An evergreen shrub up to 6 ft high; young stems clad with greyish hairs; bud-scales deciduous. Leaves obovate, elliptic or oblong, obtuse or rounded at the apex, cuneate at the base, up to 21⁄2 in. long and 1 in. wide, of leathery texture, dull medium green above, undersurface coated with a thin, continuous indumentum of branched hairs; petiole about 5⁄8 in. long. Flowers opening in May, in clusters of three to six. Calyx variable in size, up to 3⁄8 in. long but sometimes reduced to a mere rim. Corolla bell-shaped, about 11⁄2 in. long, five-lobed, crimson, with nectar-pouches at the base. Stamens ten, with glabrous or downy filaments. Ovary densely tomentose, not glandular; style glabrous. (s. Neriiflorum ss. Sanguineum)
Native of N.W. Yunnan and S.E. Tibet; discovered by the Abbé Soulié on the Se La, Mekong-Salween divide, in 1895; introduced by Forrest in about 1917. In its natural habitat it is a small shrub, usually under 4 ft high, occurring in open situations at 12,000 to 14,000 ft. It is hardy, but of little value in gardens, being very shy-flowering in most forms.
The above description is of R. sanguineum in the narrow sense. There are, however, many rhododendrons which resemble it closely in the size, shape, and indumentum of the leaves and in the general characters of inflorescence and flowers, but differing from R. sanguineum, or among themselves, in the colour of the flowers, in the presence or absence of glands on the ovary, and in having the foliage bud-scales either persistent or deciduous. Since these variations are not correlated, the group is very difficult to treat taxonomically. This problem, which concerns other species in the Sanguineum subseries also, is discussed by Dr J. M. Cowan in: 'Rhododendrons of the Rh. sanguineum Alliance', Notes Roy. Bot. Gard. Edin., Vol. 20, pp. 55-91. Dr Cowan's treatment of the variations of R. sanguineum is followed here, though it is perhaps rather too elaborate for horticultural purposes.
subsp. atrorubrum Cowan – Flowers black-crimson, differing from subsp. haemaleum in the eglandular ovary, from subsp. didymum in the deciduous bud-scales. Described from a specimen collected by Forrest on the Salween-Kiuchiang (upper Irrrawaddy) divide.
subsp. cloiophorum (Balf. f. & Forr.) Cowan R. cloiophorum Balf. f. & Forr.; R. asmenistum Balf. f. & Forr.; R. cloiophorum subsp. asmenistum (Balf. f. & Forr.) Tagg – Flowers rose-coloured, or rose more or less flushed with yellow. Foliage bud-scales deciduous. Ovary tomentose, not glandular. N.W. Yunnan and bordering parts of S.E. Tibet.
subsp. consanguineum Cowan – Resembling typical R. sanguineum in flower colour, differing in having the ovary glandular as well as tomentose. In cultivation from seeds collected by Forrest and by Rock in N.W. Yunnan and bordering parts of Tibet; also from Kingdon Ward's 6831 from the Seinghku valley, upper Burma. The type is F.25507, collected on the Mekong-Yangtse divide. Some plants agreeing with this subspecies flower more freely than typical R. sanguineum.
subsp. didymum (Balf. f. & Forr.) Cowan R. didymum Balf. f. & Forr. – Flowers black-crimson as in subsp. haemaleum but with the ovary glandular as well as tomentose. Foliage bud-scales persistent. Described from Forrest 20220, collected on the Salween-Kiuchiang divide at 14,000 to 15,000 ft and cultivated under this number and also from seeds collected later in the same area. The wild plants are of dwarf habit (up to 2 ft high) and this character has been retained in cultivated plants. Bot. Mag., t. 9217.
