A deciduous bush up to 10 ft high; shoots four-angled and, like the under-surface of the leaves, covered with a reddish-brown down. Leaves up to 9 or 12 in. long, by 3 in. wide; lanceolate, slender-pointed, tapered at the base, toothed; stalk 1⁄4 to 1⁄2 in. long. Inflorescence cylindrical, 2 to 8 in. long, about 1 in. wide, made up of shortly stalked, few-flowered, closely packed clusters; it is terminal, supplemented by others from the axils of the top pairs of leaves; all the stalks very woolly. Flowers fragrant, described by Forrest as ‘grey and maroon’, ‘reddish maroon’, ‘soft lavender rose’, ‘pale mauve, almost white’; from which it would seem that, like B. davidii (variabilis), it has a considerable range of colouring in even a wild state. Corolla 1⁄3 in. long, 1⁄5 in. wide across the lobes, more or less woolly outside; stamens inserted immediately below the mouth. Calyx 1⁄6 in. long, with four narrowly triangular lobes, more or less downy. Ovary downy, sometimes becoming smooth. Bot. Mag., n.s., t. 93.
Native of S.W. China; discovered by Forrest in 1903 and introduced by him. It has been confused in gardens with B. farreri, with which it has no close kinship; that species flowers in spring from the joints of the previous year’s growth, whilst B. forrestii flowers in late summer and autumn at the end of the current season’s growth, in the same way as B. davidii. It is not very hardy in the open ground at Kew; and I have not yet seen it in a sufficiently attractive condition to justify a recommendation of it.
The description given above is of the typical form, but in downiness as well as in flower colour it varies a good deal, some forms approaching an almost glabrous state in branchlet, inflorescence, and ovary. The insertion of the stamens close to the mouth of the corolla, and the corolla being about twice as long as wide are (in association) the most distinctive characters. In the example now at Kew, which is figured in Bot. Mag., n.s., t. 93, the lower surfaces of the leaves are covered with a thin, white indumentum and the stems are rounded, only rarely somewhat four-angled. It is grown on a wall of the Temperate House, where it is only cut in severe winters.
B. pterocaulis A. B. Jacks. B. longifolia Gagnep, not H. B. K. – Another ally of B. forrestii, differing in its narrower corolla-tubes, longer inflorescences and larger leaves; introduced by Farrer from Upper Burma. It is in cultivation at Highdown but Sir Frederick Stern recorded, ‘The flowers are a poor colour; it was only kept because Bean when staying here said I must keep it as it was the only one he had ever seen.’