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Journal American Rhododendron Society

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Volume 19, Number 2
April 1965

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Yakushimanum
A. F. Serbin, M.D., Bloomfield, Conn.
A paper delivered before the New York Chapter of the A.R.S.

        With the awakening of the rhododendron enthusiast to the singular excellence and potentially great hybridizing qualities of the species R. yakushimanum in recent years there has been a flurry of comment regarding this species within the past year some of which is factual, others presumptive and some actually speculative. In an effort to bring as much as is known regarding this species, its habitat, its mode of transfer to the western world, and in particular its superior qualities as a shrub, to the attention of the plant grower and breeder this paper is presented in an effort to "set the books straight."
        The species was first described by T. Nakai, professor of botany at the University of Tokyo (Imperial Univ.) between 1920 and 1921. Although the species is found only on Yaku Shima, very few plants are found in the gardens of Japan for it is so limited in the wild that the Japanese government forbids collectors to remove plants from its native source without their expressed permission. It may actually be an almost extinct species for it is to be found only on higher elevations of the three larger mountains on a tiny island only 15 miles in diameter.
        Plant hunting in the southern-most portion of Japan and in particular Yaku Shima has changed considerably since 1959 when the author visited the island. Yaku Shima is an almost circular island lying in the China Sea about 120 miles south of the southernmost tip of the Japanese main land. In 1959 a flight on small domestic planes from Tokyo reached Kagoshima in 5 hours, the southern tip of the Kuyusu mainland. It was from here that the prime difficulty in transportation was encountered for there were no planes or scheduled ships to Yaku Shima (recently small plane service has been instituted from Kagoshima to Yaku Shima). When one visits this island in the fall of the year, the time schedule for travel into the China Sea must reckon with the frequent typhoons which abound here. Ships wait in the Kagoshima port for subsidence of turbulence after each typhoon before lifting anchor and making the run to Yaku Shima. By the same token it is easy to become landlocked on Yaku Shima should a storm arise after arrival to the island. In 1959 there were no inns or hotels and it was mandatory that some arrangement with the Dept. of Agriculture be made in advance so that food and room could be obtained at the Forester's house. Almost no one speaks English so that an interpreter as a companion is a necessity and adjustment in diet becomes an experience. A well balanced breakfast, for instance, consists of fish soup, a raw egg, a few strips of purple seaweed, piping hot boiled rice and the ever present green tea. Unless one is handy with chopsticks it is best to carry a spoon, fork and knife in your kit of essentials.
        The island is subtropical with a unique climate. At the shoreline, somewhat reminiscent of the Bahamas, papaya, a dwarf banana, and pineapple are under cultivation. The island is a rain forest with 4,000 mm (157") of rain at the shoreline, 6,000 to 10,000 mm (235" to 393") at the mid mountain levels and as high as 12,000 mm (471") at mountain peak level. The sunshine alternates at frequent intervals with the rain and sturdy rain gear is essential. The island is completely mountainous, the slopes extending down to the sea. As one leaves the shoreline, the mist and rain become almost continuous. The hills are deeply notched with cold, clear streams which indent the island freely. Three mountaintops form the apex of the island, Mt. Kuromi, Alt. Nagata and Mt. Miyanoura. Mt. Miyanoura is the highest mountain in southern Japan (7100 ft.).

At High Elevation
        Two rhododendrons bear the island's name. R. yakuinsulare is an azalea growing at the shoreline as well as on the lower slopes. R. yakushimanum montanum is the species to which this paper is assigned. As the name indicates, it is not to be found at shore levels or on the lower slopes, contrary to recent writings, but only on the higher portions of the three mountains. As indicated in a previous article, the author was able to delineate three distinct forms of R. yakushimanum montanum. However, all three forms were found only at higher levels none of which were below 1,000 ft. elevation. A recent article suggested that plants had been stripped from the accessible parts of Yaku Island and therefore this species could be found only at the very top. According to the chief forester of Yaku Shima, this is not so and is further borne out by the fact that very few of the inhabitants of the island have a single plant of the species in their own gardens. Only a few University Botanic gardens of Japan have a plant of this species!
        The name yakushimanum makes its first appearance in the English literature in the British Rhododendron Assoc. Year Book of 1934 which was edited by Lionel de Rothschild. The laconic description under this species belies the little that was known regarding it in the western hemisphere at that time. The incorrect title of R. yakushimanum was made because it came from "Yakusima." Had the island been correctly indicated as Yaku Shima the plant today would have been known as yakushimanum. The Japanese refer to this species as R. yakushimanum montanum. The only description given in the 1934 Year Book was "a shrub with pale rose flower." This gross understatement, of course, has since been adequately corrected with the most glowing adjectives describing the species.

