Dr. Frederick W. Coe
Talk presented by Dr. Coe before the California Chapter on Dec. 8, 1966
Many of us who grow rhododendrons think of medium to large-leafed shrubs with trusses of ten to twenty outfacing flowers, preferably large, and red or pink, borne over the foliage. The more initiated may know that there are small, medium, and large-leafed species and hybrids with flowers of varying size and shape, but may not realize that there are also deciduous species, or that azaleas are members of the rhododendron family. Few of the authoritative works on the species of rhododendron provide more than a short note on a large group of little known rhododendrons (nearly one third of the genus)-most of them needing sheltered positions, or greenhouse care, even here in the San Francisco Bay Area.
These are the Malaysian, formerly known as the javanicum rhododendrons, so named because one of the early found species was introduced into England from Mount Salak in Java in 1845. What distinguishes these rhododendrons from the usual rhododendrons we grow in our gardens? First of all they are all lepidote; that is scaly leafed rhododendrons. The seeds are probably the best distinguishing characteristic, for all of this group have long tails or wings on either end of the seeds. The name commonly used, javanicum, is ambiguous, for actually only a half dozen species are found in Java, several of these being found elsewhere also.
The greatest number of the 270 species described is found in New Guinea (157), while Borneo follows with 34, Sumatra with 26, the Celebes with 24 and the Philippines with 18. Fifteen species are found on the Malay peninsula, three in Southern Tibet (the Vaccinioides series) and two or three more in southern China. There is a scattering of species throughout the East Indies and one species each is found in Australia (R. lochiae) and in Formosa. It must be remembered that many of these "species" are based on pressed herbarium specimens and the actual number of true species-not varieties or hybrids-is probably less. Comparison of live material in the field is often needed to distinguish parent species from hybrids and thus there will undoubtedly be fewer species when this work can be done. This "disappearance" of species has occurred in the Himalayan rhododendrons when thorough study was made of the living plants. There are still "species" in common cultivation which are almost certainly hybrids. Probably the commonest of these in the present genus is R. mucronatum, the common 'Ledifolia Alba' evergreen azalea which has never been found in the wild. The collector of species is caused more or less anguish when a long familiar friend is considered a variety of another species.
While we think almost entirely of the rhododendron hybrids produced in Europe during the last century as being the arboreum, ponticum and catawbiense group with the addition of the various azalea groups, there was an extensive hybrid group produced between the members of the so-called javanicum species.
The early species used in these hybrids were introduced by Thomas, Lobb, and Charles Curtis in the last half of the nineteenth century while collecting for the great Veitch nursery in Chelsea, England. The hybrids arose from crosses made between seven species and their varieties. These were R. jasminiflorum, javanicum, brookeanum and its variety gracile, malayanum, celebicum, teysmannii (now a variety of R. javanicum) and multicolor. These species ranged in color from white through yellow, oranges and reds. Hybrids between them "exceeded the original species in brilliant and varied colors, large size of truss and individual blooms" to quote Veitch. A paper presented to the Royal Horticultural Society in 1891 gave the parentage of the numerous hybrids, at that time numbering in the hundreds. The first of these hybrids, 'Princess Royal', was the result of crossing R. jasminiflorum (white) and R. javanicum, (yellow). The hybrid is a delicate pink. This plant crossed with its parent R. jasminiflorum gave a white-flowered hybrid, 'Princess Alexandra'. This hybrid crossed with R. brookeanum, which is bright yellow, gave the pink flowered 'Taylori'. From this brief outline it may be seen that it is difficult to tell what will be obtained on crossing different colors.
A great number of beautiful hybrids were raised during the last quarter of the nineteenth century and more improvements appeared. One very beautiful group arose which were called the Balsamaeflorum hybrids. These were double to semi-double flowered hybrids and covered the full range of colors found in the other hybrids. As is true of many double flowers these were very durable and would last many weeks in perfect condition. Even as cut flowers they were very long lasting. As the century drew to a close possibly the culmination of the javanicum hybrids was reached in such plants as the yellow flowered 'King Edward VII', 'The Queen', a pure white, and 'Cloth of Gold', pale yellow.
Attempts were made to cross these javanicums with the better known and hardier species, and one hybrid was raised between R. auklandii (now R. griffithianum) and 'Princess Royal' used as the female parent. This was a small flowered hybrid closely resembling 'Princess Alexandra' and named 'Pearl'. Another most interesting hybrid was raised between a red-orange javanicum hybrid, 'Lord Wolseley', and an Indian azalea 'Stella', as pollen parent. This was smaller than the javanicum parent and had some of the characters of the azalea parent. A sister seedling was grown for twenty-one years without blooming. At that time it was only five inches high, very similar to crosses made by Dr. John Creech between evergreen and deciduous azaleas. This plant had glandular hairs but no scales.
