An evergreen shrub 2 to 4 ft high; young shoots sprinkled with glands. Leaves leathery, oval, tapering to a short stalk at the base, abruptly pointed and with a mucro at the apex, 11⁄2 to 3 in. long, 3⁄4 to 11⁄2 in. wide, dark dull green and glabrous above, glaucous and with numerous small scales beneath. Flowers closely packed in compact terminal clusters of four to eight, opening in April. Calyx scaly, deeply five-lobed, the lobes 1⁄/6 in. long, rounded at the end; flower-stalks 1⁄2 to 5⁄8 in. long, scaly. Corolla rather flat and open but bell-shaped at the base, about 1 in. wide, bright yellow, faintly spotted inside, scaly outside, five-lobed, the lobes roundish ovate, 3⁄8 in. wide. Stamens ten, 3⁄8 in. long, densely clad with white hairs at the base; anthers large, reddish brown. Ovary scaly; style glabrous, 1⁄4 in. long, abruptly bent over. Bot. Mag., t. 8946. (s. and ss. Boothii)
A native of the rainier parts of N.W. Yunnan, and of bordering areas of upper Burma, on the mountains between the upper Irrawaddy and its tributaries and the Salween; discovered by Père Delavay on the Tali range in 1886; introduced by Forrest in 1910. It grows mainly at 10,000 to 12,000 ft. on moist, shady cliff-ledges, mossy rocks, etc., or occasionally as an epiphyte on the trunks and branches of trees. Although far removed from it geographically, it is allied to R. boothii from the borders between Bhutan and Assam. And it is interesting that R. dekatanum Cowan, a species not in cultivation, was found in an area not far from the type-locality of R. boothii, yet is very closely allied to R. sulfureum (R.Y.B. 1948, p. 65).
R. sulfureum first flowered at Caerhays in April 1920. Most forms are tender, but a plant at Borde Hill in Sussex has lived in a sheltered place for at least thirty years. Plants with flowers of a vivid pure yellow are very striking, and the smooth brown bark is another attraction. R. sulfureum received an Award of Merit when shown by Lord Stair, Lochinch, Wigtonshire, on April 6, 1937.