QBARS - v23n4 Collection and Cultivation of Sabah Rhododendrons

The Collection and Cultivation of Sabah Rhododendrons
E. F. Allen, Copdock, Suffolk, England

My wife and I have climbed and botanized on Mt. Kinabalu on two separate occasions - once during a fortnight's visit in February, 1958, shortly before my retirement from the Malayan Government Service and, more recently, in January and February, 1966. On the first trip our chief object was to climb the mountain from the south side but I collected some herbarium specimens and took a number of colored photographs so we were able to identify most of the more striking plants seen in flower. Of these, by far the most conspicuous were the many rhododendrons as this genus was seen from where the steep climb began, around 4,500 ft., to the very summit at a height of 13,455 ft.
Dr. W. Meijer (1963) has listed the following climatic zones and vegetation types for Mt. Kinabalu:

  1. The hill zone, up to 3,000 ft.
  2. The lower montane zone, 3,000 - 6,000 ft.
  3. The upper montane zone, 6,000 - 10,000 ft.
  4. The summit zone, 10,000 - 13,455 ft.

The hill zone is the home of important timber trees belonging to the Dipterocarp family and also of many tropical fruit trees. Rhododendrons occur here but are inconspicuous and mostly local. In the lower montane zone various oaks, chestnuts, figs and gymnosperms are some of the dominant upper-storey trees, while the aromatic-leaved genus, Eugenia, - the jambu of the Malays - is well represented amongst the smaller trees. By far the most striking rhododendron found here is the epiphytic form of R. brookeanum , which seems to favor a tree crutch perhaps 50 to 70 ft. above ground, almost always close to a stream, and we twice found it growing on huge granite boulders in the bed of a torrent. These epiphytes were never more than about 3 ft. tall and, in the wild, they seem only to bear the one huge head of large flowers. This inflorescence or umbel is roughly spherical, 8 in. or more across, with 13 to 15 flowers, each about 3 in. across. The corolla varies from Poppy Red (RHS 40D) to Mandarin Red (40C) and the throat is bright yellow, the two colors being sharply delineated. The flowers are quite scentless.
The upper montane zone, which may start at about 5,500 ft. on ridges, is characterized by the delightful moss or elfin forest, where tree branches are thickly covered with mosses, liverworts, club-mosses, ferns and orchids. On some ridges the peat and vegetation cover may sometimes be so thick that it is difficult to find mineral soil, so the distinction between terrestrial and epiphytic habit is not always clear. Here we found many fine species of Rhododendron , of which the most conspicuous was perhaps the ridge type of R. brookeanum , a small upright shrub commonly 5-8 ft. tall, growing in shade within the wild only 3 to 5 golden yellow waxy flowers with a delicious lemon scent. On Kinabalu these two types are so distinct that I suspect we are not the only field botanists to have taken them for separate species but Dr. H. Sleumer has assured us that our cultivated plants are varieties of the one species.
Quite common on ridges in the same zone was the very distinct R. stenophyllum , few flowered and with very narrow leaves, 3 in. long and about 1/6 in. or less-across. Both in the wild and in cultivation the leaves between pseudo-whorls soon drop off. The flowers are orange-red but a little too small to be of garden value.
In the same zone, but more locally, we found R. fallacinum , with leaves shining golden from the thickly clustered scales which are persistent on the lower surfaces. This species has beautiful coppery-orange flowers which we saw and photographed in 1958 but the plants seen in 1966 were not in flower.
Even more local was R. orbiculatum , with broadly elliptic or orbicular leaves and 4-inch wide umbels of 16-40 pure white, tubular, jasmine-scented flowers on bright pink stems, for all the world like an overgrown Daphne and very beautiful. This we found growing semi-epiphytically, close to the crest of a precipitous ridge in shady moss forest at 5,500 ft. on the east side of the steep ravine formed by the Mesilau East River. The root zone seemed to be composed of reddish brown peat and the stems grew out horizontally, with long internodes, until - with increasing light intensity - the internodes shortened and terminal flower heads were produced, usually only one head per plant in the wild. This species is clearly very adaptable as Sleumer has noted that, in N. Sarawak and Brunei, it occurs as low as 800 m (2,700 ft.) and is generally terrestrial on bare sandstone rocks in low scrubby vegetation. This adaptability affects its habit and probably explains why Sleumer records only 4-9 flowered umbels whereas our plants, in cultivation, much more compact than in the forest, produce, from 16 to 40 flowers per truss.
At about 10,000 ft. two very fine species are conspicuous. The smaller plant, R. acuminatum , has small elliptic-ovate leaves, very stiff in texture, deeply puckered (bullate) and dark with rusty-brown scales. The downy young shoots and leaves are silver grey and most attractive. One plant which I photographed carried four flower trusses when only 20 in. tall but it grows more leggy in shade. The attractive, deep pink, tubular flowers hang downwards like bells. In cultivation it has not yet flowered with us so perhaps it need to be kept cooler in summer.
