An important group of hybrids deriving from the European black poplar P. nigra and its relative of eastern N. America P. deltoides, the earliest of which arose spontaneously in western Europe soon after the introduction of the American species. Being more vigorous than either patent, and easily propagated by cuttings, these hybrids were widely planted from the second half of the 18th century onwards. See further under ‘Serotina’, ‘Marilandica’, and ‘Regenerata’, which are the oldest existing clones in this group. It is probable that the poplar described by Moench as P. canadensis was ‘Serotina’, which was at one time often sold under that name.
These hybrids show the characters of the parental species in various combinations. In the American parent, P. deltoides, the leaves are always ciliate and there are two, sometimes more, glands at the junction of the petiole with the leaf-blade. In the European black poplar the leaf-margins are glabrous and the glands are absent. The hybrids usually have ciliate leaf-margins, at least on the youngest leaves, and glands are usually present on some leaves. Further, in P. deltoides the leaves on the strong shoots are truncate, rounded or slightly cordate at the base, in P. nigra cuneate. Some of the hybrids, notably ‘Eugenei’ and ‘Marilandica’, are nearer to P. nigra in this respect, while ‘Serotina’, ‘Regenerata’, and ‘Gelrica’ have the leaves on the strong shoots more or less truncate.
It was Augustine Henry who, among his many other achievements, first clearly distinguished the black poplar hybrids from the two parent species, and gave a comprehensive account of them in: Elwes and Henry, Trees of Great Britain and Ireland, Vol. 7 (1913), pp. 1841-51, and in Gardeners’ Chronicle, Vol. 56 (1914), pp. 47 and 66-7. The most recent work in English is: G. S. Cansdale, The Black Poplars (1938).
‘Eugenei’. – A tree of columnar habit, producing short, comparatively weak, but spreading side branches; young shoots glabrous, somewhat angular. Leaves coppery when young, on ordinary branches 2 or 3 in. across (considerably larger on vigorous leading shoots), broadly triangular, widely tapered to nearly straight across at the base, slender-pointed, the margins set with rather coarse, incurved, gland-tipped teeth, and furnished more or less with minute hairs. It is a male tree; catkins 21⁄2 to 31⁄2 in. long; anthers red.
This fine tree originated in the nursery of Messrs Simon-Louis near Metz, around 1832, as a self-sown seedling found growing in a bed of young silver firs. Gabriel Simon, the founder of the nursery, named it ‘le peuplier Eugène’ after his infant son, born in 1829, who later carried out botanical explorations in China for the French government, during which he discovered and introduced P. simonii. The parentage of ‘Eugenei’ is uncertain, but is usually supposed to have sprung from ‘Regenerata’ pollinated by Lombardy poplar. If that is so, Gabriel Simon must have been among the first growers to plant ‘Regenerata’, which was first distributed only seventeen years before ‘Eugenei’ saw the light of day.
The original tree was 80 ft high and 9 ft in girth when twenty-five years planted; in 1904 it had reached 150 ft and girthed 23 ft at 4 ft; it died and was felled around 1945. A number of plants were procured for Kew in 1888, and one of these grew to be over 80 ft high and 5 ft in girth in a little over twenty years. Three trees of this batch still exist in the collection and the two largest measure 110 × 111⁄2 ft and 120 × 111⁄2 ft (1974). Other examples are: Edinburgh Botanic Garden, 105 × 111⁄4 ft and 98 × 11 ft (1970); Colesbourne, Glos., pl. 1903, 128 × 111⁄2 ft (1970); Forestry Commission, Alice Holt, Hants,pl. 1954, 72 × 41⁄4 ft (1971).
In Cansdale’s survey of the black poplars (1938), ‘Eugenei’ was stated to be ‘perhaps the most liable to canker of all the Black Poplars’. It was later found that the poplar thus denounced was not the true ‘Eugenei’, which is in fact one of the most resistant to canker and is one of the poplars recommended by the Forestry Commission for use in plantation. It is often planted in screens on account of its comparatively narrow and symmetrical crown. In the USA, where it has been widely planted, it is known as “Carolina poplar”.
‘Florence Biondi’. – A hybrid black poplar of great promise raised by Schreiner and Stout, who gave the parentage as P. deltoides var. virginiana crossed with P. nigra var. caudina. Until named recently, it was known by its registration number – OP 226. A tree in the Forestry Commission collection at Alice Holt, seventeen years planted, measures 72 × 31⁄2 ft (1973).
