An evergreen shrub of variable habit in the wild, usually not more than 6 ft high in this country, and of compact habit; young branches thinly white- tomentose at first, becoming glabrous. Leaves elliptic, oblong-elliptic, or lanceolate-elliptic, obtuse or bluntly acute at the apex, sometimes bluntly and broadly cuspidate, base cuneate, commonly 3 to 5 in. long and 7⁄8 to 13⁄4 in. wide, sometimes up to 7 in. long and 2 in. wide, dark green or olive-green and glabrous above when mature, covered beneath with a suede-like tomentum which is whitish at first, becoming fawn to rusty or brownish at length; petioles stout, 1⁄2 to 3⁄4 in. long. Inflorescence terminal, with up to fifteen flowers (more in some cultivated plants); pedicels more or less tomentose, up to 1 in. or slightly more long. Calyx reduced to a mere rim, or very short and bluntly lobed. Corolla between funnel-shaped and campanulate, 11⁄2 to 2 in. wide, usually either five-lobed (in typical R. degronianum) or seven-lobed (in var. heptamerum), but sometimes six- or eight-lobed, pale rose-pink or sometimes white, unmarked or with reddish spots. Stamens ten (fourteen in var. heptamerum), filaments white, anthers yellow. Ovary densely rusty tomentose; style white, just overtopping the stamens. Fruits cyhndric to ovoid, up to 1 in. long, dark brown, with remains of the tomentum persisting. Bot. Mag., t. 8403 (the typical five-lobed form). (s. Ponticum ss. Caucasicum)
Native of central and southern Japan at altitudes of up to 6,000 ft. The typical, five-lobed variety was in cultivation in Britain by 1894; Nicholson gives 1870 for the introduction of “R. metternichii”, i.e., the seven-lobed variety, but Fortune may have sent it from Japan in 1862 to Standish’s nursery. R. degronianum is a perfectly hardy species, but not much grown in this country. It is a variable species, not only in the number of lobes and other parts of the flower, but also in colour, in the form of the truss, in density of indumentum and in habit. In the 1930s K. Wada distributed ‘Wada’s Form’, described in his catalogue as having a dense, cinnamon-brown, woolly tomentum, as in R. mallotum, and ‘Metternianum’, said to be a dwarf grower with five- or six-lobed flowers. Dwarf, compact forms are found on some of the islands off the inner coast of the main island of Japan. Of these Wada listed ‘Sadoense’, from Sado Island (38° N.); a form from Oki Island, farther to the south-west, is now available in commerce.
The variations of this species in the wild have been studied by Frank Doleshy, who has published interesting reports in the Quarterly Bulletin of the American Rhododendron Society (Vol. 22 (1968), pp. 145-59; Vol. 24 (1970), pp. 68-79; Vol. 26 (1972), pp. 193-5).
f. angustifolium (Makino) Sealy, comb. nov. R. metternichii f. angustifolium Makino; R. stenophyllum Makino, not Hook. f.; R. metternichii var. angustifolium Bean; R. makinoi Tagg – Leaves strikingly narrow and sometimes sickle-shaped, up to 6 in. or slightly more long, but not more than 3⁄4 in. wide; the midrib is often deeply impressed above, and the margins are usually revolute. The taxonomic status of this variant is controversial. It is said to be confined to a restricted area in central Japan and is usually regarded as a distinct species – R. makinoi. Its specific status was upheld by the Edinburgh botanist H. F. Tagg, who pointed out that plants at Edinburgh differed from R. degronianum not only in the narrowness of the leaves, but also in flowering later, and not making their new growth until September. But Nitzelius, in the paper already referred to, points out that some cultivated plants agreeing with R. makinoi make their growth at the normal season, while some plants of R. degronianum flush late. Also that a plant agreeing with R. makinoi was raised in the Göteborg Botanic Garden from seeds collected on Mt Hito, Kyushu, far to the south of the area to which R. makinoi is supposed to be confined.
