Betula potaninii Batalin

Rarely seen in cultivation, Betula potaninii is one of three rather similar, closely related western Chinese dwarf birches, the others being Betula calcicola from Yunnan and B. bomiensis from Gansu and Tibet. The leaves of all three species are similar, being leathery and deeply veined, but the ong silky hairs beneath the leaves of B. potaninii are very distinctive. The species was described by Alexander Batalin from specimens collected near Kanding by Grigori Potanin in 1884–1886, and has been in cultivation since 1909 from E. H. Wilson’s introduction from Sichuan (Bean 1976). There appear to have been other early or mid twentieth-century introductions, since Frank Meyer found the species in cultivation at the Hesse nursery in northwest Germany in 1960 and introduced it to the United States under PI 265602 (Meyer 1963).

Bean’s date of 1909 means that Wilson’s introduction must have been W 1140 from Wa-shan, Western Sichuan at 2100–2800 m – an earlier Wilson collection from the same place in July 1902 is annotated as being in young fruit, and others (W 4490, 4209, 4299) were not made until 1910 (Schneider, in Sargent 1916). Wilson found the species to be common on the ‘moist rocks and cliffs of the forest-covered higher mountains’, noting that ‘usually all its slender whip-like branches hang down over cliffs and boulders, but often it is a shrub from 1 to 3 m high with decumbent and prostrate branches’ (quoted by Schneider, in Sargent 1916).

A more recent collection from Paoma Shan near Kangding in 1988 (SICH 347) was from what appeared to be a single shrub growing to 1 m tall in rather dry scrub together with roses, Malus sp. and young larch trees (Howick, pers. comm.). Material from this collection grows at RBG Edinburgh and Howick Arboretum. Plants from other recent collections grow at Sonoma Botanic Garden: SICH 0546 also from Paoma Shan in 1991 at 2840 m, and SICH 2208 from Shimian County, gathered in 2001 at 2896 m.

Given a situation in reasonably high light without too much shade from surrounding trees, Betula potaninii is relatively slow growing but vigorous. Although Wilson described it as prostrate and hanging, in cultivation it makes a spreading shrub with indications that it might become a small tree (see B. jiulungensis below). As in most other species of this group, the leading shoots soon bend over and grow horizontally, further increases in height being made by the growth of a lateral shoot; between one and five main stems may develop. Flowering begins after three years, the male catkins borne on the terminal shoots restricting lateral growth. After fifteen years some plants have reached 5 m in height. Young plants of SICH 347 have proved easy to grow at Ness, with no dieback and no problems with early autumn or late spring frosts. Propagation of W 1140 proved very difficult by either cuttings or grafting, but a few cuttings were – eventually – rooted at Hergest Croft and one was planted out in 1992 (cat. 4453, 4454; McAllister & Ashburner 2013).

The shrubby Betula trichogemma described from Emei Shan in Sichuan in 1985 seems to be no more than a minor variant of B. potaninii, differing primarily in its hairier buds. Its distinctness from B. jiulungensis and B. calcicola is questioned by Skvortsov (Li & Skvortsov 1999). Betula jiulungensis, described in 1979 from Jiulong Shan in SW Sichuan, differs from B. potaninii mainly in that it is described as a tree up to 12 m tall; it may be merely a tree form of B. potaninii or perhaps a distinct species.