This rhododendron, better known as R. didymum, is the most distinct of its group. The colour of the flowers is rather funereal unless lit up by a gleam of sunshine. It is hardy, and flowers very late, in June or even July. Among its hybrids are 'Carmen', 'Arthur Osborn', and the Red Cap grex. These and others are described in the section on hybrids.
subsp. haemaleum (Balf. f. & Forr.) Cowan R. haemaleum Balf. f. & Forr. – Flowers blackish crimson, otherwise not differing from typical R. sanguineum. Bot. Mag., t. 9263. N.W. Yunnan.
subsp. himertum (Balf. f. & Forr.) Cowan R. himertum Balf. f. & Forr.; R. nebrites Balf. f. & Forr.; R. poliopeplum Balf. f. & Forr. – Flowers in some shade of yellow. Ovary tomentose, not glandular. Foliage bud-scales deciduous. N.W. Yunnan. Differs from R. citriniflorum in having leaves with a thin, plastered indumentum, as in all the subspecies of R. sanguineum. In R. citriniflorum the indumentum is thick and woolly.
subsp. leucopetalum (Balf. f. & Forr.) Cowan R. leucopetalum Balf. f. & Forr.; R. cloiophorum subsp. leucopetalum (Balf. f. & Forr.) Tagg – Flowers white. Found by Forrest on the Mekong-Salween divide and introduced by him (F.14270).
subsp. melleum Cowan – Flowers yellow as in subsp. himertum, but ovary tomentose and glandular.
subsp. mesaeum Cowan R. mesaeum Balf. f. MS – Sir Isaac Bayley Balfour gave the unpublished name R. mesaeum to a rhododendron found by Forrest on the Salween-Kiuchiang divide, 2 ft high, with black-crimson flowers. According to Dr Cowan it has the ovary tomentose but not glandular, and represents an eglandular expression of subsp. didymum, but it is also very near to subsp. haemaleum, and the seeds that Forrest collected under the type-number and later numbers were in fact distributed as R. haemaleum, but this subspecies is typically taller-growing and has deciduous bud-scales (persistent in subsp. mesaeum).
subsp. roseotinctum (Balf. f. & Forr.) Cowan R. roseotinctum Balf. f. & Forr.; R. mannophorum Balf. f. & Forr.; R. torquatum Balf. f. & Farrer – Near to subsp. cloiophorum, but with the ovary both tomentose and glandular. N.W. Yunnan, bordering parts of S.E. Tibet and upper Burma. The colour of the flowers is typically creamy white edged with deep rose-crimson, but also in various combinations of pink and yellow.
From the Supplement (Vol. V)
Dr Chamberlain provides a simpler treatment than did Dr Cowan, and one better adapted to use in gardens. R. sanguineum is subdivided into two subspecies (in the modern sense of that category, which has a higher rank than the 'subspecies' of Dr Cowan's paper). These are subsp. sanguineum, subdivided into botanical varieties; and subsp. didymum:
subsp. sanguineum – Bud-scales usually deciduous. Ovary usually tomentose, not glandular. Under this come:
var. sanguineum – R. sanguineum in the narrow sense, with bright crimson flowers.
var. haemaleum (Balf.f. & Forr.) Chamberlain – Flowers blackish crimson. This includes R.s. subsp. mesaeum Cowan.
var. himertum (Balf.f. & Forr.) Chamberlain – Flowers yellow.
var. cloiophorum (Balf.f. & Forr.) Chamberlain – Flowers varying in colour from white to pink or yellow flushed with pink. This includes R.s. subsp. leucopetalum (Balf.f. & Forr.) Cowan.
var. didymoides Tagg & Forr. – Flowers as in the preceding, but resembling subsp. didymum in having a partly glandular ovary and peristent bud-scales. This includes R. s. subsp. consanguineum Cowan and R. s. subsp. roseotinctum (Balf.f. & Forr.) Cowan.
subsp. didymum (Balf.f. & Forr.) Cowan – See page 762. The characters mentioned, in combination, distinguish this from subsp. sanguineum. Dr Cowan's subsp. atrorubrum is intermediate between subsp. didymum and var. haemaleum of the typical subspecies.
For a note on the taxonomic difficulties presented by R. sanguineum, see Rev. 2, pp.399-400.