A Distribution Note on Rhododendron tamurae
John L. Creech, Director, U.S. National Arboretum
Agricultural Research Service, U.S. Department of Agriculture
Rhododendron tamurae Masamune is an important azalea native to Yakushima, Japan. Its origins and local habitat are described. Relationships to other azalea species, namely R. indicum and R. simsii, are questioned. The value of R. tamurae for future breeding programs is considered.
Rhododendron tamurae Masamune is a rare azalea, both in the wild and in cultivation. It is better known by its synonym, R. eriocarpum. Even under that name, however, this most important azalea is rarely cultivated. Rather, it is known through the popular Satsuki azalea cultivars of which 'Gumpo' is the most widely available.(1) 'Gumpo' is reportedly a hybrid between R. tamurae and R. indicum. The Satsuki azaleas have a most promising future in the United States because of a low plant habit and the large flowers that appear late in the season of azaleas.
So often, we have developed our azalea races by improving on existing Japanese cultivars. There are very few, if any, successful American races of evergreen azaleas that have been produced as the result of using selected species as parents. While this is a perfectly good practice, it often impedes our knowledge of the breeding behavior of azalea species. We can only surmise, for example, as to the origins of the Kurume and Satsuki azaleas. Yet these have been basic breeding stocks for many recent American azalea cultivars.
The National Arboretum has established additional elements of R. tamurae in cultivation as a result of a 1976 plant-collecting expedition to Japan. Although I had visited the general habitat in southern Japan in 1956, I did not collect this species. Rather, I concentrated on collecting another important azalea, R. indicum.
Both R. tamurae and R. indicum are native to Yakushima, that famous island paradise for plant hunters about 70 km. south of Kagoshima, Kyushu. I suspect that these azaleas are to be found on the several adjacent islands in the East China Sea, but these are small and uninhabited places. Wilson(2) described R. eriocarpum from the Tokara island group by specimens presented to him by Japanese forestry officers. He apparently did not know of the presence of R. eriocarpum on Yakushima. This can be readily understood because of its isolated locality on Yakushima.
Yakushima is remarkable in its climate as compared with the rest of the Kyushu island group. It has a mean annual precipitation of 3,750 mm., and it typically rains 200 days per year. The low-land is frost-free but the highest peak, Miyanoura dake (1,935 m. el.), is often snow-covered and frozen in winter. The peaks on this island rise abruptly from the ocean, so the one main road circles the island near the shore line and from this, logging roads wind part-way up the slopes, perhaps to 1,000 meters. From there on, travel is either by foot or, occasionally, a logging train.
R. tamurae (R. eriocarpum) occurs in a most unfavorable habitat for an azalea. I have been accustomed to finding azaleas in cool, moist habitats, on the slopes of volcanic cones, along granite strewn mountain streams, and in sphagnum bogs. I was totally unprepared for the unique habitat of R. tamurae.
R. tamurae grows at sea level at Nagata on the northwestern coast of the island. Beyond Nagata Light, the land drops into the ocean by means of sloping walls of granite. The vegetation is subjected to constant sea-winds, bright light, and a hot, often humid climate. It seemed a harsh, uninviting habitat for azaleas. They grow in shallow gravelly soil on the edge of trails winding down to the slabs of granite. In one place, plants of R. tamurae have secured a foothold in fissures in the rock, along with tufts of Zoysia grass. Companion plants on the ledges include Buddleia, Callicarpa, Elaeagnus, Eurya, Hibiscus, and Hydrangea.
Under these conditions, R. tamurae grows to about 1 meter high. The leaves are broadly elliptic in the spring, followed by thick shiny obovate ones in the summer, and these persist through the winter. The flowers are red to purple, with 8-10 stamens. The plants we saw at Nagata in November had a few purple flowers.
It is important to recognize that R. tamurae in no way can be related to R. indicum. The latter species is a mountain plant with narrow elliptic leaves throughout the year. The flowers are pink to red with 5 stamens. Some botanists have considered R. tamurae a variety of R. simsii, a Chinese plant, but that seems equally unlikely because of the geographic isolation and the difference in vegetative characters and habitat. As for Wilson's R. simsii var. eriocarpum, he most likely saw only the three specimens presented to him and appeared uncertain as to what they really represented. He did feel that the plant in question might be a distinct species.
R. tamurae is an important link to the origin of the Satsuki azaleas. It also offers opportunities for new directions in azalea breeding when used in combination with other species. Plants are being raised from seeds and cuttings at the U.S. National Arboretum, Washington, D.C.
1. Lee, F. P., 1965, The azalea book (second edition), D. Van Nostrand Co., New York.
2. Wilson, E. H. and A. Rehder, 1921, A monograph of azalea, The University Press, Cambridge.