Rhododendron komiyamae and Its Alliance With
John L. Creech, Director, U.S. National Arboretum
Agricultural Research Service, U.S. Department of Agriculture
A comparatively local azalea species, Rhododendron komiyamae, is described with particular reference to its habitat in Japan. Its relationship to R. kaempferi relative to naturally occurring hybrid populations between these two species is discussed.
Rhododendron komiyamae is an obscure azalea restricted to Mt. Ashitaka (1,505 m.), a volcanic cone in the Fiji-Hakone mountain range of central Honshu, Japan. This azalea species was described by Makino in 1926(1) Japanese botanists were probably acquainted with this azalea much earlier and Makino had classified an azalea from this locality as Rhododendron indicum var. mikawanum in 1909.(2) E. H. Wilson treated the Ashitaka azalea as a new combination, Rhododendron obtusum var. kaempferi f. mikawanum, arguing that the increase in number of stamens from 5 to 10 "is another instance of the extraordinary range of variation that obtains in R. obtusum.(3)"
Rhododendron komiyamae is a much-branched shrub, growing to 2 meters. The leaves are persistent to the same degree as R. kaempferi and clothed with appressed brown hairs on the upper and lower surfaces. The flowers of R. komiyamae appear before the leaves, or nearly so, and are borne in terminal clusters of 1 to 4. The corolla is reddish purple, 2 to 3.5 cm. across. The stamens of R. komiyamae number 10 and this is the chief characteristic that distinguishes R. komiyamae from R. kaempferi. The latter species has 5 stamens.
At first, one might attribute nothing unusual about R. komiyamae except that it is a new entity. Next to R. kaempferi, it is the most northern of the large evergreen azaleas of Japan. From this standpoint, R. komiyamae should be a most useful parent for breeding purposes.
The hybrid populations at Ashitaka are a significant example of the manner in which the widely distributed R. kaempferi hybridizes naturally with local species. R. kaempferi occurs from southern Kyushu to central Hokkaido, a distribution which far exceeds that of any other azalea of Japan. In doing so, R. kaempferi hybridizes and produces hybrid populations with local species encountered. This occurs at Mt. Kirishima in Kyushu where there are hybrid populations between R. kaempferi and R. kiusianum.
The same situation occurs where R. kaempferi and R. macrosepalum grow together in colonies near Ikeda and Miyakoda, Honshu. This hybrid is called R. x tectum.
At Mt. Ashitaka, it is possible to observe a similar relationship between R. kaempferi and komiyamae. R.kaempferi occurs near Jurigi village, up to 900 meters elevation. R. komiyamae occurs at the summit of Mt. Ashitaka (Eichizen-dake) at 1500 meters. Between the two localities and particularly on moist mossy hummocks where seedlings have a better chance to survive, a bewildering array of seedlings can be found. These range in flower color from pink to purple and have between 5 and 10 stamens. These are natural hybrid populations between R. kaempferi and R. komiyamae and probably the entity which Wilson chose to call R. obtusum var. kaempferi f. mikawanum.
For those who would like to visit this interesting habitat, it can be reached easily from Tokyo. The express bus takes one from Tokyo around Mt. Fuji to Gotemba in about 3 hours. A one-half hour car trip or taxi will bring the visitor to the television signal station at the foot of Mt. Ashitaka. The climb from 900 meters to the top of Ashitaka (1,505 m.) requires about 3 hours. The trail leads through a veritable sea of shoulder-high Sasa bamboo but this is soon reduced to knee-height at a clearing around 1,100 meters. One can obtain exciting views of Mt. Fuji from this spot. The hillsides are covered with colonies of Ilex crenata, Pieris japonica, Ligustrum japonicum, Stewartia, Rosa, and Deutzia as well as pines and Hinoki cypress. At the top, under the usual cool, foggy conditions that typify Japanese volcanic cones, R. komiyame is abundantly scattered across the summit of Mt. Ashitaka.
1. Makino, T., 1926, A contribution to the knowledge of the flora of Japan, Jour. Jap. Bot., lll, No. 5.
2. Makino, T, 1909, Rhododendron indicum var. mikawanum, In Tokyo Bot. Mag., XXIII, 251.
3. Wilson, E. H., and Alfred Rehder, 1921, A monograph of azalea, The University Press, Cambridge.