R. giganteum and R. magnificum are the best known of a group of three closely allied and doubtfully distinct species, which, if they were to be united, would have to take the name of the third member, R. protistum, which was the first of the trio to be described. It therefore seems best to bring all three together under the oldest name, but to describe R. giganteum rather than R. protistum in the narrow sense, which is little known in cultivation, but which is discussed below. The group is studied by Dr J. Cowan in Notes Roy. Bot. Gard. Edin., Vol. 21 (1955), pp. 279-88.
R. giganteum Tagg – A tree up to 80 ft high and almost 8 ft in girth at 5 ft from the ground, or a large shrub; young stems stout, grey-felted. Leaves elliptic to oblanceolate, broadest just above the middle, 5 to 14 in. long, up to 5 in. wide, mostly about three times as long as wide, obtuse or subacute at the apex, narrowed to a slightly auricled base, with twenty to twenty-four pairs of impressed lateral veins, glabrous above when mature, clad beneath with a dense buff or brown woolly indumentum (but the undersides glabrous in young plants); petiole stout 1 to 2 in. long. Inflorescence a racemose umbel of up to almost thirty flowers, opening in early spring. Calyx very small, with eight wavy teeth. Corolla elongate-campanulate, fleshy, 21⁄4 to 23⁄4 in. long, 2 to 21⁄4 in. wide, eight-lobed, deep rose-purple shading to a paler tint, with eight darker-coloured nectar-pouches at the base. Stamens sixteen, glabrous. Ovary about 3⁄8 in. long, densely coated with a fawn tomentum partly persisting on the capsule; style stout with a large discoid stigma. Bot. Mag., n.s., t. 253. (s. Grande)
R. giganteum was discovered by Forrest in the autumn of 1919 in S.W. Yunnan, near the border with Burma, some 50 miles north of Tengchung (Tengyueh), at between 9,000 and 10,000 ft. Only three trees were found, the largest (which was felled) 80 ft high, more than 40 ft in spread, with a girth of 73⁄4 ft at 5 ft. The trees were in fruit and a fair quantity of seed was gathered (F. 18458). Two years later, when on his way east from Burma in March, Forrest made a special detour to get flowering specimens, and the species was described in 1926 from the material he collected during these two visits. Further specimens and seed were collected later in the same region and as far north as 270 on the Nmai Hka-Salween divide, but some of these later findings were of no great size.
R. giganteum first flowered in 1934 with Edgar Stead of New Zealand, and two or three years later at Arduaine in Argyll. The original plant at Arduaine is 20 ft high and 25 ft wide (1966) and the truss portrayed in the Botanical Magazine is from it. R. giganteum has proved to be a very tender species, suitable only for the mildest parts and rare even there. Another Scottish garden where it thrives is Brodick in the Isle of Arran, where it is represented by Forrest's number 27730, collected in 1926 on the Shweli-Salween divide, not far from the type-locality. This form received a First Class Certificate when exhibited on February 17, 1953 (the number F. 19335 given for this plant in the present R.H.S. Handbook is erroneous, being in fact the number attached to the flowering type-specimen collected by Forrest in 1921). See further in the articles by the Duchess of Montrose and J. P. T. Boscawen in R.Y.B. 1951-2, pp. 9-10 and fig. 2; and by J. Basford in R.C.Y.B. 1966, p. 25.
R. magnificum Ward – This species was discovered by Kingdon Ward in 1931 in the Adung valley, a western feeder of the Nmai Hka in the far northwestern corner of Burma. He described it in his field-note as follows: 'A big tree, up to 50 ft high, and 6 ft in girth towards the base; may bear flowers as a shrub 10 ft high. Leaves with a coating of snow-white felted hairs beneath, which slough off later; on young plants, often 18 ins. long by 9 ins. wide, on maturer trees 12 ins. long by 6 ins. wide. Trusses hemispherical, about 30-flowered, corolla 3 ins. long, darker or lighter rose-purple, with 8 dusky purple honey pockets at the base, but no flash, the colour variable, but uniform for each tree; some have almost crimson flowers. Blooms from mid-February to the end of March, according to altitude. Never gregarious like R. sinogrande. . . . Common along the river bank in mixed forest between 6,000 and 7,000 ft, where the finest specimens were seen. A tree by the river, 50-60 ft high, carried over 500 trusses, and in mid-February was a gorgeous spectacle.'