var. heptamerum (Maxim.) Sealy, comb. nov. R. metternichii Sieb. & Zucc., nom. illegit.; Hymenanthes japonica Bl.; R. metternichii var. heptamerum Maxim.; R. hymenanthes Makino var. heptamerum (Maxim.) Makino; R. japonicum (Bl.) Schneid. (1912), not (A. Gr.) Suringar (1908) – Corolla seven-lobed; stamens fourteen; ovary seven- or eight-chambered. From the limited number of specimens available for study, it is impossible to decide whether the heptamerous flower of this variety is correlated with other characters. But T. Nitzelius, who examined a wide range of material and carried out field-studies in Japan, concluded that there was no other difference between the five-lobed and seven-lobed plants, and refused to recognise them as even varietally distinct, pointing out that several Chinese species vary in the number of corolla-lobes (Act. Hort. Gotoburg., Vol. 24 (1961), pp. 159-67). It certainly seems doubtful whether the species can be usefully categorised into five- or seven-lobed varieties, since some plants are intermediate in this respect, and eight-lobed forms are known.
R. yakushimanum Nakai R. metternichii var. yakushimanum (Nakai) Ohwi – Yakushima, where this rhododendron occurs, is a small island lying at the southern end of the Japanese archipelago, some 90 miles south of Kagoshima in Kyushu. The island, best known as the home of the conifer Cryptomeria japonica, has several peaks rising above the tree-line, the highest of which is Mt Miyanoura, some 6,5000 ft above sea-level. On these mountains the R. degronianum complex occurs in several forms. Some are tall-growing (up to 25 ft high) and grow in the forests; at the other extreme are forms from exposed places, which are dense, low-growing or even prostrate, with narrow, strongly revolute leaves. Owing to the inadequacy of Nakai’s original account of R. yakushimanum (1921), and the lack of a type specimen, it is impossible to know for certain what he regarded as the typical state of his species, nor the location of the plants on which his description is based. But from his diagnosis, and from a later account (in Nakai and Koidzumi, Trees and Shrubs Indigenous in Japan, rev. ed. (1927)), typical R. yakushimanum has short leaves (up to 23⁄4 in. long according to the original diagnosis), with revolute margins, and differs from R. degronianum, which too has dwarf forms, in being more densely hairy in all its parts. This is not much on which to found a species, but it is convenient to retain R. yakushimanum as such, at least in garden nomenclature.
The following articles contain interesting information on R. yakushimanum in the wild: R.C.Y.B. 1961, pp. 52-8, by A. F. Serbin on plants growing on Mt Hanano-Ego, reprinted from Qtly Bull. Amer. Rhod. Soc., where there is a later article by the same author in Vol. 19 (1965), pp. 75-82, and another by Frank Doleshy in Vol. 20 (1966), pp. 79-85; Gard. Chron., Vol. 156 (1964), pp. 112, 115, by K. Wada; Intern. Dendrol. Soc. Yr Bk 1972, pp. 28-9, by R. de Belder, with a striking photograph of R. yakushimanum on Mt Miyanoura. For the taxonomy of this species see: J. M. Cowan, Journ. R.H.S., Vol. 72 (1947), pp. 391-3, and T. Nitzelius, Act. Hort. Gotoburg., Vol. 24 (1961), pp. 158-9,162, 166-7.
R. yakushimanum owes its fame in Britain to two plants received by Lionel de Rothschild at Exbury from K. Wada’s Hakoneya Nurseries, Numazushi, Japan, in 1934 (1936?). One of these, moved to the Royal Horticultural Society Garden at Wisley, was exhibited at the Chelsea Show in 1947 and received a First Class Certificate. Ever since, the Wisley clone – now named ‘Koichiro Wada’ – has been one of the most admired of rhododendrons. It is of very dense habit, with dark green doubly convex leaves about 3 in. long, thickly tomentose beneath (the indumentum at first white, becoming brown) and felted with white above when young; flowers twelve in a dense truss; corolla campanulate, about 21⁄4 in. wide, of an exquisite apple-blossom pink in bud, ageing to pure white with faint speckling. In 1947 the original plant was 21⁄2 ft high and 31⁄2 ft across; its present dimensions (1975) are 4 ft 8 in. by 7 ft. The Exbury clone is very similar to ‘Koichiro Wada’ and scarcely less beautiful. It is a trifle more erect-growing, with rather smaller leaves, and there is less spotting in the flower. Both are perfectly hardy, flower towards the end of May, and will stand full sun. Other plants are in cultivation which were imported as seedlings collected in the wild.
During the past twenty years, R. yakushimanum (in particular the clone ‘Koichiro Wada’) has been more used in hybridising than any other species, except R. griersonianum. See further on p. 826.