‘Gelrica’. – A tall tree resembling ‘Serotina’ in habit; trunk remaining white for many years; young stems glabrous. Leaves unfolding at about the same time as those of ‘Regenerata’ and ‘Marilandica’, at first reddish brown, light green when mature, deltoid in outline, shallowly cordate at the base on long shoots, rounded to broad-cuneate on short shoots; petioles red-tinged. There are apparently at least two clones under the name ‘Gelrica’, one male, the other female.
This hybrid, selected in Holland at the end of the last century, was not introduced to Britain until much later and is still uncommon. Although not especially vigorous in its early years, its radial growth rate surpasses that of the older hybrids.
‘Marilandica’. – A large tree with a rounded, almost spherical crown, and a corrugated, not burred, trunk; young shoots glabrous. Leaves light green, triangular-ovate, inclined to diamond-shaped, nearly always widely tapering at the base, and with a longer slender point, those on the long shoots 3 to 6 in. long, three-fourths as wide, margins ciliate on young leaves; glands at base none to two (usually none on those of the short shoots); stalks compressed, 11⁄2 to 3 in. long. Catkins female, ultimately 4 to 6 in. long; stigmas two to four.
After ‘Serotina’ this is the oldest of the black poplar hybrids. It was described in 1816, and is believed to have arisen about 1800 from a cross between P. nigra and P. × canadensis ‘Serotina’. It was at one time confused with the American black poplar, of which it was thought to be a female form; it is really nearer to P. nigra, differing in the burrless trunk and showing the influence of the American parent in the presence of stray cilia on the leaf-margins and, usually, of glands at the junction of the leaf-blade with the petiole.
Although common in some parts of the continent, ‘Marilandica’ has never been much planted in this country, owing to its poor stem shape and comparatively slow growth. The oldest and largest tree in the country grows at Kew on the lawn near the Water Lily House. Planted around 1846, it measures 115 × 17 ft (1974). Although handsome and imposing, it is not a perfect lawn tree, owing to the litter its cottony seeds make on the ground around midsummer.
‘Regenerata’. Railway Poplar. – The leaves of this clone resemble those of ‘Serotina’ in their shape, toothing, and other characters. But they are pure green, not sea-green, expand about a fortnight earlier and are scarcely coloured when young. ‘Regenerata’ also differs from ‘Serotina’ in being female, and the two trees are also very different in habit: in ‘Regenerata’ the branches arch outwards at the ends, vase-fashion, and are snagged, while the shoots are slender and pendulous; in ‘Serotina’ the bole and branches are clean, the branches curve inwards, forming a goblet-shaped crown, and the shoots spread upwards.
‘Regenerata’ arose in France, at Arcueil, in 1814, probably from a spontaneous cross between ‘Serotina’ and ‘Marilandica’. It was bought by the nurseryman Romanent of Montirail, who named it ‘le peuplier régénéré’ and propagated it. But it was first put into commerce by Bujot of Chateau-Thierry and soon became common in the valley of the Ourcq, north of that town (Carrière, Rev. Hort. (1865), pp. 58, 276).
The date of introduction of ‘Regenerata’ to Britain is not known for certain, but it is almost beyond doubt that it is the poplar which the Knap Hill nursery introduced around 1870 and distributed as ‘the new Canadian poplar’ or ‘P. canadensis nova’, recommending it for its very fast growth and its ability to grow well in a smoky atmosphere. It is still a very common tree in the London suburbs, especially near the railways, where it was used to screen goods-yards. It is no longer a recommended poplar in this country, being very susceptible to bacterial canker, and its planting in France north of the latitude of Paris has been banned for the same reason. Although female, it produces little cotton, as most of the flowers are sterile.
Some specimens of’Regenerata’ are: Kew, Queen’s Cottage Grounds, 105 × 13 ft (1973); pl. 1887, 85 × 8 ft (1967); Hyde Park, London, by the Serpentine bridge, 90 × 11 ft (1967); Regents Park, London, four trees 70 to 85 ft high and 83⁄4 to 103⁄4 ft in girth (1967-8); Wildwood Road, Hampstead, London, 80 × 103⁄4 ft (1968).
