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This hybrid poplar was first raised at Kew in 1912 by crossing the female P. deltoides ‘Cordata’ with pollen from P. trichocarpa. The seed was ripe by the end of June and, being sown immediately, germinated the following October. The cross was repeated in 1914. The plants so raised made extraordinary growth and in seven years from sowing the seed were up to 37 ft high and 22 in. in girth of trunk.
The young shoots are glabrous, shining grey-green, and angled (but by no means so markedly as those of the seed-parent); winter-buds awl-shaped, about 1 in. long, freely supplied inside with yellowish, viscid, balsamic resin. Leaves deciduous, triangular-ovate, truncate or slightly heart-shaped at the base, pointed, the translucent margin set with regular, incurved, gland-tipped teeth; rather pale green above, greyish beneath but not so pale as in P. trichocarpa; they vary much in size, the largest 12 or 13 in. long, 9 or 10 in. wide, the smaller ones one-third those dimensions; leaf-stalk 3 to 4 in. long, more or less flattened, with two or three conspicuous glands where it joins the midrib. Many of the leaves turn a good yellow in autumn. Male catkins 4 to 5 in. long: stamens with reddish anthers and long white stalks; female catkins rather longer, ovary glabrous, stigmas usually three. The flowers open in April.
As a timber tree, P. × generosa has not fulfilled its early promise. The rapid growth of young trees is sustained provided they remain free from disease, but usually development is checked owing to infection by one disease or another, or by loss of height due to breakage. P. × generosa has no ornamental value. It is very susceptible to bacterial canker, and is usually the first to be attacked in a mixed planting. It is, however, of historic interest, as the first deliberate cross between a black poplar and a balsam poplar.
Two of the original trees at Kew, pl. 1914, measure 92 × 71⁄2 ft (1970) and 75 × 61⁄4 ft (1967). A tree at Albury Park, Surrey, pl. 1928, is about 120 ft high and 83⁄4 ft in girth (1966).
Whether the name P. × generosa is applicable to all hybrids of the parentage P. deltoides × P. trichocarpa depends on the precise status of the female parent, originally distributed as P. angulata cordata (see P. deltoides ‘Cordata’, page 311). On this pretext J. T. M. van Broekhuisen has published the name P. × interamericana for P. deltoides × P trichocarpa. However, there is really little doubt that ‘Cordata’ belongs to P. deltoides in the wide interpretation of that species which is now accepted.
specimens: Kew, pl. 1914, 92 × 71⁄2 ft (1970) and, pl. 1919, 66 × 53⁄4 ft (1976); Thorp Perrow, Bedale, Yorks., 98 × 51⁄2 ft (1981); Edinburgh Botanic Garden, 95 × 7 ft (1981).