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).
From the Supplement (Vol. V)
This species, described in 1911, is near to B. pendula, differing in the bark, which remains white and smooth even on old trees, in the larger leaves, 2 to 21⁄2 in. long and 13⁄8 to 21⁄4 in. wide, more commonly truncate at the base than in its ally, and in the relatively broader fruiting catkins, with the bracts more tapered at the base. According to Lindquist, it is really very close to the race of B. pendula which he named var. lapponica (q.v. in this supplement). It has a wide range in eastern Russia, from Lake Baikal to the Pacific, and is said also to occur in Sakhalin and in parts of northern China.
The established taxonomic convention has been to treat all the Asiatic white birches as a single species with several varieties. At first B. japonica was used as the name for this species (as in Schneider's treatment of the Asiatic birches in Plantae Wilsonianae), and after this name was found to be invalid B. mandshurica (Reg.) Nakai (1915) was substituted. This in turn had to give way to the earlier, overlooked name B. platyphylla (1911), when this was taken up by the Japanese botanist Hiroshi Hara.
It is, however, not at all clear what character there is, except perhaps the bark, that unites all the birches placed under B. platyphylla and at the same time differentiates them from B. pendula. The first to challenge the prevailing orthodoxy was the Swedish botanist Lindquist (Svensk. Bot. Tidskr, Vol. 41, pp. 68-72 (1947)), and later in a more detailed study, C. A. Jansson (Act. Hort. Gotoburg., Vol. 25, pp. 103-56 (1962)).
The latter's treatment of the group is in the main followed in the present supplement, where the birches currently placed under B. platyphylla as varieties will be found under B. mandshurica and B. szechuanica.