Poplars similar to ‘Regenerata’, and probably of the same parentage, were also known as ‘peupliers régénérés’ and are cultivated in France. Two of these now have distinguishing names: ‘Régénéré de l’Yonne’ and ‘Bâtard d’Hauterive’. The original clone is sometimes distinguished in France as ‘Régénéré de l’Ourcq’ (Pourtet, La Culture du Peuplier (1957), pp. 182-7).
‘Robusta’. – A very vigorous tree, with ascending branches; young shoots ribbed and minutely downy; winter-buds sticky, pointed, reddish brown. Leaves rich bronze when young, triangular-ovate, pointed, truncate or nearly so at the base, the margins set with even, rounded teeth which become wide apart towards the base; 2 to 5 in. long, 11⁄2 to 4 in. wide; stalk 1 to 2 in. long, finely downy. The glands where the stalk joins the blade are either two, one, or absent altogether. This is a male tree and its catkins are 21⁄2 in. long.
This hybrid arose in the nursery of Messrs Simon-Louis at Plantières, near Metz, in 1895, the seed-parent being P. deltoides ‘Cordata’ (P. angulata cordata) and the pollen-parent most probably P. nigra ‘Plantierensis’, which would have imparted to the hybrid its erect branching and downy young shoots. The pollen-parent was at first thought to be P. × canadensis ‘Eugenei’, but in that poplar the young shoots are glabrous, as they are in the seed-parent.
‘Robusta’ has been widely used in Britain since the second world war in commercial forestry and in screens and shelter-belts. It is perhaps the most ornamental of the hybrid black poplars, with its glossy, sea-green leaves, brilliantly coloured when they first unfold.
Being of comparatively recent origin, and fairly new to cultivation in this country, ‘Robusta’ is rarely seen as a large specimen. The most notable are: Bowood, Wilts, 105 × 9 ft (1964); Tortworth, Glos., 113 × 111⁄2 ft (1974); Shinners Bridge, Totnes, Devon, 103 × 81⁄4 ft (1967); National Botanic Garden, Glasnevin, Eire, pl. 1900,102 × 8 ft (1974). A tree in the Forestry Commission collection at Alice Holt, pl. 1951, is 66 × 43⁄4 ft (1971).
‘Serotina’. – A large tree, always male, frequently over 100 ft high, with an open, rather gaunt habit, and extremely vigorous; young shoots glabrous, green, slightly angled. Leaves ovate-triangular, with a broad, straight base, and a short, abrupt, slender apex, 2 to 6 in. wide and long, regularly round-toothed, the margin translucent and at first minutely hairy; one or more glands occur at the base near the stalk, which is 11⁄2 to 21⁄2 in. long, glabrous, compressed. Catkins 3 to 4 in. long; flower-stalks glabrous; anthers rich red.
This hybrid, perhaps the oldest of the group, arose early in the 18th century, probably in France, though it became known there as the Swiss poplar (peuplier suisse). In this country, to which it was introduced late in the 18th century, it was known until recently as the “Canadian poplar” or the “black Italian poplar”. It was once the most commonly planted of all poplars except the Lombardy, and is still a feature of the landscape both in the countryside and in suburbia, easily recognised by its goblet-shaped crown, bare of leaf until the first or second week of May, and its grey-green leaves. It responds well to pollarding and quickly regains its characteristic shape. Being of comparatively slow growth, it is no longer much grown in plantation, but is still of value in low-lying, frosty areas owing to its late flushing. It attains greater dimensions than any other exotic deciduous tree. Among the largest recorded specimens are: Fairlawne, Kent, 140 × 203⁄4 ft, with a clean bole of 45 ft (1965); Chelsworth, Suffolk, 118 × 213⁄4 ft, clean bole 30 ft (1968); Alresford, Hants, by the watercress beds, 112 × 20 ft (1965); Hyde Park, London, 105 × 163⁄4 ft (1967).
‘Serotina Aurea’ (‘Aurea’). – Leaves very yellow in spring and early summer, becoming yellowish green later. This variety originated in the nursery of van Geert at Kalmthout, Belgium, as a branch-sport, and was put into commerce in 1876.
‘Serotina de Selys’ (‘Serotina Erecta’). – A tree of fastigiate habit, but in other respects resembling ‘Serotina’. It arose in Belgium before 1818 and was first described by Baron de Selys-Longchamps in 1864. It would make an excellent substitute for the Lombardy poplar were it not that, like ordinary ‘Serotina’, it leafs late and therefore lacks the screening and shelter properties of the Lombardy.