An evergreen or semi-evergreen shrub up to 10 ft high; young twigs covered with appressed, forward-pointing bristles. Leaves of two forms: spring leaves (i.e., those on the lower part of the fully developed shoot) lanceolate, ovate, or elliptic, up to 2 in. long, 3⁄8 to 11⁄8 in. wide, mostly deciduous, at least in cold localities; summer leaves (i.e., those formed later in the season and situated nearer to the terminal bud) smaller and elliptic or obovate, mostly persisting through the winter; both types glossy green above, paler beneath, with scattered bristly hairs on both surfaces. Flowers produced about mid-May (later in some forms), two to four in terminal clusters on silky-hairy stalks about 1⁄4 in. long. Calyx-lobes five, narrowly ovate or elliptic, hairy outside and at the margins. Corolla funnel-shaped, five-lobed, 11⁄2 to 2 in. across, coloured some shade of red, with faint purplish or crimson spotting on the upper lobes. Stamens five, hairy in the lower part, anthers yellow or sometimes purple. Style glabrous. (s. Azalea ss. Obtusum)
Native of Japan, where it is widespread. According to Wilson it is a sun-loving plant, 'seen to best advantage in open thickets on mountain slopes'. It was introduced to the United States by Prof. Sargent in 1892, thence to Kew two years later, but there may have been an earlier introduction by Charles Maries to Messrs Veitch in 1878.
R. kaempferi, at least in the forms commonly cultivated, is a perfectly hardy species. It varies greatly in flower-colour; some plants have flowers in hard shades of orange-scarlet; others, with some pink in the colouring, are very beautiful. Always the colour tends to fade quickly in the sun, so light shade is desirable. Although the normal flowering-time is May, some clones flower later but appear nevertheless to be pure R. kaempferi, not hybrids of the naturally late-flowering R. indicum. Two such clones bear the names 'Daimio' and 'Mikado'.
A clone named 'Eastern Fire' was awarded a First Class Certificate on May 24, 1955, when exhibited by the Crown Estate Commissioners, Windsor Great Park, and had received an Award of Merit two years earlier. The flowers are described as Camellia Rose, darkening towards the tips of the lobes.
R. kaempferi is a parent of many of the hardiest evergreen azalea hybrids.
R. kaempferi f. multicolor Wils. and f. mikawanum (Makino) Wils. are considered by Hara to represent for the most part hybrids between R. kaempferi and R. macrosepalum (H. Hara, R.Y.B. 1948, p. 120).
R. 'Obtusum'. – The plant named Azalea obtusa by Lindley was introduced by Fortune from the Poushan nursery, Shanghai, China, in 1844, and is figured in Bot. Reg., Vol. 32 (1846), t. 37. There is no doubt that the original stock came from Japan, where according to Wilson the same azalea is much cultivated around Tokyo and Yokohama. There was a later introduction by Veitch from Japan, which differed slightly from Fortune's plant, but curiously enough neither seems to be in cultivation in Britain at the present time. From Lindley's figure and description it is evident that 'Obtusum' is a hybrid of R. kaempferi. The small flowers, described by Lindley as glowing red, have oval, acute, widely spaced lobes, the upper one spotted; the summer (persistent) leaves are obovate, obtuse, dark green, the spring leaves bright green, elliptic-lanceolate, acute. The small flowers and dense habit suggest that the other parent was R. kiusianum, and that, like 'Amoenum' and 'Ramentaceum' (qq.v. below), it came originally from the mountains of Kyushu. 'Obtusum', then, is simply one of the many garden clones deriving mainly or wholly from R. kiusianum or from natural hybrids between it and R. kaempferi. Wilson's view, however, was that R. kaempferi and R. kiusianum were states of one and the same variable species, and that plants such as typical R. obtusum were linking forms, uniting the two extremes, which he admitted were very distinct (Rehder and Wilson, Monograph of Azaleas (1921), p. 40). For this polymorphic “species” he therefore adopted the name R. obtusum (Lindl.) Planch, and under it he placed R. kaempferi as a variety, while R. kiusianum appears as a synonym of R. obtusum f. japonicum (Maxim.) Wils. He failed to notice that the name R. obtusum (Lindl.) Planch, is illegitimate, since it is long antedated by R. obtusum Wats., used in 1825 for some form or hybrid of R. ponticum. However, the illegitimacy of the name is really a minor consideration, of importance only to a botanist who considers that Wilson's taxonomic treatment of the group is correct. If the type of R. obtusum (Lindl.) Planch, is a hybrid between two distinct species, as is now generally accepted, the name, even if legitimate, could be used only for other hybrids of the same parentage.
The botanical synonymy of R. 'Obtusum' is: R. obtusum (Lindl.) Planch. (1854), nom. illegit., not Wats. (1825); Azalea obtusa Lindl.; R. indicum var. obtusum (Lindl.) Maxim.
