An evergreen shrub or a small tree up to 20 ft high; young shoots, leaf-stalks, and undersurface of leaves covered with a velvety, cinnamon-brown felt, which is especially richly coloured beneath the leaves; these are obovate, tapered at the base, rounded at the apex, and with a short mucro there, 5 to 9 in. long by 11⁄2 to 31⁄2 in. wide; stalk 1⁄2 to 1 in. long. Flowers borne in April, crowded, fifteen to twenty-five together, in a truss 5 or 6 in. across, each on a downy stalk 1 to 11⁄2 in. long. Calyx very small. Corolla bell-shaped, 11⁄2 to 2 in. wide, eight-lobed, in colour either white, creamy yellow, or yellow tinged with rose, with a dark crimson blotch or streaks of crimson at the base. Ovary densely downy but not glandular; style glabrous. (s. Falconeri)
In the wild, R. arizelum often occurs with R. sinogrande though at higher altitudes, and the range of the two species is very similar – from the Salween westward through upper Burma to the eastern end of the Himalaya. Forrest's sendings are from various localities along the divide between the Salween and the eastern Irrawaddy, from the Shweli-Salween divide in the south, where he first found it in 1917, to as far north as 28° 50′ N. Near the north-western end of its range it was found by Kingdon Ward in 1924 on the far (southern) side of the Doshong La. The most recent collections in the Himalaya are Ludlow and Sherriff's 2753 from the upper Subansiri, not far east of the Bhutan frontier, and Cox and Hutchison 427, collected in 1965 in the same region but nearer to the plains of Assam.
In the wild R. arizelum is a gregarious species: 'One cliff, which faced the cold east, was painted pink and yellow for 2000 ft with flowers of R. arizelum. Looking squarely at this cliff from the river, one saw about 250 acres of rhododendron blossom. . . . The trees stood touching one another, and there must have been fifty thousand of them in bloom together' (Kingdon Ward, Plant Hunter's Paradise, p. 206, referring to the Adung valley, a feeder of the Irrawaddy). The variation in flower-colour was also remarked on by Farrer: 'Typically, its blossom is of a creamy white. . . . But I have seen it vary to citron and once even to apricot-yellow, while there is no end to its developments into the loveliest rosy shades' (Gard. Chron., Vol. 69 (1921), p. 274, referring to plants on the Burma-Yunnan frontier). There is also variation in the size and form of the truss, some plants in cultivation having very poor, flat-topped trusses. However, even when its flowers disappoint, R. arizelum makes a fine specimen, striking both in leaf and bark. Of the young foliage of R. arizelum in the Mishmi Hills Kingdon Ward wrote: 'In August the young leaves project like silver spears powdered with bronze-dust from amongst the tawny red and bottle green of the old leaves' (Plant Hunting on the Edge of the World, p. 278).
R. arizelum is hardy in a sheltered place near London but, like all the big-leaved rhododendrons, it attains its greatest size in the Atlantic zone. An Award of Merit was given to a form with a globular truss of about twenty flowers, coloured Solferino Purple with a dark crimson blotch in the throat, shown from the National Trust Gardens, Brodick, Isle of Arran, April 9, 1963.
var. rubicosum Cowan & Davidian – Under this name is distinguished a form with bright crimson flowers, raised by Lord Stair at Castle Kennedy, Wigtownshire, from Rock 59550. The seed was collected in 1923 on the slopes of Kenichunpu, Irrawaddy-Salween divide, just north of 28° N., at 12,000 ft. Award of Merit April 9, 1963.
From the Supplement (Vol. V)
This becomes R. rex subsp. arizelum. See further under R. rex in this supplement.