An evergreen shrub up to 10 ft or 15 ft high in the wild, described by Ward as of 'tall, loosely knit' habit. Leaves oblong-oval, often inclined to obovate, mostly rounded at the apex, 4 to 6 in. long, 1 to 3 in. wide, strongly veined, dull light green and glabrous above, densely furnished with sunken, yellow-brown scales beneath; stalk 1⁄2 to 3⁄4 in. long, with a shallow groove on the upper side. Flowers drooping, opening April to early June in loose clusters of three to five, with a nutmeg-like fragrance. Pedicels stout, up to 11⁄4 in. long, glabrous. Calyx very large, bell-shaped, 3⁄4 to 11⁄8 in, long, with five deep, rounded ovate lobes, glabrous and without scales. Corolla funnel-shaped, 4 in. long and as much wide, pure white or flushed with pinkish purple, stained with pale yellow at the base inside. Stamens ten, shorter than the corolla, downy at the lower part. Ovary and base of style scaly. Capsule very short, about 3⁄4 in. long, surrounded by the persistent calyx. Bot. Mag., t. 9326. (s. Maddenii ss. Megacalyx)
Native of the borderland between China and upper Burma westward across the upper Irrawaddy to the region of the Tsangpo bend, at the eastern end of the Himalaya; discovered by Kingdon Ward in 1914 in the valley of the Nmai Hka, Burma, at 7,000-8,000 ft; introduced by Forrest in 1917 from the Shweli-Salween divide, about 20 miles south of the type-locality. Two years later Farrer and Cox collected seeds a few miles north of the original locality and at the same altitude. 'It is a tall shrub, or small spindly tree, with large, loose heads of blossom, passing over by the middle of May. The long stout pedicels are clothed in a sort of blue bloom, the big conspicuous calyx is crimson and pink and green. The flowers are of enormous size, pure white, flushed with pink, orange-anthered, and limp in texture, so as to suggest some floppy, snow-white flowered Gloxinia. Add to all this an intense fragrance of clove, and you may imagine with what acclamations I gathered in this new recruit . . .' (Farrer, Gard. Chron., Vol. 66 (1919), p. 161). The fragrance is usually likened to that of nutmeg, whence Kingdon Ward's nickname for the species – ”Nutmegacalyx”.
R. megacalyx is too tender to be grown outdoors in the London area, but succeeds in some gardens of the Atlantic zone. It received an Award of Merit when shown by Adm. Heneage-Vivian of Clyne Park, Swansea, on June 7, 1937.