An evergreen shrub of thin loose habit up to 8 ft high; young shoots 3⁄16 in. thick, clad with a thick, tawny felt. Leaves thick and leathery, narrowly oval-ovate, rounded or broadly tapered at the base, pointed, 11⁄2 to 51⁄2 in. long, 3⁄4 to 2 in. wide, glossy green and without down at maturity above but with a network of deeply sunken veins giving it a puckered (bullate) surface; lower surface thickly covered with tawny felt; stalk up to 1 in. long. Flowers produced during May in terminal clusters usually of two to four, but occasionally five or even six, richly fragrant; flower-stalk 1⁄2 to 3⁄4 in. long, shaggy. Calyx deeply five-lobed, the lobes rounded, 1⁄2 in. long, usually scaly outside, shaggy on the margins and often on the surface also. Corolla fleshy, scaly outside, waxy-white with a yellow stain inside and tinged with pink outside; shallowly trumpet-shaped, five-lobed, 21⁄2 to 41⁄2 in. wide, the lobes 11⁄2 to 2 in. wide. Stamens ten, shorter than the corolla, shaggy with down at the base. Ovary very woolly; style scaly and slightly downy towards the base, rarely without scales. Bot. Mag., t. 4936 (s. Edgeworthii)
R. edgeworthii is a species of wide range from E. Nepal to Yunnan and S.W. Szechwan. It was discovered by J. D. Hooker in Sikkim and described by him in 1851. 'The majority of my specimens were obtained from land-shoots, or -slips, in the rocky ravines, which bring down in their course those Pines on the limbs of which this species delights to dwell.' Hooker also introduced it. R. bullatum, now included in R. edgeworthii, was described from specimens collected by the French missionary Delavay in 1886. Forrest later found it on the eastern, drier, flank of the range growing on humus-covered ledges and along the base of cliffs at an altitude of 8,000 to 10,000 ft or even higher and published an account, illustrated by his own photographs, in the Gardeners' Chronicle for 1909 (Vol. 46, p. 378, fig. 166 and Supplementary Plate). He first sent seeds in 1910 and subsequently there were many further sendings by him and other collectors from various parts of the range of the species and from an 8,000 to 13,000 ft span of altitude. Where there is forest, and where the climate is very humid, R. edgeworthii is usually epiphytic; but in drier parts, or in scrub, it is ground- dwelling.
The old Himalayan form of R. edgeworthii, although tender, has been grown outdoors in the milder parts for over a century. The plants once known as R. bullatum are variable in hardiness. Tender forms are Farrer 842 from above Hpimaw on the Salween-Irrawaddy divide; and Forrest 26618 from the same area, both from epiphytic plants. The Award of Merit plant of 1923 was from the former number and the F.C.C. plant of 1937 from the latter (in both instances as shrubs for the cool greenhouse). The hardiest form is considered to be Dr Joseph Rock's 59202, with pink-tinged flowers, found as a low shrub 2 to 3 ft high among rocks on Mt Shenzi, at 13,000 ft; this form was introduced in 1923 and first exhibited by Lord Stair at the Rhododendron Show in 1930. Forrest's 21564 from the Chienchuan-Mekong divide has also proved to be moderately hardy. Both these forms are from near the eastern end of the range of R. edgeworthii. There is also a fine hardy form of the species at Bodnant, Denbighshire, known as the Kingdon Ward Pink Form. This received an Award of Merit on May 5, 1946.
R. seinghkuense Ward – This uncommon and tender species, at present placed in the Edgeworthii series, was discovered by Kingdon Ward in the Seinghku valley, north-west Burma, in 1926. The leaves are not unlike those of R. edgeworthii but in other respects it is very distinct, the corolla being sulphur-yellow, rotate-campanulate, with a short, bent style. The seed sent by Kingdon Ward under his no. 6793 was mixed with that of R. edgeworthii, which occurs in the same valley, and the true species is rare in cultivation. It has also been found in the neighbouring Adung valley, and in Yunnan. It received an Award of Merit when shown by the Crown Estate Commissioners in 1953.