QBARS - v25n4 New Horizons for Rhododendrons

New Horizons For Rhododendrons
P. H. Brydon, Salem, Oregon
Reprinted from the Brooklyn Botanic Garden Handbook
"Rhododendrons & Their Relatives"

The considerable interest in rhododendron species and hybrids from Malaysia has been brought into focus by Dr. Sleumer's book "An Account of Rhododendron in Malesia": In this authoritative work, he states that of the 850 species in the genus, 250 occur in that area of the S. W. Pacific which stretches eastwards from the Malay Peninsula encompassing Sumatra, Java, Borneo, Celebres, New Guinea, Philippines, various smaller islands and the northern tip of Australia.
Despite the fact that rhododendrons have been in cultivation for over 150 years, very few growers have had the experience of growing species or hybrids of Malaysian origin. This is understandable since in recent years plants have not been available and, even if they had, they would require glasshouse protection in most parts of the United States. With the great increase in rhododendron hybridization, the potential of these Malaysian species should provide a source of desirable characters to include in hardier garden varieties. Quite apart from their use as parents for new varieties, the Malaysian species are a fascinating group for those who have glasshouse space and could prolong the season of interest in the genus with flowers in summer, fall, and winter.
In 1843, the English nursery firm of Messrs. Veitch sent Thomas Lobb to collect ornamental plants in Malaya, Java, and Borneo. Among the species collected by Lobb were R. javanicum, R. brookeanum, R. jasminiflorum and others from which Veitch created over 200 hybrids. Because of their geographical origin, these were called "Javanicum" hybrids. They possessed brilliant shades of orange, yellow, red, and pink but strangely enough, no blues and, as Dr. Sleumer observes, there does not seem to be blue shades among species native to the Malaysian areas. The records show that in 1897 Veitch exhibited "Javanicum" hybrids in flower at the Royal Horticultural Society's shows in London every month of the year, revealing an important character of Malaysian rhododendrons which may be put to good advantage in the production of hybrids which will flower throughout the year under proper conditions. This is explained by the fact that the species are found within altitudes of 20 degrees N. and 20 degrees S where daylight almost equals that of darkness, and where there is little variation in seasonal temperatures. It has been observed that plants under cultivation grow, produce flowers, and grow again to produce more flowers the same year. In their wild state, there is not the definite resting period that is found in plants from a more northerly latitude. Many of the species are epiphytic as well as terrestrial and Veitch discovered that if plants were cultivated in open beds under glass, they flowered sparingly and vegetative growth was excessive. When they began to grow them in pots with root growth restricted. flower buds appeared in greater abundance.
The species and hybrids became popular in the late 19th and early 20th centuries with gardeners of sufficient affluence to afford a large conservatory wherein the plants could be displayed. It is surmised that with the changing economy in Great Britain during World War 1, the large estate owners were unable to keep up the costly glasshouses and many of the "Javanicum Hybrids" were lost to cultivation. However, it is also possible that the tremendous and exciting influence of new species from S. W. China and the Himalaya brought back by collectors as Wilson, Forrest, Kingdon Ward, Farrer, and Rock turned rhododendron enthusiasts away from the tender varieties to the hardier relatives which proved so amenable to the British climate.
In 1961, the California Chapter of the American Rhododendron Society participated in sponsoring Dr. Sleumer's expedition to Malaysia. Seeds and cuttings were sent back to the Strybing Arboretum in San Francisco where they were grown on and eventually disseminated to members of the California Chapter. From these, various amateur breeders are busy creating new forms, some of which have flowered and show great promise for climates comparable to San Francisco and, of course, for those who live in less equable climates but do have the advantage of a glasshouse.
The color range includes striking orange shades, golden yellow, delicate pastels, glowing pinks and brilliant reds. One of the remarkable features of the flower colors in many species is that the color tends to intensify with age rather than fade as is the case in many of our garden hybrids. Fragrance is much sought after by breeders and this is a noticeable feature of many of the Malaysians. The floral shapes are often unusual. For example, in R. wrightianum war. cyclopense the flowers are reminiscent of a large Abelia floribunda and in R. culminicolum war. angiense , the corollas are oblique and resemble a large Salvia flower. The flower substance is often heavy and fleshy with great staying power particularly in hot weather. In a letter from Arthur Headlam of Australia, he states that R. laetum , a golden yellow species from New Guinea flowered for him in mid January when the temperatures ranged from 105, 94, and 110 deg. F. Yet, when the flowers dropped in mid February, they were still fresh and the color had intensified! Their ability to flower more than once a year has been mentioned and it is possible that this character may play an important part in the production of plants capable of flowering any time during the year.
In Dr. Sleumer's treatment of the genus, he divides the Malaysian species into five subgenera; namely, Rhododendron , Hymenanthera , Penanthera , Tsutsutsi , and Azaleastrum . This account is concerned primarily with the subgenus Rhododendron which is separated from the other four by the fact that they are lepidote (covered with scales) and the seeds have appendages like tails or wings at each end. The subgenus is again divided into Sections and in the Vireya Section, Dr. Sleumer further identifies the species by the type of scales, i.e. "disk shaped," "star shaped," "dendroid," and "sessile." While species of this Section cross freely within themselves, there seems to be a lack of compatibility between them and other species of the genus. The fact that the species in the Vireya section are lepidote has suggested that they might cross with members of other lepidote species, as for example members of the Maddenii Series. One hybrid between R. maddenii and R. javanicum has been reported by Don Stanton in Australia but so far there is no account of it having flowered.
To my knowledge, there are no hybrids in existence today between species or hybrids of the Malaysian group and others in the genus which have flowered. However, in 1885, a hybrid by the name of R. 'Pearl' received a First Class Certificate in London and was reported to be the result of a cross between R. 'Princess Royal' (a hybrid between R. jasminiflorum and R. javanicum ), and R. griffithianum . Another cross is recorded between R. 'Duchess of Teck' ( R. griffithianum hybrid) with R. javanicum , made by Harry Noble of England and called R. 'Lord Wolseley'. Neither of these are in cultivation today.
There is a member of the Vireya Section which occurs in Formosa called R. kawakamii and Michael Black of Grasmere, England, an authority on Malaysian Rhododendrons, suggests that this species may prove to be a suitable parent for a hardier race of Malaysian hybrids. R. kawakamii occurs at 7000 ft. altitude and from our limited experience with its cultivation on the Pacific Coast, it seems to be hardy to at least 10 degrees F. Hadley Osborn notes that R. kawakamii occurs in company with R. morii , a species presently rated as hardy to zero.
There is one other species belonging to the Vireya Section and which occurs on the Chinese mainland. It is R. vaccinioides , a native to Sikkim and S. E. Tibet at from 6,000 to 12,000 ft. altitude. It is a shrub to 5 ft. with campanulate pinkish flowers singly or in pairs. There are no reported hybrids with this species to date.
Propagation of the Malaysian species is much the same as for other members of the genus. The seed is short lived and should be sown immediately on screened sphagnum moss, in flats or pots, enclosed within a polyethylene covered frame where the temperature range should be 70 and 75 degrees F. Germination may take from 20 days to 45 days and, as a general rule, growth after germination is much slower than in the hardier species. Hadley Osborn has been successful in speeding up growth by decapitating the seedlings at ground level after the true leaves have been formed and when the seedlings are about ½ to ¾" high. All but the two top leaves are removed from the cutting which is then inserted in screened moss and the resulting plant outgrows the seedlings by a considerable margin. As soon as the seedlings, or rooted seedling cuttings, are large enough to handle, they should be pricked off into flats or pans of equal parts of screened sphagnum moss, moist peat and washed sand. It may be necessary to re-flat the seedlings twice in the same year, before moving them on to pots. For glasshouse cultivation, a minimum night temperature of 45 degrees F. would be best although many of the species in this article have survived to 27 degrees F. out of doors in San Francisco. But just survival is not enough if you want your plants to look well, hence the suggested warmer temperature. Because of their epiphytic nature, a coarse well drained soil mixture is advised, such as 40% screened leaf mold, 40% redwood bark chips, and 20% sandy loam. Do not over pot and as long as the plants hold their foliage and make a normal growth, do not move them up to larger containers; otherwise flowering may be delayed. Many of the species and hybrids in this group tend to be leggy and awkward in pots. If a free standing well formed plant is desired, training should commence when the young plants are a few inches high by "pinching out" the apical bud just as it begins to elongate. Lateral branches will be induced to appear from lower down and in the event that only two or even one should appear, then pinch again until an evenly spaced framework is produced. Most of the Malaysian species and hybrids will respond to small quantities of liquid fertilizers such as fish emulsion applied during the spring and early summer months. Increasing the day length by means of artificial lights during the short days of winter will assist in the production of flowering wood. Frequent syringing with water is beneficial in keeping down most insect pests and providing the moist conditions which most of these plants enjoy.
From my observations on the more than 50 species now being grown at the Strybing Arboretum in San Francisco, I have selected the following ten which have done reasonably well and when they became available should provide an interesting and attractive collection for under glass in colder climates than is found in the San Francisco region.

