JARS v37n3 - Some Notes on Rhododendron forrestii and R. chamaethomsonii

Some Notes on Rhododendron forrestii and R. chamaethomsonii
Alleyne R. Cook, Vancouver, B.C.

During the winter of 1981-1982, a start was made on moving some two thousand rhododendron plants from Stanley Park to the new Sino-Himalayan garden at the VanDusen Botanical Garden. For the first time since 1965 when the Greig Collection of rhododendrons was moved from Royston on the east coast of Vancouver Island, it has been possible to bring together the many variants of each species in one place. As part of this process, all the taller variants of R. forrestii and the numerous specimens of R. chamaethomsonii were relocated. The VanDusen site faces the northeast in what must be one of the coldest areas of Vancouver. It is hoped that the north facing slope, subject as it is to the coldest winds, will delay the opening of the flowers thus saving them from late spring frosts.
In Stanley Park, the peak of bloom for the R. forrestii group is the last week in March and the first week of April. This garden is considered to be in the mildest area of the B.C. mainland. Sheltered as it is from the northeast outpourings of cold interior winds, and beside the ocean, the early blooming is about two weeks before VanDusen with its higher elevation and more exposed location. About 50 plants of R. forrestii ranging downward in size from a span of 8 feet are ready to be moved to VanDusen Botanical Garden when suitable preparations have been made to receive them.
To protect these early flowering species, the Greigs always grew them in covered frames during the winter. This was not an indication of a tender plant, but was necessary to protect the opening buds from the late frosts sent down from the Cole-man Glacier located 12 miles above Royston. At the VanDusen site it is perhaps fortunate that there are no trees to give any shelter on the exposed slope where the R. forrestii group are planted. We found in Stanley Park that the losses that occurred after the move from Royston occurred without exception under tree cover. These plants apparently must be in the open in approximate simulation of sub-alpine conditions.
It is a very great pity that the name "repens" is not completely discarded from rhododendron literature. If it were, we would then constantly commemorate the great George Forrest ¹ . If the red colored underside of the leaf is the only difference between R. forrestii and var. repens and if grown from seed, this characteristic is not constant, then surely the varietal name is superfluous.
When Mrs. Stevenson of Caerhays reviewed this Species in the R.H S Year Book 1951, R. forrestii was not mentioned. It did not exist in what was the greatest collection of rhododendron species ever grown. The type plant (F.699) sent to Tower Court from Caerhays Castle in 1922 had a green underside to the leaves. Forrest described this Species as resembling English ivy because of its habit of climbing up rock faces, rooting as it climbed on the underside of the stems. The original R. forrestii was discovered in 1905 on the divide between the Mekong and the Salween rivers. Twelve years later, R. forrestii var. repens was discovered in the same locality. The type seems to be constitutionally weak.
My introduction to the great Tower Court Collection of R. forrestii with its many shapes and forms was the removal of a large specimen to Sunningdale Nurseries. At first sight, the planted area consisted of a north-sloping bank, upwards to 40 yards long, covered entirely with a prostrate green mass. About every 6 feet there erupted a low mound, 2 feet across and maybe 9 inches high. These domes marked the centers of the plants. Each plant had a small china label on which was recorded species, series, and collectors' number. Before digging our plant, it was necessary to tease the outer runners from those of the surrounding bushes. It was then possible to dig under the dome. I remember well that the foreman and I had arrived expecting to dig a plant 2 feet across. We returned with a specimen 6 feet across and needing 4 men to lift it.
In Vancouver we have found that with the defoliation of the central dome, the original structure begins to break down. The larger specimen becomes several very healthy independent units. The largest specimen from Royston completely filled the back of a 1965 pick-up truck. It has now naturally divided into 14 healthy plants which we re-spaced in a new area. Our second largest, given to the park by Dr. Hawthorn, is now losing the central domes' foliage. Photographs taken in the wild do not show neat rounded bushes. They show a number of different plants each vying for advantage in the struggle to cover the ground. It would appear that the death of the central plant after the rooting of spreading shoots is a natural form of growth.