Exbury plant, 1964 R. yakusimanum at Exbury R. yakusimanum F.C.C. at Wisley
    Fig. 17.  Exbury plant, 1964.
    Photos by author
    Fig. 18.  Close up of
    F.C.C. plant of
    R. yakushimanum
    at Wisley, 1964.
  Fig. 19.  Free flowering
  R. yakushimanum at
  6000 ft. on Mt.
  Miyanoura.

First Plants from Wada
        In 1931 Lionel de Rothschild had written to Mr. K. Wada (now of Yokohama) requesting plants of unusual character and high quality, not specifying any particular species. Mr. Wada responded by sending two 10" plants of R. yakushimanum. Mr. Wada states that one of these plants came from the plateau of Hanano-Ego. He could not be certain of the precise source of the other plant. These small plants arrived in England by ship without difficulty or damage (the species travels very well and is resistant to wilting and drying.) Both plants were established at Exbury. One of these two plants made its way ultimately from Exbury to Wisley and the modus operandi responsible for this transfer remains shrouded in some mystery. The two plants are quite similar in flower as are so many of the variants grown from seeds both from the wild and under cultivation.

The F.C.C. Award
        Both plants are superb specimens and to the author's eye the difference lies in the growth habit as well as in the distinctive foliage. The excellence of the Wisley plant is probably attributable to the fact that it has attained a height of only 3-3½ ft. after 30 years and has grown in breadth so that at the present writing it probably is 6-7 ft. in diameter. The Exbury plant has a more vertical habit. The Wisley plant received its F.C.C. award in 1947 when it was 2½ ft. in height and 3½ ft. across. Its description read "there was a flower truss at the tip of every stem. The pink buds opened to bell-shaped, 5-6 lobed white flowers 2¼" in width. The opening leaves were covered with a grayish white tomentum on both sides. The more mature leaves were a dark, glossy green averaging 3" in length with mild degrees of curving." It immediately captured the F.C.C. award when first shown and created a storm of enthusiastic interest.

        The Exbury plant, however, was first exhibited in 1950 at the Chelsea show with equally devastating effect. I am quite certain that had the Exbury plant been exhibited first it would have easily taken the F.C.C. award. Without doubt both of these plants are equally worthy of the highest rating. In both specimens the leaves are heavily covered with wooly beige indumentum beneath the leaf and the new leaves present a snow-white coat on both surfaces resembling the texture of suede. Visualize then such a plant completely draped with creamy, coral pink buds which open pure white on a mid May morning and you will at once realize how well this plant deserves the highest accolade which it first acquired in 1947. It is the author's personal feeling that the foliage is even more attractive in the Exbury plant since the leaves are doubly convexed, somewhat shorter in length, darker green and the plant has a more vertical tendency in growth pattern. To the non-botanist's unpracticed eye the flowers of both plants appear quite similar.

Distinct Forms
        The author, in 1959, described three distinct variations which were striking after viewing several thousand plants in the wild and numerous cuttings derived from these plants. A large flat leaf form was found in the plant variety one first encounters upon climbing the mountains. This form is seen at timber levels between 4,000 and 5.000 ft. This brings to attention another error noted in recent writings, namely, that the larger leafed forms flower more freely at lower elevations and that smaller leafed forms which are found at highest attitudes were less floriferous. This has not been the experience of the author. The large flat leaf variety flowers very scantily in the wild since it is very leggy, growing in heavily shaded areas, and under great competition with other flora indigenous at this level. Large leaved plants lifted from their bed have very small root systems due to the tremendous competition. This variety attains an average height of 6-8 ft. in the wild. Very old plants with great diameters were not apparent during the author's stay on Yaku Shima and it is possible that their life span is short due to the competitive struggle for food and growing room.
        The small, round leaved variety is the rarest of all forms of this species. These plants as adults average 15-18" in height and are very uniform in appearance growing on a plateau at about 5,000 ft. in a soggy bed of dense sphagnum with almost no competition from trees. This is a distinct variety and has the unique adverse quality of being difficult to grow as well as to propagate. I have seen no plants of this variety under cultivation either in England or in the United States. The third variety previously described was that from which the F.C.C. and the Exbury plants undoubtedly are derived. This form is noted when one ascends almost to the peaks of the mountains in the most windswept and exposed areas approximately 800 to 1.000 feet above the timber line. The area is constantly doused in mist or heavy rain and much filtered sun. This is the double convex form typified especially by the Exbury plant. In the wild the leaves are almost tubular in their transverse convexity and form almost one third of a circle in their horizontal convexity. This interesting, quality in the leaf which I presume to be nature's way of protecting the stomas of the under leaf from the constant wind loses some of its curvaceous characteristics when cultivated under more favorable conditions. The convexity in both planes becomes less severe but never quite becomes flat.
        In the past few years still a fourth distinct form has been noted in seedlings obtained from the wild. This is a long, narrow, flat leaf variety which closely resembles in shape the leaf of Rock's form of R. metternichii. Its indumentum, however, is quite characteristic of yakushimanum. As one studies several hundred seedlings obtained from the wild, it is clearly possible to categorize the various individuals into these four groups although variants are noted quite frequently which lie in form and habit between the four major groups outlined above. It is doubtful whether any other species in the genus offers as many variants as does R. yakushimanum. Is it then a natural hybrid?
        The varieties described above are distinctively different in their ability to strike roots from cuttings. The long flat leaf forms root easily while the small, round leaved forms are most difficult to root vegetatively.

