Shrub, to 2.5 m; young shoots and petioles with a floccose indumentum though soon glabrescent; bud scales deciduous. Leaves 8–12 × 2.5–3.5 cm, elliptic, apex more or less cuspidate, upper surface glabrous, lower surface with persistent punctate hair bases, otherwise glabrous, or with some persistent dendroid hairs, especially towards base and on midrib. Flowers c.10, in a lax truss; calyx c.3 mm; corolla white, with reddish flecks, funnel-campanulate, without nectar pouches, 35–45 mm; ovary densely glandular, style glandular below. Flowering April-May. Royal Horticultural Society (1997)
Habitat 1,000–1,300 m
RHS Hardiness Rating H7
Awards AM 1976 (Capt. c. Ingram, Benenden, Kent) to a clone 'Omo'; flowers white.
Conservation status Data deficient (DD)
Taxonomic note R. hyperythrum is a distinctive species without close allies. Royal Horticultural Society (1997)
A dense evergreen shrub usually not more than 6 ft high in cultivation, but twice that in width; branchlets stout, glabrous. Leaves leathery, oblong-elliptic or oblong-oblanceolate, acute and mucronate at the apex, margins strongly recurved (at least in the cultivated form), 31⁄2 to 6 in. long, 1 to 13⁄4 in. wide, dark green and glabrous above, with an impressed midrib, undersurface dotted with minute punctulations, which are more evident on dried specimens than on live material, glabrous, but the prominent midrib at first covered with a floccose tomentum; petiole at first hairy like the midrib, up to 1 in. long. Inflorescence a racemose umbel of about ten flowers opening in April or May; pedicels up to 13⁄4 in. long, floccose. Calyx small, the lobes triangular, ciliate, about 1⁄8 in. long. Corolla five-lobed, funnel-campanulate, about 13⁄4 in. long and wide, pink or white and more or less speckled in wild plants, pure white and almost unspeckled in the cultivated form. Filaments of stamens downy at the base. Ovary coated with short, spreading or appressed hairs, scarcely glandular; style hairy at the base. Bot. Mag., n.s., t. 109. (s. Ponticum ss. Caucasicum)
Native of Formosa, where it is said to be very local. It was described from a specimen collected on the central ranges, while the type of R. rubropunctatum was collected at the northern extremity of the island, where the species occurs north and east of Taipei. It was described by Hayata in 1913 and introduced by Lionel de Rothschild to Exbury in the 1930s. It is of uncertain taxonomic position, but seems to be very distinct from R. morii, in which it was included by Kanehira. It is at present placed in the Caucasicum subseries of s. Ponticum. R. hyperythrum is perfectly hardy, needs only light shade and a sheltered position, grows quickly and flowers when quite young. In the form usually seen in cultivation the flowers are pure white and almost unmarked, in this respect differing from most wild plants (see the note by K. Wada in Rhododendrons 1972, pp. 33–4). R. hyperythrum is one of the best species for a small garden, being of neat habit, with striking foliage and beautiful flowers.