An evergreen, glabrous shrub up to 14 ft high and more in diameter in the Cornish gardens. Leaves roundish oval, 2 to 4 in. long, two-thirds as wide, round at the apex except for a short, abrupt tip, and rounded or slightly heart-shaped at the base, dark green above, blue-white or glaucous green below; stalk about 3⁄4 in. long. Flowers borne in March or April, six to ten in a loose terminal cluster; pedicels up to 1 in. long. Calyx 1⁄2 to 3⁄4 in. deep, rarely shorter, cup-shaped, often tinged with red. Corolla bell-shaped, fleshy, rich blood-red, 2 to 3 in. across, five-lobed. Stamens ten, glabrous. Ovary and style glabrous. Bot. Mag., t. 4997. (s. and ss. Thomsonii)
Native of the Himalaya as far west as Nepal; introduced by J. D. Hooker in 1850 from Sikkim, where it grows at 10,000 to 13,000 ft. It first flowered in 1857 with Messrs Methven of Edinburgh, who had taken a scion from one of their seedlings and grafted it on R. ponticum.
R. thomsonii is hardy at Kew, but needs a sheltered position, and even then its flowers and young growths are very liable to injury by late frost. In more favoured gardens it is magnificent, and really deserves a place even in a garden with the climate of Kew, since it makes a handsome specimen and the flowers are so splendid that a display one year in three is ample return for the room it occupies. It received an Award of Garden Merit in 1925.
Crossed with other species, R. thomsonii has given rise to some of the finest and best known hybrid grexes, notably Luscombei (with R. fortunei), Shilsonii (with R. barbatum), and Cornish Cross (with R. griffithianum), 'Ascot Brilliant' is an old hybrid between R. thomsonii and a hardy hybrid, raised by Standish over a century ago; more recent commercial hybrids with R. thomsonii as one parent are 'Sir John Ramsden' and 'J. G. Millais'.
var. candelabrum (Hook, f.) C. B. CI. R. candelabrum Hook, f. – Flowers paler than in the typical variety. Calyx smaller. Ovary glandular. Described from Sikkim.
var. pallidum Cowan – Flowers rose-pink. Calyx large. Ovary glandular. Described from specimens collected by Ludlow and Sherriff in the Tibetan Himalaya.
R. lopsangianum Cowan – Allied to R. thomsonii, but not so tall-growing, and with smaller flowers and leaves. The leaves are markedly papillose beneath and the calyx is small, not forming a distinct cup. Described in 1937 from specimens collected by Ludlow and Sherriff in the Tibetan Himalaya and introduced by them. It is named after the then Dalai Lama of Tibet.
R. viscidifolium Davidian – In describing this species in 1966, Davidian remarks that it agrees with R. thomsonii var. pallidum in some respects, differing in the copper-red flowers, the small calyx, and the tomentose or glandular ovary. The leaves are glandular and sticky beneath, whence the specific epithet. It was discovered by Ludlow, Sherriff, and Taylor in the Tibetan Himalaya on the Lo La and was introduced by them in 1938. It is in cultivation at Glenarn in Dunbartonshire.
From the Supplement (Vol. V)
subsp. lopsangianum (Cowan) Chamberlain R. lopsangianum Cowan – The distinctive characters of this subspecies are given on pages 786-7, under R. lopsangianum.
The two varieties of R. thomsonii mentioned are considered to be natural hybrids between R. thomsonii and R. campylocarpum – R. × candelabrum Hook.f. – a cross occurring occasionally where the two species are in contact.