An evergreen shrub up to 4 ft or perhaps more high, of rounded compact shape; young shoots densely scaly. Leaves oval or obovate, rounded or bluntish at the apex, tapered at the base to a very short stalk, 3⁄4 to 11⁄8 in. long, 1⁄3 to 1⁄2 in. wide, dull grey green or glossy glaucous green above, brownish with a dense layer of overlapping scales beneath, edged when young with bristles. Flowers terminal, usually in pairs, opening in May, each on a scaly stalk 1 in. or so long. Calyx 1⁄4 in. long, with five broadly ovate, purplish lobes, scaly, sometimes densely so. Corolla open and flattish, 11⁄2 in. wide, pale pink to magenta purple with deeper spots on the upper side, five-lobed, the lobes overlapping, downy outside. Stamens ten, 1⁄2 in. long, purple with a dense tuft of hairs at the base. Ovary covered with pale scales; style purple with a few hairs at the bottom. Bot. Mag., t. 9001. (s. Saluenense)
R. calostrotum was discovered by Kingdon Ward in 1914 on the eastern spur of the Imaw Bum, an isolated peak in upper Burma on the Nmai (eastern Irrawaddy)-Salween divide. When he reached the summit in 1919 from the western side he found, and collected seed of, what he thought was the same species (KW 3390), but it appears that what he found on that occasion was really R. keleticum. So the credit for its introduction belongs to Farrer and Cox, who collected seeds in the same year on the top of the Hpawshi Bum, about twenty miles to the south-east of the type-locality. Here it 'covers the barest braes and tops of moorland in a close, flat carpet of dark foliage, from which on pedicels (in pairs) of an inch or so, rise large, round blossoms of a rich, warm magenta-rose' (Farrer 1045; Gard. Chron., Vol. 66 (1919), p. 289). Forrest later sent seeds from other localities on the same divide, whence it ranges westward as far as the eastern end of the Himalaya. In the wild it is usually an erect shrublet up to 2 ft high, but mat-forming in exposed positions.
R. calostrotum flowered at Kew in April 1923 when only two or three inches high. No more delightful rhododendron for the rock garden has been introduced. So large are the flowers and so freely are they borne that a small plant may be literally hidden beneath them. It produces fertile seed in plenty and is easily raised from cuttings. Provided the soil is moist it can be grown in full sun.
Award of Merit May 8, 1935, to a form raised at Nymans from Forrest 27065.
cv. 'Gigha'. – Flowers described as deep claret-red (International Rhododendron Register, Additions 1968-9). A plant under this name received a First Class Certificate May 4, 1971; flowers described as light rosy-red.
var. calciphilum (Hutch. & Ward) Davidian R. calciphilum Hutch. & Ward – Leaves smaller than in the type, up to 1⁄2 in. long. Described from Kingdon Ward's 6984, collected in the Seinghku valley, N.W. upper Burma and introduced under that number. He found it forming tight tufts on limestone screes (whence the epithet calciphilum and the nickname “Limestone Rose” which he gave to it). In the wild it flowers later than the normal form (which occurs in the same valley) and has kept this character in cultivation, where it flowers late May. The leaves are somewhat glaucous.
From the Supplement (Vol. V)
This species has been enlarged by Dr Cullen to include R. keleticum and R. radicans as a single subspecies, while R. riparium is resurrected from synonymy and given specific rank. He also describes the new subsp. riparioides.
subsp. calostrotum – The characters of the typical subspecies, to distinguish it from the others mentioned, are: low and spreading or erect shrub; leaves more or less obtuse, up to about 1 in. long, 3⁄8 to 7⁄8 in. wide, lepidote above, the scales on the undersides in three or four tiers; flowers one or two in each inflorescence, on pedicels 5⁄8 to 1 in. long. In cultivated plants the leaves are usually grey above, but this is not a constant character of subsp. calostrotum.
subsp. keleticum (Balf.f. & Forr.) Cullen R. keleticum Balf.f. &. Forr.; R. radicans Balf. f. & Forr. – Habit prostrate or semi-prostrate. Leaves acute, not lepidote above, up to 3⁄8 in. wide. It will be seen that in effect R. keleticum and R. radicans have been united, to the indignation of some growers. It is true that some forms grown as R. radicans are completely prostrate and self-rooting (e.g., Forrest 19919), but as pointed out under R. radicans on pages 698-9, some plants distributed as this species (and presumably deriving from seed-collections identified as such) are more like R. keleticum in habit. Peter Cox writes: 'I have seen plants in cultivation that would easily pass as either keleticum or radicans using the old descriptions. I have also seen groups of seedlings ranging completely from typical keleticum to typical radicans. So the fuss made about this amalgamation stems from ignorance' (The Smaller Rhododendrons, p. 125).
subsp. riparium (Ward) Cullen R. riparium Ward; R. rivulare Ward, not Hand.-Mazz.; R. calciphilum Hutch. & Ward; R. calostrotum var. calciphilum (Hutch. & Ward) Davidian; R. nitens Hutch. – The leading characters of this subspecies, to distinguish it from subsp. calostrotum, are: two to five flowers in each inflorescence on shorter pedicels (about 1⁄2 in. long). South-east Tibet to north-west Yunnan.
The type of R. riparium (hitherto included in R. calostrotum in synonymy) was collected by Kingdon Ward on the Doshong La in the region of the Tsangpo Bend. For R. nitens, see on page 698 under R. keleticum, to which (i.e., to subsp. keleticum) it is really quite as close as it is to subsp. riparium. For R. calciphilum, the other inclusion, see var. calciphilum on page 617. The subspecies is not horticulturally uniform, and there is no reason why it should be, so long as it holds together botanically. Peter Cox has proposed that two groups should be distinguished: the Nitens and the Calciphilum groups – to accommodate the horticultural variations (op. cit., p. 124).
subsp. riparioides Cullen – Near to subsp. riparium, but leaves larger, 1 to 11⁄2 in. long, and scales of undersurface in one tier (cf. subsp. calostrotum), and flowers larger. Described from Forrest 25503 and introduced by him (as R. calostrotum), but probably now mainly represented in cultivation by Dr Rock's 178, collected during his expedition to Yunnan for the American Rhododendron Society in 1948-9. This subspecies has a limited distribution in Yunnan (mostly around Weihsi) and Dr Cullen suggests that it may be a stabilised hybrid between R. calostrotum and R. saluenense subsp. chameunum.