Kingdon Ward collected seed under his number 9200 and in 1935 described this rhododendron as a new species. As it happens, he had found something similar five years earlier in the neighbouring Seinghku valley, growing with R. sinogrande, but the plants apparently made no impression on him at the time, probably because he arrived too late to see them in flower. He collected seed under KW 6782, which was distributed under the name 'R. giganteum var.', this being the identity given in his field-notes. But his herbarium specimens were identified as R. sinogrande, and consequently the plants raised from KW 6782 have been grown either as 'Ward's giganteum' or as R. sinogrande. However, Dr Cowan considered that these plants are R. magnificum. KW 6782 is represented at Brodick Castle, where it flowers regularly. For further details see the articles in the Year Book cited above.
As to the botanical characters of R. magnificum, the difference between it and R. protistum would seem to be the rather denser indumentum and the colour of the flowers (see below); from R. giganteum, in its mature state, it differs in the thinner, cobwebby indumentum, but we cannot be sure that the type trees in the Adung valley did not, out of reach, bear leaves with an indumentum similar to that of R. giganteum.
R. protistum (in the narrow sense) was discovered by Forrest in 1918 as a shrub 20 to 30 ft high, growing at 13,000 ft on the Mekong-Salween divide in latitude 28° N. Between 1919 and 1925 he sent at least six batches of seed, gathered on the Salween-Irrawaddy divide between 26° 20′ and 28° 24′ at altitudes of up to 14,000 ft, but no plant under its original field-number has been traced in Britain – at least none is mentioned by Cowan. Plants grown under the name R. protistum without collector's number could as well have been raised from Kingdon Ward seed (see below). Young grafted plants of R. protistum F.24775, received from the USA in 1968, are growing at Wakehurst Place, Sussex.
Essentially, R. protistum is the same as R. giganteum, differing in the undersides of the leaves being dull green and glabrous, or coated with at most a thin veil of hairs. It may simply be a state of the species in which the glabrous, juvenile foliage is slow to give way to the adult type seen in typical R. giganteum. In the type from the Mekong-Salween divide the flowers are more campanulate than in R. giganteum and creamy white flushed with rose, but all Forrest's later collections are of foliage and fruit only, so little is known about its flowers, and nothing at all about those borne by the plants from which Forrest collected seed.
Among other introductions of the R. protistum complex the most interesting are KW 21498 and 21602, collected by Kingdon Ward in The Triangle, N. Burma, during his last expedition (1953). These have not yet flowered. The wild specimen under 21498 appears to be near to R. protistum, while 21602, with rather broad leaves covered beneath with a fluffy, buff indumentum, suggests R. giganteum. Others are: KW 7427 from the Seinghku valley (where R. magnificum KW 6482 was collected); KW 7642 (from the Di Chu valley, Tibet, distributed as R. sinogrande, but R. magnificum according to Cowan); and KW 8069 (from the Delei valley, Assam, distributed as R. protistum aff.). KW 13681, from the Poshing La, Assam Himalaya, is cultivated at Nymans in Sussex as R. magnificum, but the identity of the plants is uncertain.
From the Supplement (Vol. V)
var. giganteum (Tagg) Chamberlain R. giganteum Tagg – See page 748. Leaves of mature plants with a continuous indumentum (but glabrous on juvenile plants). R. magnificum (page 748) is recognised by Dr Chamberlain as a distinct though related species, known only from the Kingdon Ward gatherings (Rev. 2, p. 250).