half of England it can increase in length by more than 5 feet annually and attain a height of 100 feet in twenty years. In the northern and western parts of Britain, which are rather too cool and wet for most poplars, this rate of growth is only achieved in sheltered localities in warm summers. It is a mistaken belief that poplar thrives on wet soils, and vigorous growth cannot be expected where there is water-logging and poor drainage. Growth is also likely to be poor on very acid soils and on thin or dry soils, and in the most extreme conditions the trees may actually die. In exposed, upland localities planting should not be attempted at all.
Largely because of its rapid growth-rate poplar has been much used to shelter farmland, orchards, and playing fields, and to screen factories and other industrial development. It has also been planted extensively in gardens in urban areas to obscure eyesores, and to provide protection against wind, noise, and intruding neighbours. The Lombardy poplar has been used for screening and shelter much more than any other cultivar, although a few other narrow-crowned forms, notably P. nigra 'Plantierensis' and the hybrid black cultivars P. 'Eugenei' and P. 'Robusta' (q.v. under P. × canadensis), have sometimes been preferred and found local favour. Partly because black poplars are difficult to distinguish in the nursery, mixing of plants has occurred from time to time, and two or more cultivars are often found in the same planting. Usually the trees are planted in a single row and, in an effort to obtain early benefits, only a few feet apart.
In the last decade the fast-growing artificial balsam hybrid, P. tacamahaca × trichocarpa 32 has been increasingly used in line plantings as an alternative to Lombardy poplar, which it resembles in angle of branching and crown shape. This hybrid, like other balsam poplars, breaks into leaf much earlier than most black poplars, and on this account it has been much planted around orchards to protect early-flowering fruit varieties. It has also been planted around market gardens and arable farmland to protect tender crops against cold, drying spring winds, but since it is a female tree there is a risk that the cottony down surrounding the seed will adversely affect the appearance of certain types of market produce. Lettuces are probably the most likely crop to suffer in this way.
As poplars are regarded as comparatively short-lived trees they are seldom used in major landscaping. However, because many grow rapidly and reach large dimensions within a relatively short time they can be usefully planted to obtain an early effect whilst slower-growing species become established and attain a respectable stature, and then felled later. Some cultivars such as Lombardy poplar that always develop narrow, cylindrical crowns are useful in mixed plantings because of their contrasting shape.
Poplars are not trees to plant in confined spaces or close to roads, walls and buildings. They ordinarily have an extremely wide spreading root system, whose radius can easily exceed the height of the tree, and this can extract large quantities of water from the soil. As a result the soil shrinks and there is a consequent settlement of the foundations of walls and buildings. In the same way the foundations of roads may be affected. The risk of damage is very much greater in clay soils than in light soils, and in general it is advisable to plant not closer than 120 feet to any structure on a clay soil.
In parks and gardens female cultivars can be a nuisance because of the large quantity of cottony down that can be dropped from ripened catkins. Cultivars that sucker may also cause annoyance both for the damage they do and for the effort required to control them. Nevertheless, poplars are valuable trees for open spaces in towns and cities because of their tolerance of atmospheric smoke pollution. In the past, forms of the black poplar, masquerading under such names as the Manchester poplar, were planted in our smoky urban parks and squares to the almost total exclusion of other species and hybrids. In the past twenty years, as more and more cultivars have become available to the nursery trade, it has been demonstrated that many poplars other than the black poplar are able to thrive in a smoke-polluted environment. Poplars also grow tolerably well at the seaside, though only the white poplar, and to a lesser extent the grey poplar, can withstand blast from salt-laden winds. It is very doubtful if any poplar will succeed if its roots are subjected to saline conditions due to flooding by sea water.
Poplars are also planted on a commercial scale in this country to produce industrial wood. Prior to 1950 only a handful of landowners and foresters had undertaken any worthwhile planting for timber, but in the last two decades or so (partly due to the encouragement provided by planting grants) successful plantations, many already yielding merchantable produce, have been established throughout the country. Much of the planting has been undertaken by the private forestry sector, and by far the most productive stands are to be found in the southern half of England, where the best conditions prevail for sustaining high growth rates.
Poplars are notoriously intolerant of competition, both above and below ground, and attain their fastest rates of radial growth as single specimen trees, removed from other trees that might cast shade or compete for moisture and nutrients. The hybrid black poplars planted for timber are particularly bad in this respect and, in plantations, soon lose vigour at the onset of competition. Partly because of this problem, stands established primarily for the production of wood have the trees at wide spacings throughout the life of the crop. More often than not the trees are planted at a distance apart that will allow maximum growth rates until time of felling without the need to remove competing trees. Spacings at planting have increased from 18 feet to 24 feet during the past twenty years; at present, to attract planting grants, spacing may not be wider than 26 feet.