The decline of these hybrids of the Malaysian rhododendrons came before or with the first World War when many conservatories and large greenhouses were without heat or used to raise food crops. Between the wars, and at present, only a limited number of these hybrids remain in cultivation.
So much for the past and its beautiful hybrids. Following World War II interest was aroused in all of the species of rhododendrons and Dr. Sleumer of the Rijks-herbarium in the Netherlands has worked on the identification, classification, and taxonomic placement of the Malaysian species as well as other members of the southern Asiatic rhododendrons. In 1961-62 he collected and explored in New Guinea. He sent back seeds and cuttings to the Strybing Arboretum in San Francisco, Golden Gate Park, and obtained herbarium specimens of many new and old species as well as natural hybrids. He has placed all of these plants in a large section, the "Vireya," and has several subsections which contain plants as variables as those found in our more familiar sections of rhododendrons.
Just as the Himalayas are the center for our more familiar rhododendrons. New Guinea is the center for this tropical group and evolution is going on rapidly in both regions. Probably nowhere else is there such tremendous variation in foliage, growth habit, and flower size and shape, as in the Vireya section of the rhododendron genus.
The foliage of some of the mountain species is amazing, in some resembling a heather, in others boxwood or even the crowded leaves of a saxifrage.
Flowers too can vary from funnel-shaped, azalea type flowers through short or long tubular flowers to the enormous tubular funnel-form flowers of R. leucogigas, almost six inches long and nearly as much across. In addition this flower has a strong carnation odor. Although the Strybing Arboretum has a well rooted cutting from this original collection it has not as yet bloomed.
Hybridization does occur in the wild and Dr. Sleumer could see this frequently in cleared or devastated areas where two or more parent species are able to intermingle and produce intermediate seedlings. This may be difficult to see in the usual un-cleared region but in mountain regions, such as those of North Carolina in this country, it is most easily seen on ridges where adjacent species will form hybrid swarms on the ridge. During his last few days in New Guinea, Dr. Sleumer was able to fly over the mountain ridges in a helicopter and could see this same condition occurring in many places.
Many of the better known and showy of the Malaysian species have flowers shaped very much like those of the oleander.
As these plants are tropical, buds are formed and plants bloom with no particular regard to season. The day, in the tropics, is a relatively short day so that in many of the hybrids and species there is more tendency to bloom during the period between September and April.
A large part of this section is epiphytic which makes these ideal plants to grow in containers. They do not in general have a large root ball and in a loose mixture of bark, peat moss, sand and perlite will grow well with occasional feedings of dilute liquid fertilizer. Too large a container may be the cause for little or no bloom in certain species.
Obviously this section of the rhododendron genus offers a gold mine to the hybridizer and the surface hasn't been scratched. Many excellent species have yet to be introduced and much work is needed to incorporate the color and scent of this group into hardy hybrids. The possibility of making crosses with members of the Maddenii series to create colorful, sweet-scented hybrids is still in the beginning phase. Some attempts were made in England, but when the first generation was of poor quality nothing further was done.
The Strybing Arboretum has quite an extensive collection of both species and hybrids of this section. Some of the species are rapid in growth and bloom quite profusely, and in most cases the hybrids are floriferous. Seedlings of this section are very slow in growth for about the first year or until they reach about three inches in height. Cuttings are easily rooted in most cases and usually grow rapidly, particularly if placed in a loose mixture as previously mentioned. The only real fly in the ointment in the cultivation of this series is the general inability to stand temperatures much below freezing. Many species are from mountainous regions and can probably stand light frost, but temperatures much below 28°F. for more than a brief time are fatal. They share this tenderness with some members of the Maddenii series. There are species that are found over 12,000 feet in the mountains of New Guinea and Borneo which might be quite hardy in the Bay Area region. In most cases these are alpine, creeping, or tufted species with small or curious flowers. These would be excellent for a rock garden in foggy regions around the Bay.
The following slides give a fair cross section of the Vireya section, the subsections and hybrids. Most hybridizing has been confined to the Javanicum series with only one species in another sub-section. (The following slides were shown.)
R. retusum R. wrightianum R. longiflorum R. zoelleri R. invasorium R. vanvuurenii R. javanicum 'Taylori' R. superbum R. aurigeranum R. macgregoriae 'Taylori' seedling R. dielsianum R. comparabile R. triumphans 'Princess Alexandra' R. leptanthum R. culminicolum R. villosulum 'Souvenir de J. H. Mangles' R. maius