Indumentum Varies
        Forms of the species without indumentum have not been previously described. However, the author has received cuttings from the specimen plant at the Royal Botanic Garden at Kew as well as from the garden of Mrs. Frazer Mackie of Northern Ireland, both of which have no indumentum but represent true forms of R. yakushimanum. The plant at Kew was a seedling derived from the F.C.C. plant at Wisley. I have under cultivation some 30 or 40 plants without indumentum, obtained from seeds in the wild, representing perhaps 1 or 2% of the total number of plants from a given batch of seed collected on Mt. Miyanoura.

Small, round leaf form. Long narrow, flat leaf form
  Fig. 21  Small, round leaf form,
  most dwarf and rarest variety.
  Fig.22.  Long narrow, flat leaf
  form, a seedling from the wild.
Leaf variation Leaf undersides
   Fig. 23.  Leaves from the plant
   variants.
   Fig. 24.  Under side of leaves;
   note the leaf without indumentum.

Yakushimanum as a Parent
        In the 1959 R. H. S. Year Book the John Waterer Sons and Crisp Nurseries of Bagshot, England, reported the first results of R. yakushimanum crosses with 'Corona', 'Doncaster', 'Britannia' and 'Fabia.' Since then the Commissioners of Crown Lands have extensively produced hybrids of this species with excellent results. It was at once noted that R. yakushimanum had a tendency to produce dwarf offspring. In any event, it certainly dwarfed the larger parent. Furthermore, it was frequently easy to recognize identifying features of both parents as it pertains to color and habit although foliage varied considerably. Most of the offspring were compact, a feature certainly attributable to yakushimanum's virtue of retaining its foliage for as long as 5 or 6 years or longer. These new hybrids, as well as those produced at Windsor Great Park under the astute guidance of Sir Eric Savill, and Mr. T. H. Findlay, attracted wide interest.
        As a subject for breeding, R. yakushimanum is ideal as a donor, as well as a recipient, for pollen. The seed capsules are five chambered and glabrous. Its anthers, when ripe, are richly laden with pollen and a single floret easily permits several crosses. When utilized as the recipient parent, it produces large seed capsules from which a wealth of seeds are harvested. I have found most "yaku" hybrid seedlings to be vigorous and sturdy. Unlike many species it is a freely flowering shrub and this trait is carried over into its hybrids. To date, however, the author has not seen a yakushimanum hybrid that has equaled its parent for beauty and excellence in growth habit. David Leach has observed that yakushimanum hybrids are relatively short-lived, possibly due to premature senility. It will be interesting to note whether the short-lived character of the "yaku" hybrids is uniform or perhaps selective for certain crosses only. Certain it is that a new family of hybrids is in the making with R. yakushimanum as the major parent because of its striking qualities.
        The flowers of the species vary especially in the large, broad leaved forms. The florets here are larger than in the smaller leaved plants and are frequently frilled at the margins. Some trusses show no pink whatever before opening all white. I have not as yet. seen the small round leaved forms in flower.