Poplar wood is soft, rather woolly in texture, and pale in colour. It has no smell or taste, and so is valuable for making into food containers, and has a low flammability. It is not durable in the open or in damp ground, and thus cannot be used for fence-posts unless previously treated with preservatives. The wood usually takes paints and glue well, and has a high resistance to abrasion, making it suitable for wagon bottoms, wheelbarrows and colliery tubs, and can be nailed without splitting, although some types of nails tend to pull through or pop out. The wood is ideally suited for the floors of oast houses and for brake-blocks because of its low flammability. The biggest use in this country is for matches, the best-known product from poplar wood, and the splints are dipped at one end in paraffin wax to give the required degree of flammability. Large poplar logs can be rotary peeled for veneers without special treatment. Uses for small-sized logs include fibreboard, wood chipboard, and certain types of pulp.
Cultivars planted for timber production have to be carefully chosen, as in addition to being fast growing they must be reasonably straight-stemmed, round and free from splits, insect damage and other serious defects. Trees grown for rotary peeling must also be easy to prune to obtain knot-free timber. The most commonly planted cultivars are: P. 'Eugenei', P. 'Gelrica', P. 'Robusta', and P. 'Serotina', all described under P. × canadensis. Others sometimes planted are P. 'Heidemij' and P. 'I – 78', a cultivar of Italian origin recommended for the southern half of England. All of these are hybrid black poplars, which to produce timber of high quality must be cultivated on better sites. On wetter, more acid ground, notably in the cooler and wetter regions of the country, balsam poplars are better suited for growing for timber. The only cultivars available at the present time, however, are P. trichocarpa 'Fritzi Pauley' and the hybrid 'Tacatricho 32' (see under P. balsamifera).
Due to the interest abroad in poplar culture, a great deal of work is being conducted at research stations to breed and select new cultivars. Several new poplars have already been chosen for release to growers from the many thousands tested, and more are expected to be selected in the next few years for use for both amenity and timber production. Most of the tests are concerned with finding cultivars sufficiently resistant to the many diseases prevalent in western and central Europe to be grown to maturity free of infection. In practice all of the cultivars raised by the nursery trade in this country and its neighbouring states are susceptible to one poplar disease or another.
The most serious disease of poplar in Britain is called bacterial canker, caused by the bacterium Aplanobacter populi. This produces cankers on the branches and main stem, varying in size and appearance depending on the reaction of the cultivar to infection by the bacterium. Although trees are seldom killed outright by the disease, their growth rate may be seriously retarded and their amenity value greatly lessened due to die-back of the crown. Cankers on the stem markedly affect the value of the timber. All the cultivars raised by our nursery trade other than P. trichocarpa 'Fritzi Pauley' are susceptible to bacterial canker, but many can be grown with little or no risk of serious infection. The cultivars named above as suitable for planting for timber production can usually be cultivated free from damage if care is taken to ensure that there are no infected poplars in the locality. Infection can occur through exit-holes made by wood-boring insects or through leaf-scars after the leaves fall in the autumn. Pruning is unlikely to increase the risk of infection.
The most serious foliage disease is Marssonina brunnea, first observed in Britain as recently as 1967. It is causing great concern on the continent of Europe where infection has led to premature defoliation in several successive seasons in the last decade and the death of thousands of previously healthy trees. In this country attacks have so far been local and periodic, and most serious in East Anglia. P. 'Gelrica' and P. 'I – 78' have proved to be the most susceptible cultivars. The related leaf-spot fungus M. populi-nigrae is the cause of death of branches of Lombardy poplar, defoliation and die-back most often occurring in the lower crown.
The only other fungi seriously attacking the leaves are the rusts (Melampsora species). These cover the undersurface of the leaves with small orange pustules and heavy infection can cause early defoliation. Severe damage occurs mainly in the nursery, where growth may be reduced, but the intensity of attack varies from year to year, and permanent damage is rare. There is no satisfactory remedy. A similar disorder, caused by the fungus Taphrina aurea, does much less harm although the orange blisters are very conspicuous and appear quite early in the growing season.
Die-back of poplar associated with the fungus Dothichiza populea is fairly common and often confused with bacterial canker, but is much less serious. The fungus attacks poplars weakened by overcrowding in the nursery or by poor planting and after-care. Infection can occur also through pruning wounds on young poplars, and pruning of nursery plants and of recently planted trees should be done in the early summer to allow the wounds to heal. Any measures that encourage the rapid establishment of newly planted trees will lessen the risk of Dothichiza damage.
Another fungus attacking the stem of poplar is Cytospora chrysosperma. This is found very commonly on dead wood but is not capable of invading a healthy tree. It should not be held responsible for the death of the wood on which it is found. It is conspicuous because it exudes long orange tendrils.
These diseases cannot usually be controlled other than by planting resistant cultivars and by ensuring that sources of infection are rigorously destroyed. There are very few resistant or slightly susceptible cultivars available at the moment but research workers are striving to select or breed new resistant clones.
This section has been contributed by Mr J. Jobling, Forestry Commission, Forest Research Station, Alice Holt
The book Poplars and Willows, published by the Food and Agricultural Organization of the United Nations in 1979, is intended primarily for the commercial grower, but the sections on poplar cultivars, propagation, pests and diseases are of more general interest.