An evergreen shrub or small tree, the bark peeling from the branches and leaving them blue-grey and smooth; winter-buds viscid; branches yellowish, sometimes glabrous, sometimes bristly. Leaves in a terminal cluster, oblong, heart-shaped at the base, terminated by a short, fine point, 4 to 9 in. long, 1 to 3 in. wide, dark dull green and ultimately glabrous above, pale and usually woolly at first beneath; stalk 1⁄2 to 1 in. long, conspicuously bristly on the upper side and at the base of the midrib. Flowers densely packed in a hemispherical truss about 4 in. wide, rich scarlet. Calyx with five glabrous ovate lobes, 1⁄4 in. long. Corolla bell-shaped, 11⁄2 in. across, five-lobed. Stamens ten. Ovary clad with long-stalked glands; style glabrous. (s. and ss. Barbatum)
Native of the Himalaya as far west as Kumaon, at 8,000 to 12,000 ft; introduced probably in 1829 (it was flowering in at least three gardens in 1848). This rhododendron is hardy in a sheltered spot at Kew, where it flowers in April. It is somewhat gaunt of habit, but worth growing for its marvellous richness of colour. It is, of course, much finer in Cornwall and similar places. There is some variation in the bristliness of the stems and leaves.
R. argipeplum Balf. f. & Cooper – This differs from R. smithii (see below) only in having a whiter and more persistent leaf-indumentum. It was described from specimens collected by Roland Cooper in Bhutan in 1915, and has also been found in Sikkim and in the Assam Himalaya. Cooper's specimens were in fruit only, but in the Ludlow and Sherriff collections from Bhutan the flowers are described as bright rose or deep pink. For R. argipeplum at Glenarn see R.C.Y.B. 1968, p. 197.
R. imberbe Hutch. – This was described in 1928 from a plant at Kew of unknown origin, raised at the end of the last century. The differences from R. barbatum were given as: petioles and stems entirely without bristles, leaves relatively broader and shorter, calyx-lobes ciliate. It is matched by wild specimens from Kumaon, but is probably no more than a form of R. barbatum.
R. smithii Hook. R. barbatum var. smithii (Hook.) C.B. CI. – Related to R. barbatum, but with the leaves clad beneath with a loose, greyish-white indumentum which becomes patchy by late summer. Native of the E. Himalaya. As seen in cultivation it is of more compact habit than R. barbatum, the leaves darker, with more impressed veins. Bot. Mag., t. 5120. It was introduced by Booth from just east of Bhutan and first flowered in 1859.
It has been overlooked by botanists that the name R. smithii Hook. (1859) is illegitimate, being antedated by R. smithii Sweet (1831).
From the Supplement (Vol. V)
[R. argipeplum] – Included in R. smithii.
[R. imberbe – Probably a natural hybrid between R. barbatum and R. arboreum (Rev. 2, p. 430).