More About R. yakushimanum
Much has been said about R. yakushimanum and undoubtedly there will be more to say, particularly when more of its hybrids begin to appear in the shows and in the nursery trade. Being a relatively new species, so far as botanical description and naming is concerned, its history is just now in the making. We are indebted to Mr. Leach for his articles in the January and April issues of the Bulletin, and the light he threw on the species and its use in breeding.
While we are on the subject, perhaps it would be well to review an article by Mr. K. Wada in the Gardeners Chronicle (England) August 1, 1964. In an' article headed "An Interesting Rhododendron," Mr. Wada outlines some of the background of the species which is of particular interest because it was Mr. Wada who in 1936 sent the first plants to the late Mr. Lionel de Rothschild.
R. yakushimanum is native only to the island of Yakushima which lies south of the main islands of Japan. This island is quite small, only about ten miles across, but apparently has an extremely interesting flora. The climate ranges from sub-tropical along the coast to peaks over 6,000 feet high which are covered with snow during the winter. The highest, Mr. Miyanoura, is 6,400 feet high.
Mr. Wada points out that there is considerable variation in plants of this species growing at different elevations. He groups them in three general types. As one goes up towards Miyanoura Peak the first plants of R. yakushimanum will be found at about 3,300 feet, growing among mixed forests. This form has leaves that are 5 to 6 inches long, with a thin white indumentum. It has particularly good trusses with up to 20 flowers per truss. Mature plants are as tall as 25 feet and 3 feet in circumference at breast height. Apparently there was an erroneous assumption by one Japanese botanist that this might be R. metternichii although he made no personal observations.
From about 4,000 feet up to 5,500 feet there is another form of about the same height, the leaves are about as long, but somewhat narrower than the lower altitude form, and the flowers are essentially the same. The main difference seems to be that this form has a thick brownish indumentum below and a very deep glossy green color above.
Between 5,300 feet and the summit, which is at 6,400 feet, the plants are much dwarfer, the leaves are shorter, broader, and more convex with thick indumentum, brown or tawny in color. The individual flowers are essentially the same as the others, but there are fewer to a truss which is a fault, horticulturally speaking. These plants in general are dome shaped, and these characters of size and form are retained even when the plants are propagated and grown under different cultural conditions.
It will be recalled that Dr. Serbin wrote in the Bulletin sometime ago about the different forms he found on a trip to the island of Yakushima.
The main point of Mr. Wada's article, perhaps, is his comments concerning the origin of the F.C.C. form. There is the implication, at least, that each of the three forms mentioned breed essentially true as to plant characters although the difference is presumably not enough to give them individual specific rank. The F.C.C. form, according to Mr. Wada, does not fit into either one of the three groups. It has the dwarf spreading form of the type from the higher elevation but the type of truss and color of bud of the tall growing form with the tawny indumentum which occurs part way up the mountain. Mr. Wada makes a suggestion that it may have been a natural hybrid between these two forms. The plant and flower characters would seem to bear out that hypothesis. Furthermore, according to Mr. Wada, the F.C.C. form does not come true from seed with respect to general horticultural characters. For that matter the other forms on the mountain do not "breed true" either although conforming to the general group from which they come.
It would seem from Mr. Wada's remarks that rhododendron fans desiring a particularly good form of this species are not too likely to obtain it by raising seedlings. Fortunately the F.C.C. clone roots rather easily from cuttings. The fact that it is difficult to get plants as good as the F.C.C. form from seed takes nothing away from the value of R. yakushimanum as a parent for use in crossing with other suitable species. A number of very fine forms have been grown and named, and undoubtedly there will be many more appearing as additional breeders bring their crosses to the flowering age. Truly this is a rather remarkable species because we have reasonable hardiness combined with the low, spreading habit so much in demand nowadays, and flowers which are both plentiful and beautiful.