R. laetum
R. laetum , Strybing Arboretum, 67-283
Photo by John P. Evans, M.D.
  • R. christianae : S. E. New Guinea, 2,000-4,500 ft. alt. shrub to 9 ft. Flowers 3" long 2" wide in clusters of 3-4, deep yellow, also variable shades of salmon, orange, etc. Withstands heat.
  • R. konori New Guinea 5000-7000 ft. alt. shrub to 12 ft. flowers, large, 5" long 4" wide, in clusters of 3-8, white, strong carnation fragrance.
  • R. laetum : New Guinea, 4500-5000 ft. alt. shrub to 6 ft. flowers 3" long 2" wide in clusters 6-8, deep yellow. Withstands heat.
  • R. leucogigas : New Guinea, 4500-5000 ft. alt. shrub to 6 ft. Flowers very large, 5½" long and 5" wide in clusters of 5-8 white, edged carmine on lobes, strongly scented. A spectacular new species discovered by Dr. Sleumer.
  • R. lochiae : Queensland, Australia, 3500-4000 ft. alt. shrub to 6 ft. Flowers 2" long and 1½" wide in clusters of 2-7, rich scarlet. The only species in Australia.
  • R. macgregoriae: New Guinea, 10009000 ft. alt. shrub to 15 ft. Flowers about 1" long in clusters of 15, color variable from pale yellow to orange flushed pink. Most common species in New Guinea.
  • R. retusum : Sumatra, Java, 500010,000 ft. alt. shrub 1 to 10 ft. Flowers in clusters of 4-10, tubular, scarlet, A neat little species.
  • R. culminicolum var. angiense : New Guinea, 3500-7000 ft. alt. shrub to 15 ft. Flowers oblique, tubular, 2" long in clusters 4-6, reddish pink.
  • R. wrightianum var. cyclopense : New Guinea, 4000-5000 ft. alt. shrub to 3 ft. Flowers 2-3" long in pendulous trusses, glossy red.
  • R. zoelleri : New Guinea, 6000 ft. alt. shrub to 18 ft. Flowers 3" long and 3" wide, in clusters of 5-8, shades of yellow, orange and salmon, sweetly scented. One of the best ornamental species.