QBARS - v13n1 Additional Notes on R. yakushimanum

Additional Notes on R. yakushimanum
By Alleyne Cook, Vancouver, B.C.

I read with interest the article "An Outstanding Rhododendron" by Maurice H. Sumner in the October Bulletin. This article about R. yakushimanum as he saw it growing in England was most interesting and brings about the following point. I have yet to see anything in this country resembling R. yakushimanum as it is in England.* I think that before long a lot of incorrectly named plants are going to appear at shows and this might help to head off or at least make exhibitors think a little before exhibiting seedlings of the species. The fine plant in England is the award plant, and seedlings raised here and abroad are by no means equal to that plant. The following two articles appeared in the R. H. S. Journal 1947, and may help to bring about my point.

Rhododendron yakushimanum
By Francis Hanger, curator
Wisley Gardens

Two feet six inches high and nearly four feet through, this rhododendron carried hundreds of flowers in perfect upstanding trusses which formed a complete dome from the ground upwards. The Rhododendron Committee of the Royal Horticulture Society was unanimous in awarding the plant a First Class Certificate.

This species appears to be most rare in this country and we are indebted to the late Mr. Lionel de Rothschild for its introduction. Always keen to import any new plant, especially unknown rhododendrons, Mr. de Rothschild obtained the first plant from the late M. N. Wada of the Hakoneya Nurseries, Numazushi, Japan. Before the last war this nursery was the source of many garden treasures and M. Wada's catalogue always repaid a handsome dividend for time spent in searching its pages for new and rare oriental plants.
I well remember R. yakushimanum arrival together with many other interesting eastern novelties, including R. metternichii the true form with lovely pink seven-lobed flowers, also a native of Japan.
This first R. yakushimanum had only four small leaves when received, but although tiny, attention was immediately drawn to it because of its distinct appearance. To make certain of establishing this plant after its long journey, it was placed in a small pot and grown under glass and was later planted in the small rock garden on the north side of the then large Rhododendron House at Exbury. When last seen at Exbury about two months ago, the plant was still in the same position, growing well and had become approximately two feet high and about three feet through.
The Wisley plant is a layer from the original introduction, given to the Wisley garden two seasons ago. It seems to have outgrown its parent, appreciating the Battleston Hill soil with a liberal supply of spent hops and water. Planted in a more or less open position it lived through the past winter unharmed, to reward us the following May with an abundance of flowers, with trusses at the tip of practically every growth.
Although this plant with its white flowers commanded appreciation at the recent Chelsea Flower Show, I feel certain that had the plant been seen a week earlier it would have been more beautiful as the buds of the upstanding compact trusses were a rich pink fading to a pale pink as they developed to be finally pure white when opened.
The Rhododendron Society's book, "The Species of Rhododendrons," gives us very little information concerning this new species other than the statement that it is figured in the book by Professor Nakai and Professor Koidzume entitled "Trees and Shrubs Indigenous in Japan Proper."
R. yakushimanum is a native of Yaku Shima a small island due south of Kyushu one of the main islands of southern Japan four to five hundred miles north of Formosa. This island is almost circular, about twenty miles across and very mountainous, the highest peak being about 6,560 feet. It is covered with forest vegetation and the finest Cryptomerias in Japan are said to grow there. The Oriental Gulf Stream passes by its shores and gives it a mild climate. The flora includes a number of endemic species and it is reported that R. yakushimanum occurs from 1,600 feet up to the highest peak of the island.

R. yakushimanum
By J. Macqueen Cowan

R. yakushimanum Nakai - recently awarded a First Class Certificate when exhibited at the Chelsea Show by the Director of the R. H. S. Gardens, Wisley - is one of a group of four closely allied species which have been placed in the ponticum series, caucasicum subseries. It comes from Yaku Island and was named by the Japanese botanist Nakai, who described the new species in the Tokyo Botanical Magazine in 1921 (vol. 35, p. 135). I have seen only one herbarium sheet which was collected by G. Masamune, in June 1928, on a windswept ridge, near the summit of Mt. Yaedake. The type is, or was, in the Tokyo Museum.
Each of the four allied species - R. degronianum Carriere, R. metternichii Sieb. & Zucc., R. makinoi Tagg and R. yakushimanum Nakai - appears to have its distinctive facies and to have its own restricted distribution, but differences other than habit of growth are by no means well marked. It is therefore not easy to distinguish the species when plants have been dried and preserved; moreover the criteria given in the literature as diagnostic seem to be by no means constant and reliable. At different times I have seen all four species in cultivation but not together and I have had no opportunity of making a close comparison between them. Herbarium material is scanty.
R. degronianum and R. makinoi have long been in gardens, and R. makinoi is readily recognized by its long, narrowly lanceolate leaves. Most plants in cultivation under the name R. metternichii are, in fact R. degronianum ; the former species has a 7-partite, the latter a 5-partite corolla. The only plant of the true R. metternichii I have seen was at Exbury; it had been obtained by the late Mr. Lionel de Rothschild direct from Messers. Wada's nurseries in Japan. It appeared to be a straggly-growing bush, lacking the compact habit of R. degronianum . Another similar plant from Japan, sent by Messrs. Wada at the request of Mr. de Rothschild and Mr. J. B. Stevenson to Edinburgh, was supposed to be true R. metternichii , and had the same straggly habit but when it flowered it did not have a 7-partite corolla. The corolla was 5-partite as in the three allied species.
In "The Species of Rhododendron", p. 581, R. yakushimanum is placed along with R. makinoi , as having narrow leaves, but the evidence of such herbarium specimens as we have (an insufficient number for any conclusive finding) is that the leaves of R. yakushimanum scarcely differ from those of R. degronianum and are not nearly so narrow in relation to their length as those of R. makinoi .
R. yakushimanum and R. degronianum are certainly very closely akin, but would seem to differ markedly in their habit of growth, the former, as described by Mr. Hanger, a compact, dome-shaped shrub, about 2 feet 6 inches high and nearly 4 feet across - the latter, a more or less hemispherical, close-growing shrub, 4-5 feet high and of about the same breadth. The flowers of R. yakushimanum are in a compact truss and eventually become pure white, whereas those of R. degronianum , usually at least, retain a pinkish tinge.
Without much more material than is at present available, it is not possible to give reliable diagnostic criteria by which the four allied species can be separated, nor is it possible to come to a definite decision as to whether we have, in fact, under consideration four distinct species, or rather only one variable species with a number of distinctive varieties and forms.
All four species are separately listed and described by Nakai in "Trees and Shrubs Indigenous in Japan Proper" (vol. 1, pp. 58-67). He adds, besides, two varieties of forms of R. degronianum and three of R. metternichii .
The text is written in Japanese characters and I am indebted to the Rev. Scott Morton who translated the descriptions and appended notes for me, but my hope that these would furnish further characters for discrimination has not been fulfilled.
The features given to distinguish R. yakushimanum cannot be considered as entirely reliable-thickened capsules covered with a brown velvety indumentum which in other species is continuous, but in this one occurring only in patches.

*Editors Note. The photos Fig. 43 and Fig. 44 by Cecil Smith that accompany Mr. Sumners article, are pictures of the Award of Merit form of R. yakushimanum from Exbury that are growing in Mr. Smith's garden.