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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Rhododendron japonicum' from the website Trees and Shrubs Online (treesandshrubsonline.
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RHS Hardiness Rating H5
A deciduous bush, of rounded habit, 4 to 8 ft high, with stiff, erect branches; young shoots sparsely hairy. Leaves narrowly oval or obovate, 2 to 4 in. long, 3⁄4 to 11⁄4 in. wide; dark green and sparsely hairy above, more or less glaucous and slightly hairy beneath when young, especially on the midrib and margins. Flowers six to ten in a cluster, produced during May on the end of leafless shoots, the corolla being of various shades of soft rose, salmon-red, and orange-red, 21⁄2 to 31⁄2 in. wide, the lobes oblong; calyx-lobes oblong to linear-oblong, as long as the ovary, conspicuously edged with long whitish hairs; flower-stalk 1⁄2 to 1 in. long, hairy. Bot. Mag., t. 5905 (s. Azalea ss. Luteum)
Native of the main island of Japan and of Hokkaido, in open grassland and scrub, never in woods or dense thickets (Wilson); introduced to the Continent in 183o(?) and to Britain in the 1860s. It was taken up by the Ghent nurserymen, who used to raise it in large quantities from seed, mainly for forcing, and in that way many colour variants came to be selected, some of which come true from seed if self-pollinated. See further in the section on hybrids, under Mollis Azaleas, under which it is customary to group the named selections of R. japonicum as well as the hybrids between it and the closely allied R. molle (sinense), q.v.
This member of the Azalea branch of Rhododendron at present lacks a valid name at the species level, but this hardly matters, since it is doubtfully distinct enough from its Chinese counterpart R. molle to need one. Considered as a variety of that species, it would become R. molle var. glabrius Miq.
R. japonicum (Bl.) Schneid. var. japonicum Hymenanthes japonica BL; R. metternichii Sieb. & Zucc., nom. illegit.; R. degronianum var. heptamerum (Maxim.) Sealy; R. metternichii var. heptamerum Maxim. – This is the typical heptamerous state of the species.
var. pentamerum (Maxim.) Hutch. R. metternichii var. pentamerum Maxim.; R. degronianum Carr. – Corollas five-lobed.
See R. degronianum, page 644; and in this supplement.
It should be noted that the type of R. japonicum (Bl.) Schneid. is the heptamerous form, so it is the pentamerous form that has to be distinguished, whereas when the name R. degroninum is used it is the other way about.