R. arboreum attains in the wild a height of 40 ft or more and may be narrow and erect with a single stem or many-branched from the base and more spreading girths up to 15 ft at the base have been recorded. Leaves stiffly leathery, oblong-lanceolate to oblong-oblanceolate, 4 to 8 in. long, 11⁄4 to 21⁄4 in. wide, obtuse to subacute at the apex narrowed gradually or more abruptly to the base, dark green and, when mature, glabrous above, clad beneath with, in the typical state, a close indumentum composed of rosulate hairs, but in other forms, where an upper layer of dendroid hairs is present, it may be thicker and more woolly or spongy; typically it is silvery white, but more commonly brown; lateral veins about twenty on each side, they and the minor reticulations prominent, but often obscured by the indumentum; petiole 1⁄2 to 3⁄4 in. long (rarely longer). Inflorescence a dense corymb of about twenty flowers on a rachis up to about 1 in. long, borne normally in March or April; pedicels very short, about 3⁄8 in. long, downy or glandular or both. Calyx very small, with the same covering as the pedicels; lobes broadly triangular. Corolla tubular-campanulate, 13⁄4 to 2 in. long and wide, blood-red in the typical form, with darker markings on the usually indented lobes and nectar-pouches at the base. Stamens ten, glabrous. Ovary conoid, downy and often glandular also, ten-chambered. (s. and ss. Arboreum)
R. arboreum, in its typical state, appears to be confined to the Himalaya, though in slightly different forms it extends farther to the east and south. The first account of it was published in 1799 by Capt. Thomas Hardwicke, who had seen it flowering three years earlier in Kumaon, south-east of Dehra Dun. It was first formally described and named in 1804 by James Smith, using Hardwicke’s notes and drawings, and introduced to cultivation some ten years later (see further below). In the Himalaya it ranges as far west as Kashmir, where it is found only on the outer and rainier side of the mountains that border the Vale of Kashmir on the south, but is best known from Kumaon and the Himalaya of Nepal and Sikkim. The typical state of the species is, at least for the most part, confined to low elevations, from 5,000 to 8,500 ft. The natural vegetation at these altitudes is temperate forest, much of which has been destroyed by fire, but R. arboreum survives, even on hot, sunny, grass-clad slopes, or in secondary forest of oak or pine; whether it is capable of regenerating in such habitats is not clear. At higher elevations, at least in the Nepal and Sikkim Himalaya, it gives way to the following variants, usually treated as subspecies:
subsp. campbelliae (Hook, f.) Tagg R. arboreum var. roseum Lindl.; R. album Sw.; R. campbelliae Hook, f.; R. arboreum var. campbelliae (Hook, f.) Hook. f. – In typical R. arboreum the indumentum is a thin silvery felt that covers the lateral veins and minor reticulations but, these being prominent, does not actually obscure them; also the flowers are blood-red, or at least commonly so. But at altitudes above 8,000 ft the prevailing form has a thicker, usually brown indumentum and the flowers are crimson, pink, or white, sometimes pink at the edge and white at the centre. There appears to be no constant difference in shape of leaf. The altitudinal overlap between typical R. arboreum and this subspecies is considerable and the correlation between flower-colour and indumentum is far from perfect, but it remains true that the richest coloured forms do not occur much above 8,000 ft and that there is a tendency for the indumentum to become thicker and darker with altitude.
subsp. cinnamomeum (Wall. Cat.) Tagg R. cinnamomeum Wall, ex Lindl. – This resembles the subsp. campbelliae but the leaf-indumentum is cinnamon-coloured or rusty and sometimes rather loosely woolly. The flowers are typically white, with coarse spotting, but can be pink or crimson. It is apparently confined to high altitudes and its distribution outside Nepal, whence it first became known, is uncertain.
Outside the Himalaya, R. arboreum extends across the Brahmaputra into Burma and south through the Khasi Hills and other hills of Assam, Nagaland and Manipur to Mount Victoria in eastern Burma (Arakan). But the plants in these areas are mostly untypical: the indumentum is often composed of rather long, interwoven hairs and is honeycombed with holes (alveolar); the leaves are relatively short and broad in some forms from Manipur and the Khasi Hills and in some respects recall R. nilagiricum and R. zeylanicum. It is also interesting that in these eastern and southern forms the richly coloured flowers of typical R. arboreum are sometimes combined with an indumentum of the type characteristic of subsp. campbelliae. However, these variations are of no importance horticulturally. A very distinct form was introduced by Bailey and Morshead in 1914 from the Nyamjang Chu on the border between Bhutan and Assam and was named R. morsheadianum by Millais; it was reintroduced by Kingdon Ward from the same area ten years later (KW 6403). The leaves have a very close, burnished indumentum resembling that of R. insigne and straight, deeply impressed lateral veins.
