An evergreen shrub up to 4 or 5 ft high, of rounded shape and usually wider than high, keeping close the ground, densely and intricately branched; young shoots slender, glaucous, thinly furnished with gland-tipped bristles. Leaves orbicular to ovate, usually heart-shaped at the base, 1 to 2 in. long, at first bronzy, finally dark green above, glaucous beneath; stalk 1⁄3 to 5⁄8 in. long, purplish when young, glandular-bristly. Flowers two to (rarely) four in a loose terminal cluster, opening in April. Calyx minute, glandular at the margin; flower-stalk 3⁄4 to 11⁄8 in. long, glandular. Corolla bell-shaped, 21⁄4 in. wide, soft rosy red, five-(rarely six-) lobed, the lobes notched; stamens ten, 11⁄4 in. long, glabrous; style overtopping the stamens and, like the ovary, sparsely set with stalked glands. Bot. Mag., t. 8935. (s. Thomsonii ss. Williamsianum)
Native of W. Szechwan, China; discovered and introduced by Wilson in 1908; he describes it as very local and occurring only in isolated thickets on the cliffs of Wa-Shan. It was later found on Mt Omei by Chinese botanists, growing in similar habitats. According to Fang, the plants there have rather larger leaves than in the typical form and up to fourteen stamens.
This species is named in honour of J. C. Williams of Caerhays and is one of the most distinct as well as beautiful of Chinese rhododendrons. It flowers when quite small, attracting notice then by the curiously disproportionate largeness of the flowers. It is quite hardy at Kew, but is slow-growing and dwarf. In more favoured gardens it makes dense hemispherical bushes not only attractive for its delicately coloured blossom but also for bronzy young foliage. Easily increased by late summer cuttings. It must be regarded as indispensable to any collection of rhododendrons. Its only fault is that both the flowers and the young growths are often killed by frost, but despite that it must be planted in an open, sunny position. Given the protection of a tree-canopy it does not set flower-bud and loses its characteristic shape.
R. williamsianum has proved to be a potent parent, imparting a compact habit and a short rounded leaf to its offspring in the most diverse crosses. All the hybrids between it and other species are charming, though some are inclined to be shy-flowering. Several crosses between R. williamsianum and Hardy Hybrids are now available, raised in Holland and Germany. See further in the section on hybrids.
From the Supplement (Vol. V)
† R. leishanicum Fang & S.S. Chang – A new species, described from a specimen collected in Kweichow in 1959. 'This interesting new species is probably most closely allied to R. williamsianum, though with a dense indumentum on the petioles and the lower surface of the leaf midribs as in subsection Maculifera, that is however not as dendroid and matted as is usual in that subsection' (Rev. 2, p. 261).
Seed of what will probably prove to be this species was collected in Kweichow by Simmons, Fliegner and Russell in September 1985.