Extremely Hardy
        The hardiness of R. yakushimanum comes as a pleasant bonus since it is probably one of the most hardy of rhododendron species extant. Its thick leathery leaves are not subject to burning and it is capable of withstanding severe drought and hot summers as well as the harshest of winters in central Connecticut. For us in New England yakushimanum is a knight in shining armor and holds great promise in producing hybrids of dependable hardiness so necessary in our killing winters. One has but to review the newer hybrids being registered in our northwest and in England to realize that very few crosses are being produced with an eye to hardiness. We may now perhaps add to our meager list of hardies with the help of this tough species.
        Seedlings tend to flower at from 5 to 7 years of age. Cuttings and grafts, of course, can flower within 1 or 2 years after establishment. A shortcoming noted by the author is that during a warm Indian summer flower buds will frequently open in the month of October to the dismay of the owner who may have but one plant with 1 or 2 flower buds.
        Sizeable plants of the species are so few under cultivation that one may review almost all of them in a modest paragraph. The parent plants at Exbury and Wisley are the largest and best known. Kew, Edinburgh, Windsor Great Park and private gardens in England, Scotland and Ireland, as well as a few in New Zealand, have plants exceeding 18 to 24" in height and probably none of these are a match for the pioneer specimens from which most are derived at Exbury and Wisley. In the U.S. I believe the finest specimen is to be found in Cecil C. Smith's garden although several fine seedlings are to be found in other private gardens on the west coast. Very few sizable specimens are to be found elsewhere in this country. The species therefore is truly scarce largely because of limited growing stock and because it is a slow grower and difficult to propagate vegetatively.
        In the wild one is struck by the absence of very old plants. The oldest shrubs I have seen on Yaku Shima were no more than 3" to J" thick at their base and these were only seen in the tall growing flat leaved varieties protected by surrounding taller trees and shrubs at lower levels. One may infer from this that the life span is considerably shorter in this species than in others. The oldest plants under cultivation are but 30 to 32 years of age.
        On the mountain tops growth is indeed slow averaging ½" to 1" per year. Unlike the majority of rhododendrons which retain their leaves but 2 years R. yakushimanum retains its foliage from 5 to 7 years which accounts for its wondrous dense appearance, a virtue duplicated by very few other species or hybrids. Under non-competitive and ideal conditions such as occur in our gardens growth is much more rapid with from 3 to 5" added each year.
        The leaves are leathery and handsomely dark. The leaf of R. yakushimanum bears close resemblance to that of R. makinoi and Rock's form of R. metternichii, both natives of Japan suggesting a common heritage. Histologically, the microscopic appearance of the hairy structures of these species bear a close resemblance to one another.

Growing Seedlings
        When grown from seed on sphagnum, peat moss, or combinations of these rnedia mixed with sand, perlite or Styrofoam, best sown in January, germination can be expected in two to three weeks depending upon the degree of warmth in the propagating frame. I use milled fresh sphagnum only, placed in pans and sterilized with boiling water. The seed is sown on the thoroughly moistened media without further covering. A clean glass plate covers the pan excluding all air. Once every two weeks seedlings are sprayed with a fine mist suspension of fungicide. As the first leaves appear, the glass plate is applied in such a manner as to permit air to enter and is finally completely removed when four leaves become evident. This is followed by pricking off the seedlings, transplanting into flats containing a mixture of two-thirds sandy loam mixed with one third peat moss. From here they can be transplanted to cold frames or open benches as they begin to crowd one another.
        The indumentum of young seedlings is port-colored until they attain a height 1½" to 2" at the end of one year (under lights the growth, of course, is much more rapid). The indumentum then becomes pure white and as the leaf matures it assumes a velvety brown-egg shade which remains throughout its life. As indicated previously, the seedlings from the wild as well as from selfed plants produce great variations in the offspring. This is notable not only in the type of leaf but also in the growth habit. Almost all will ultimately be dwarf or semi dwarf plants.
        Propagation By Cuttings Cuttings strike roots reluctantly in this species but once rooting is started the ball becomes rapidly luxuriant with feeder rootlets and can be transplanted without fear of breakage. I have found that a much higher percentage of rooting occurs when cuttings are taken in June rather than in the fall. I must confess, however, that my batting average in successful rooting at best runs only 60%. Some cuttings have been reset over and over again for almost three years before becoming rooted individuals. Some cuttings have remained alive, forming abundant callous, but have not rooted after five years in a propagating bench! A strong hormone (Hormodin #3 or 2-3% I.B.A.) is recommended.
        Most plants of this species from England have been grown either from seed or from grafts. The species lends itself to grafting, and although R. ponticum remains the favorite under stock of the English, I personally prefer 'Cunningham's White' or 'Roseum Elegans' because of their freedom from Phytophthora. Air or soil layering is not practical because of the density of growth and the rigidity of the wood.
        In a recent issue of the Bulletin I noted with some concern that two seedlings of R. yakushimanum had already been named and one of these had already been registered. Before long, these variants will be propagated and marketed. Bearing in mind the large variety of seedlings already under cultivation, and the tendency for almost all seedlings of this species to excel as individuals, I foresee in the near future a babel of named variants of this wonderful species. Should we not keep in check our desire to tag each individual plant with a new name? Would it not be a very confusing situation if all the plants that differ from one another in the English and the American gardens were to be named?


Volume 19, Number 2
April 1965

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