The first recorded flowering of R. arboreum in cultivation was of the typical form and took place at The Grange, Alresford, Hants, in 1826. When the seed was introduced and by whom is not known, but a possible source is Francis Buchanan Hamilton, who made important botanical collections in India and is known to have sent seeds of R. arboreum to the Calcutta Botanic Garden in 1810 from near the borders of Nepal (where he had discovered the white-flowered form of the species during a visit in 1802-3). It is also recorded that he sent seeds of a red- and a white-flowered form to the Edinburgh Botanic Garden in 1820. Neither date agrees well with a first flowering in 1826, the first being too early and the latter too late. Another possibility is that the seeds were collected during the military expedition to Nepal in 1814-16, and is borne out by the date of introduction given by Loudon – 1817 (Prain and Bean, Rhod. Soc. Notes Vol. I, pp. 175-9; Cowan, R.Y.B.1953, pp. 38-9). Three years after the conclusion of peace with Nepal, Dr Nathaniel Wallich arrived in Katmandu and sent seeds to Britain in 1820 of the subsp. campbelliae and cinnamomeum, which were widely distributed. The rose-coloured form, sown in 1821, flowered at Knight’s nursery, Chelsea, in 1828 and was named R. arboreum var. roseum by Lindley; the first recorded flowering of the white form of subsp. campbelliae was in a garden at Chester, and of the subsp. cinnamomeum at Rollisson’s nursery, Tooting, in 1836.
Both typical R. arboreum and the subsp. campbelliae were reintroduced by Hooker in 1849-50 from the interior of Sikkim, the former from near Chungthang (Hooker’s ‘Choongtam’), at the confluence of the Lachen and Lachung rivers, and both it and the subsp. campbelliae from the Laghep valley, an eastern feeder of the Tista (Himalayan Journals, Vol. II, pp. 186, 197). The oldest extant specimens in the British Isles are probably from these seed-collections. Those at Stonefield in Argyll are remnants of the plantings made by John Campbell, a friend of the elder Hooker and a relative of Dr Campbell, the Political Resident at Darjeeling, who did more than anyone to make the younger Hooker’s Sikkim expedition a success (the subsp. campbelliae, originally described as a species, is named after his wife). The famous colony of R. arboreum at Lochinch in Wigtownshire, in which pink and white forms predominate, was planted by the 10th Earl of Stair in the early 1860s, and is also from the Hooker seed.
The typical state of the species, usually distinguished in gardens as ‘blood-red arboreum’, is not reliably hardy outside the milder parts, and is by no means common even there. South of London the best-coloured forms, though they may survive quite severe winters once established, are often cut by spring frost and rarely attain a good size. The Tregothnan form, raised from seeds collected in the Himalaya by a former Viscount Falmouth, was tried at Exbury by Lionel de Rothschild and found to be hardier there than other blood-red forms.
The subsp. campbelliae is much hardier, but, like all forms of R. arboreum it flowers dangerously early and does not reveal its full beauty until twenty or so years old. Typically the flowers are pink, but both in the wild and in cultivation they may be white at the centre and pink at the edge, or wholly white except for the spotting. Many seedlings were raised by the Cornish nurseryman Gill, either from plants at Tremough (the oldest of which were from the Hooker seed) or from seed collected in the Himalaya, and some of these he propagated and named, such as ‘Blushing Beauty’, with white, pink-tinted flowers, and ‘Mrs Henry Shilson’, a fine pink with larger flowers, fewer in the truss, than normal, and almost certainly a hybrid.
The subsp. cinnamomeum is also hardy, but slow-growing and usually more bushy than the subsp. campbelliae.
The following clones of R. arboreum have received awards: ‘Goat Fell’, from Brodick Castle Gardens, Isle of Arran, A.M. May 5, 1964; ‘Rubaiyat’, from Exbury, A.M. April 2, 1968 (both these of the blood-red type); R. arboreum ‘Tony Schilling’, from Wakehurst Place, Sussex, F.C.C. April 2, 1974 (a form of subsp. campbelliae).
R. delavayi Franch. R. pilovittatum Balf. f. & Forr. – This species was described from specimens collected in Yunnan, China, by the French missionary Delavay, and as usually understood it is a native of that province and of parts of Burma, Thailand, and the former Indo-china. How it differs from R. arboreum has never been made clear. H. F. Tagg, in The Species of Rhododendron, p. 17, says: ‘The best distinguishing feature is in the indumentum; in R. arboreum it is usually thin and more or less plastered, whereas in R. delavayi it is of a somewhat spongy texture, the surface more or less fissured.’ This is not much to base a species upon and is not even reliable. Some specimens in the Kew Herbarium, collected by Delavay and others in Yunnan, have an indumentum differing in no way from that of R. arboreum, though the leaves tend to be shorter than in that species. Other specimens from Yunnan have the ‘spongy’ type of indumentum, but this is also to be seen on some specimens from the eastern Himalaya and the Khasi Hills (and in R. nilagiricum and R. zeylanicum).
Delavay discovered this rhododendron in 1884; it was introduced to France at about the same time and thence to Kew in 1889. The first flowers seen in the British Isles were borne on a plant at Kilmacurragh, Co. Wicklow, in 1904. It was later reintroduced by Forrest from various parts of Yunnan, where it extends as far to the north-east as the Lichiang range and into the southern parts of the province. Like R. arboreum, it occurs in drier habitats than most rhododendrons, and according to Forrest is at its best on the margins of pine forests.
R. delavayi is uncommon in cultivation and tender in its best forms, which are mostly to be found in Scottish collections (Lochinch, Brodick, and Crarae). It has reached 10 ft in height at Wakehurst Place in Sussex. Bot. Mag., t. 8137.
R. peramoenum Balf. f. & Forr. – Closely allied to R. delavayi, differing in the longer, relatively narrower leaves 3 to 6 in. long, up to scarcely 1 in. wide, less rugose above, and with a closer indumentum beneath. It was discovered by Forrest on the Shweli-Salween divide, Yunnan, near the frontier with Burma and was introduced by him.