R. kiusianum Makino R. indicum var. amoenum f. japonicum Maxim.; R. indicum var. japonicum (Maxim.) Makino; R. obtusum f. japonicum (Maxim.) Wils.; R. kaempferi var. japonicum (Maxim.) Rehd.; R. amoenum var. japonicum (Maxim.) Bean – Although in its essential characters this species is quite near to R. kaempferi it is very distinct in general aspect. It is a low, dense, sometimes prostrate shrub, usually under 3 ft high in the wild. The spring leaves are variable in shape, mostly broad-elliptic to obovate, obtuse and up to about 3⁄4 in. long, glossier and darker than those of R. kaempferi (at least on most cultivated plants); the summer leaves are obovate, oblanceolate, or elliptic, mostly obtuse, and at least the uppermost ones are persistent through the winter; all the leaves are covered with appressed hairs. Flowers in twos or threes. Calyx well developed, with ciliate lobes. Corolla about 1 in. wide, varying in colour from mauve to light or dark magenta-pink, sometimes white, usually unspeckled (at least in cultivated plants), with a short tube and spreading limb. Stamens five, as in R. kaempferi.
R. kiusianum is an endemic of Kyushu, the southernmost of the four main islands of Japan, where it occurs on the mountains and volcanoes in open places above 2,500 ft. It was first described in 1870 by the Russian botanist Maximowicz, as R. indicum var. amoenum f. japonicum, from specimens collected on Mt Unzen in central Kyushu. The Japanese botanist Makino raised the plants to the rank of variety – R. indicum var. japonicum (Maxim.) Makino – and in 1914 gave them specific rank, but had to find a new epithet, since the name R. japonicum had been applied to two other species. Thus R. kiusianum and R. obtusum f. japonicum (Maxim.) Wils. are both typified by the Unzen specimens and Maximowicz's description.
R. kiusianum was introduced from the wild by Wilson, who collected seed in 1918 on Nishi-Kirishima and sent it to the Arnold Arboretum. The seed was widely distributed, and the older plantings in Britain derive from it, but there have been other importations of seed and plants since then. The Kyushu azalea is not much planted in gardens but deserves to be more common, as it is hardy in most forms and the flowers, though small, are borne abundantly in May or early June. Owing to its dense habit it is an excellent weed-smotherer and would not be out-of-place in the heather garden.
R. 'Amoenum'. – A shrub of dense habit, building up tiers of branches and eventually 5 or 6 ft high (occasionally even taller), more in width. Spring leaves rich green and rather thick, elliptic or elliptic-obovate, persistent on strong shoots, summer leaves smaller, obovate to spathulate, both sorts clad with appressed hairs on both surfaces, but especially above and on the margins. Flowers in twos or threes, borne in May. Corolla funnel-shaped, rosy purple, 3⁄4 in. wide. Calyx-lobes narrow-oblong, 3⁄8 in. long, resembling the corolla in shape and colour, and giving the appearance of one flower growing out of another – a character known to gardeners as 'hose-in-hose'. Bot. Mag., t. 4728.
R. 'Amoenum' was introduced by Fortune in 1850 from the Poushan nursery near Shanghai, China, for Standish, and Noble. Like 'Obtusum', which he had found in the same nursery six years earlier, it is of Japanese origin; and is not specifically distinct from R. kiusianum. The botanical synonymy is: R. amoenum (Lindl.) Planch.; Azalea amoena Lindl. (1852); R. indicum var. amoenum d. genuinum Maxim.; R. obtusum var. amoenum Wils. It will be seen that the name R. amoenum has long priority over R. kiusianum, which could take the name R. amoenum var. japonicum (Maxim.) Bean. The only reason for retaining the name R. kiusianum in the present revision is that there is a still earlier name to be considered (see 'Ramentaceum').
Although long treated as a greenhouse shrub, 'Amoenum' is quite hardy and a very pleasing evergreen, flowering most profusely and at all times a neat shrub. It grows slowly, and may be only 3 ft high after twenty years, though more than twice that in width. This is a valuable characteristic in many positions, especially where a permanently low evergreen mass is desired without the trouble and perhaps unsightliness of a periodical cropping. There is a pink-flowered branch-sport 'Amoenum Coccineum', which is apt to revert to the normal form.
R. 'Ramentaceum' ('Obtusum Album'). – This azalea was introduced by Fortune from Hong Kong in 1846 and described by Lindley in 1849. It never became established in this country but, in common with many other azaleas grown in Britain in the 1840s and 1850s, it found its way to the USA, and was cultivated in the Holm Lea Collection at Brookline. It was reintroduced to Britain early this century but apparently again lost. From Lindley's original description and figure, and from herbarium specimens of cultivated plants, 'Ramentaceum' would seem to be an azalea similar to those members of the Kurume group that have white or pale pink flowers. The corolla is white, 11⁄4 in. long and wide, and the spring leaves are broad-elliptic to roundish, up to 11⁄2 in. long. Lindley's epithet ramentacea refers to the chaffy, awl-shaped hairs to be seen on the stems of R. kiusianum, R. kaempferi, etc. The fact that the original introduction was from Hong Kong need not be a cause for surprise, since there is abundant evidence for the cultivation of Japanese azaleas in the coastal cities of China. Wilson considered 'Ramentaceum' to be no more than a white-flowered, large-leaved form of R. obtusum f. japonicum, i.e. of R. kiusianum, and if he is right then R. ramentaceum would be the correct name for R. kiusianum, over which it has long priority. But this is a matter best left until the Kyushu azaleas have received the thorough taxonomic study that they deserve. The botanical synonymy of R. 'Ramentaceum' is: R. ramentaceum (Lindl.) Planch. (1854); Azalea ramentacea Lindl. (1849); R. obtusum f. album Schneid.
It should be added that R. sataense Nakai, described in 1949, seems to be near to 'Ramentaceum', judging from an authentic specimen in the Kew Herbarium, collected on Mt Kaimon, near Kagoshima in Kyushu.