Mrs. Stevenson in her 1951 review mentions how once between 1922 and 1950, the type specimen flowered magnificently on only one occasion. Only once have I seen this Species literally covered with flowers. Dr. Hawthorn's specimen was 6 feet across. We rolled it up like a carpet and carried it to its new home, which consisted of 6 inches of gravel. The next spring, it was a sheet of scarlet. This was so out of character that I telephoned Dr. Wade, informing him that he might never see the equal of it again in his life, and that he had better come and photograph this magnificent sight.
Dr. Hawthorn's gigantic plant grew in the open, on a steep slope facing the north, on clay. Most people do not like clay as it is wet, cold and sticky. However, we should consider the fact that in the rhododendron growing area of the Pacific Coast, the rain falls in the winter and the summers are often dry. We are told that plants from the Sino-Himalayan area need copious quantities of water during the growing season. Some wild rhododendrons live where 400 inches of rain cascades to earth during the summer monsoons. In other regions upwards to 300 inches is not infrequent. Most of it comes during the growing season. In the winter the ground is either under snow or relatively dry.
I am fond of clay as a substrate for rhododendrons provided one can keep the roots above it and provided there is plenty of humus for mulching and preventing the clay from drying and caking. When one considers that one ounce of clay spread one grain thick will cover 5 acres and every grain is surrounded by a film of water, the potential for storing water for use during the growing season is considerable. Dr. Hawthorn's specimen was planted in Stanley Park on a 6 inch layer of drain rock, with no apparent soil or fertilizer.
I believe alpine rhododendrons to be short lived in gardens and to grow out of character because of the rich diet of garden soil. It is unlikely that in the conditions of a high mountain habitat they receive the nutrients found in a normal garden soil. When planted either on gravel or in gravel, with no added nourishment, they apparently receive sufficient nutrients for they grow happily. It is possible that sufficient nutrients from the soil are being carried upwards in the capillary film of moisture surrounding each rock particle. It would appear that the near perfect situation for the prostrate forms of R. forrestii var. repens would be as follows - in the open - facing north - a clay base - and several inches of gravel. There they would grow happily but probably flower as poorly as ever!
R. forrestii var. tumescens was named, from plants growing at Tower Court and Edinburgh, by Dr. Mac-Queen Cowan and Mr. Davidian in the R.H.S. 1951 Year Book. Mrs. Stevenson, in her 1951 article, divided the Tower Court specimens into 5 distinctive growth groups. It must be remembered that all their plants were grown from wild collected seed. Each plant was marked with the collectors' number. Her article was immediately followed by the MacQueen Cowan-Davidian article. It was in this revision that the easily-grown R. repens was reduced to varietal rank. At the same time, they made the split of R. forrestii var. tumescens . "It differed from R. forrestii var. repens in its dome shaped habit, with the outer branches creeping, and the larger leaves." The height of the specimens moved to the VanDusen Botanical Garden are considerably greater than those growing at Tower Court. It is possible that the former are now considerably older than were the Tower Court plants when the review was written. It is also certain that the care and attention these plants received in British Columbia was far greater than at Tower Court. A dry sand bank and no watering does not compare with the pure leaf-mold of Stanley Park plus ample watering.
If we did not know that the VanDusen plants came from three different collections, we would conclude that all the R. forrestii var. tumescens came from one large plant. This beauty is 4 feet high and 7 feet across. It required 4 men to lift it on to a pick-up truck. This plant habit may be contrasted with another that divided naturally into 9 segments each 6 inches high and 2 feet across.
This is the only Rhododendron Species we are aware of where the downward facing growth shoots grow as vigorously as any upward and outward pointing shoots. When these descending shoots reach the ground, they continue outwards. In this manner, the "dome" and the "fan" grow outwards and upwards. From the outward growths at ground level came a considerable number of layers. At the conclusion of the move, we had increased our 11 specimens to 55 plants - a remarkable and unexpected feat of propagation.
The foliage of R. forrestii var. tumescens is longer and narrower than R. forrestii . Two and one half inches long and one and one half inches across the lobes would be the average size. The largest measures 3 inches by 1 inches. Dr. Chamberlain has now concluded that this variety is of uncertain status and has temporarily set it aside. It is, he writes, very close to a new variety, R. forrestii ssp. papillatum .
In the 1951 R.H.S. review, Mrs. Stevenson drew attention to some wild collected numbered plants, which although labeled R. forrestii , were upright in habit. The height of this group varied at Tower Court from 18 inches to 39 inches. The leaves 2 inches by 1 inches are about the size and shape of R. thomsonii . Whereas this Species' leaves are smooth, those of all the R. forrestii group are heavily veined. The flat scarlet truss bears a resemblance to R. thomsonii . Because of these differences, Dr. Cowan and Mr. Davidian established a new species, R. chamaethomsonii .
Six specimens of this species were lifted in Stanley Park and transplanted to VanDusen's Sino-Himalayan garden. There, three plants were propagated by division and they now have a total of 10 plants. The tallest is 36 inches high with a 4 foot spread. The widest is 5 feet across and a little over 2 feet high. We have noted that this species does not need full exposure to flower freely.
Among the collection is one plant labeled K.W. 2\013 repens 'Rogue'. The word 'Rogue' was added by Mrs. Greig before this plant left Royston. The collectors' number refers to R. forrestii repens and was collected by Kingdon Ward in North Burma in 1953. Mrs. Stevenson had noted in her article that very often the plants grown from wild collected seed did not fit the collectors' field notes. The above specimen is an example of this tendency. In its very pendent flowers it is quite different from all our other collections of R. chamaethomsonii .
The Greig specimen of R. chamaethomsonii var. chamaethauma is represented by a sprawling plant 2 feet high and 5 feet wide. After 17 years it still retains its Royston metal label. Its habit encouraged layering and we now have a number of small plants. The flowers, instead of being the usual scarlet, are pink.
This is not quite the extent of our collection of these species. There are about 25 other plants that are different. The tallest is labeled R. repens forma . The flowers and foliage in size, shape, color and markings agree with R. forrestii . The height is 6 feet - hardly within the accepted range of size for this species.
There was another specimen only 9 inches high but nearly 3 feet across. It has small foliage like that of R. forrestii but upright growth like that of R. chamaethomsonii . About three years ago, this plant on being moved, broke up naturally into 15 sections. Each of these has sprouted from the base and these are now forming attractive bushes.
Another completely prostrate plant is nearly 3 feet across. It has never shown any tendency to grow as a compact mat but emerges sporadically from the mossy surface of the ground. The leaves are 2 inches long - somewhat larger than other forms. This plant has never flowered. The remaining are all small neat bushes around 1 2 inches in height and width, and have flowered only occasionally. It is to be hoped that at the VanDusen Botanical Garden, they will be encouraged to be more floriferous in a micro climate that is less benign. These 200 odd plants may some day provide a dazzling display in early April.
If any conclusion may be reached from the study of these many and diverse variations of R. forrestii , it is the rather obvious view that the Species is a genetically unstable group of interbreeding individuals. The shyness in flowering that is characteristic of many forms may be related to the efficiency by which the plant increases by vegetative means.
Further study might pursue two aims: first, a field study and collection of new material, and secondly, perhaps more feasible, would be a program of inbreeding within the many varied forms of Rhododendron forrestii , followed by an objective observation of the results.