An evergreen shrub of rather stiff habit up to 12 ft high; occasionally a tree twice as high in the wild; young shoots minutely downy and glandular. Leaves of firm and rather leathery texture, narrowly elliptical, sharply pointed, tapered at the base; 2 to 51⁄2 in. long, 1 to 13⁄4 in. wide; ultimately glabrous on both surfaces; veins in thirteen to sixteen pairs; stalk 1⁄3 to 3⁄5 in. long. Flowers in a terminal hemispherical truss, 4 in. wide, of about fifteen blooms opening in April. Calyx with five shallow triangular teeth, covered like the flower-stalk with minute glands; the latter is 1⁄2 to 1 in. long. Corolla bell-shaped, with the five rounded lobes notched and wavy at the margin; 11⁄2 to 2 in. long and wide; the colouring varies, and although the ground is white, sometimes pure, sometimes yellowish, sometimes suffused with rose, it is always or nearly always more or less spotted, sometimes conspicuously so. Stamens ten, white or yellowish, about 1 in. long, minutely downy towards the base; anthers brown. Ovary densely glandular, as is also the style the whole of its length. Bot. Mag., t. 7361. (s. and ss. Irroratum)
Native of N.W. Yunnan at altitudes of 8,000 to 10,000 ft. It was discovered by Père Delavay near Lankiung in 1886 and introduced by him to France, thence to Kew, where it first flowered in 1893. However, the species is now mainly represented in cultivation by plants from the seeds collected by Forrest and by Dr Rock in the region from the Tali range northward to the Chungtien plateau and eastward beyond the Yangtse to Yungpeh. It seems to be a common species on the Sungkwei pass N.E. of Lankiung, in scrub and thickets.
R. irroratum is variable in the colour of its flowers. The form with white or blush-coloured flowers is represented in gardens by the lovely Rock 59220 and also by his 59212, 59620, and 59581, all represented in the Species Collection, Windsor Great Park. A form with pale creamy-yellow flowers was introduced by Forrest from the Sungkwei pass under his number F.5851. According to his field notes, F.21323 from south-west of Lichiang had orange-yellow flowers. Most remarkable are the forms with heavily spotted flowers, of which one was growing at Kew in 1911 and is figured in the Gardeners’ Chronicle for that year (Vol. 49, facing page 350). This was presumably from the Delavay introduction. A similar form received an Award of Merit in 1957 when shown from Exbury on March 19 under the clonal name ‘Polka Dot’, but this plant has a very large number of flowers per truss for R. irroratum and could be a hybrid between it and R. arboreum. The normal white-flowered R. irroratum received an Award of Merit on the same occasion, exhibited from Minterne by Lord Digby.