B. platyphylla, in its typical form, is found on the mainland of N.E. Asia as far west as Mongolia and south to N. China. It is unlikely to be a success in the British Isles, where it is represented by its Japanese variety. From this variety, typical B. platyphylla differs chiefly in its leaves, which are glabrous beneath except for axillary tufts, and dotted with glands.
var. japonica (Miq.) Hara B. alba var. japonica Miq.; B. japonica (Miq.) Winkler; B. mandshurica var. japonica (Miq.) Rehd. B. pendula var. japonica Rehd.; B. verrucosa var. japonica Henry – A deciduous tree up to 85 ft high in a wild state, with thin spreading branches and pure white bark on the trunk; young shoots slightly glandular-warty and either glabrous or slightly downy. Leaves 11⁄2 to 3 in. long, downy or nearly glabrous, reddish. The young, expanding leaves are of a pretty, red tinge. Fruiting catkins 11⁄2 to 31⁄2 in. long, cylindrical, 1⁄4 in. diameter, borne singly; scales very small, the middle lobe several times larger than the side ones. Nutlets with wings broader than themselves.
Native of Japan and the Okhotsk peninsula; in cultivation since 1887, probably before. Botanically, B. platyphylla is closely related to the silver birch (B. pendula) but this variety is easily distinguished from it by its larger, broader leaves, with axil-tufts beneath, their more numerous veins and usually single toothing. It thrives well in cultivation but I do not know that it has a greater value in the garden than our common silver birch. There is a large specimen of this birch at Tortworth, Glos., 72 × 5 ft (1965). At Westonbirt it is 46 × 2 ft in Holford Drive (1964).
var. szechuanica (Schneid.) Rehd. B.japonica var. szechuanica Schneid. – Introduced by Wilson from W. Szechwan in 1908 (W. 983 and 4088); his specimens were at first treated by Schneider under the name B. japonica var. mandshurica. As seen in cultivation, Wilson’s trees have rather bluish-green leaves and spreading crowns. The variety is easily distinguished from var. japonica by its leaves, which are dotted with glands beneath. It is a vigorous but rather graceless tree, with a silvery-white bark. ‘This var. szechuanica is the only birch I know where the white comes off on the hands like old whitewash’ (A. F. Mitchell). Examples recorded are: Kew, 39 × 21⁄2 ft (1965); Westonbirt, Glos., in Mitchell Drive, 41 × 21⁄4 ft (1965); Tortworth, Glos., 58 × 43⁄4 ft (1966).