R.yedoense is the name given by Maximowicz in 1886 to a double-flowered garden form of an azalea that grows wild in Korea. Unfortunately this azalea was not described as a species until many years later and must therefore be treated as a variety of its own cultivated offspring:
var. poukhanense (Lévl.) Nakai R. poukhanense Lévl.; R. coreanum Rehd. – A deciduous or nearly deciduous azalea 3, 4, or occasionally up to 6 ft high; young shoots clothed with appressed bristles. Leaves lanceolate, oval-lanceolate, or oblanceolate, pointed, tapered at the base to a stalk 1⁄8 to 1⁄/6 in. long, 11⁄2 to 3 in. long; both surfaces bristly, especially at the margins. Flowers fragrant, produced in April and May usually two to four together in a terminal cluster; Flower-stalk 1⁄3 in. long, bristly. Calyx five-lobed; the lobes ovate, about 1⁄4 in. long, very bristly, especially on the margins, green. Corolla rosy purple, funnel-shaped, 11⁄2 in. long, rather more wide, five-lobed, freely spotted on the upper lobes. Stamens ten, about as long as the corolla, downy on the lower third; anthers purple. Ovary bristly; style 13⁄4 in. long, usually glabrous. Bot. Mag., n.s., t. 455 (s. Azalea ss. Obtusum)
Native of Korea; introduced to the Arnold Arboretum by J. G. Jack in 1905, thence to England in 1913. It flowered at Kew in April 1914. Although now known by the above varietal name, it is a genuine wild type and according to Wilson is the common azalea of Korea from about the latitude of the capital, Seoul, southward, but is uncommon on Poukhan-san from which it derives its name. 'It is partial to open country and on grassy mountain slopes and in thin Pine-woods it forms dense matlike masses from a few inches to a yard high . . . but in thickets the plants are more loosely branched and often two metres high' (Monograph of Azaleas, p. 66). It also occurs on Daghelet Island (Quelpaert). It is perfectly hardy at Kew. The plant which provided the material figured in the Botanical Magazine is from a plant there raised from seeds collected in Korea by Mr Moorcraft in 1951, while serving with the British forces. The flowers in this form are rosy pink, but more commonly they are a vivid shade of lilac purple.
R. yedoense var. poukhanense received an Award of Merit when shown by Capt. Collingwood Ingram, Benenden, Kent, on April 11, 1961 (flower-colour described as Mauve).
The double-flowered type of the species is also known as 'Yodogawa', and was introduced to Europe from Japan in 1884.