The other notable species at this altitude is R. lowii , perhaps Kinabalu's finest rhododendron. We found it common only from 9,600 to 10,000 ft., where it occurs as a large epiphyte on low crutches of dwarfed trees, sometimes almost at ground level. It makes a much more massive plant than R. brookeanum , with which it is frequently confused, but it has much broader leaves, which are flatter and less acutely pointed. The very large flower heads, each with 15-17 wax-like, funnel-shaped flowers, are not carried so gracefully as in R. brookeanum , but the flowers are orange-yellow, flushed apricot, and very strongly scented. This description refers to plants seen and photographed in the Paka Cave area: it should be noted that Sleumer has recorded fewer flowers per umbel (6-12), a greater altitudinal range and the flowers as being scentless. We have three small, healthy plants in cultivation. All are slow growing and none has flowered yet. I suspect that, for glasshouse cultivation, it will always prove more difficult to manage than R. brookeanum . As against that factor, it is the more likely to survive near freezing temperatures out of doors in mild districts.
From about 11,300 ft. almost to the summit there occurs a massive tree species, albeit much dwarfed at the highest altitudes. This is R. buxifolium robustum , apparently always terrestrial and often growing amongst huge granite boulders. Some flat topped specimens seen from a distance appeared to be 20-25 ft. tall but it will flower when only 6-8 ft. high. A large specimen can be a magnificent sight as it carries hundreds of umbels of bright crimson, scentless flowers. The small ovate leaves, little more than an inch across, are clustered terminally and very densely on thick shoots. It seems to be characteristic of this species that the older and leafless branches are often densely covered with a black growth of the epiphytic alga Trentepohlia . This rhododendron has proved to be very difficult to cultivate: all our seedlings have died and none of my seed collections has germinated.
Another species which is conspicuous on the high rocky peaks is R. ericoides - well named as both its foliage and its small scarlet flowers are heath like. To a botanist it is a fascinating plant but clearly it is of less value to a gardener and I did not collect it. At the altitude where it occurs one has little excess energy and small seedlings are difficult to find while larger plants are less likely to survive transplanting. Perhaps a successful technique, if ripe seed cannot be collected, would be to root tip cuttings in situ in a suitable peat bed but that would require two visits and special permission from the Director of what is now the Kinabalu National Park.
Rainfall on Kinabalu is so heavy and frequent, and dry spells so unpredictable, that fresh ripe seed of rhododendrons is difficult to collect. We made a practice of looking for the smallest seedlings, often only some 2 in. tall, and dug them up with a fern trowel and then put them in a labeled polythene bag which was then sealed with plenty of air inside. Some collections survived in these bags for over a month in spite of the tropical heat of three days in Singapore and a very inclement February on our return to London.
As far as cultural technique is concerned we learnt by trial and error - errors being usually fatal. All are grown in a wooden-framed glasshouse, at a minimum temperature of 50°F with the maximum kept reasonably low in summer by shading and with the help of an automatic extractor fan. Before our second expedition I had acquired some experience of pot cultivation with the Maddenii hybrid 'Lady Alice Fitzwilliam'. This thrived in a potting compost made up from moss peat, perlite and topsoil from an oak wood. However, experience since then has shown that the Kinabalu species dislike perlite and we now use equal parts of chopped bracken fronds, moss peat and oak leaf soil, with or without a little pelleted bone meal. Occasional interveinal chlorosis of young plants led me to treat some seedlings with a dilute solution of iron sequestrene but this proved to be rapidly harmful and often fatal. It was most damaging to seedlings of R. orbiculatum and R. lowii , the leaves often dropping off within a day of treatment. We still have a little chlorosis of young leaves, especially with R. orbiculatum , and I now think that it is caused by a toxicity and not by a nutrient deficiency. It is well to remember that epiphytes grow slowly and need very little fertilizer, while terrestrial species from Kinabalu come from a soil which is exceptionally low in bases - the percentage base saturation of the topsoil in the moss forest being as low as 1-2 (G. P. Askew, 1964) and the pH of the order of 2.0-2.8.
To date we have flowered two specimens of R. orbiculatum , one of R. stenophyllum , three plants of the epiphytic type and one of the ridge form of R. brookeanum and one other undetermined species. Five out of five cuttings of R. orbiculatum have rooted and one out of one of R. acuminatum . Material is insufficient as yet to propagate other species from cuttings and this will always be a limiting factor applicable to R. lowii . Fortunately single node cuttings cut just above the second whorl of leaves root well and, once established, seem to grow quite rapidly.
There are said to be 58 species of Rhododendron recorded from Mt. Kinabalu (W. Meijar, 1963) but many are of interest only to a botanist. All are lepidote. Of those which we identified but failed to collect living specimens I would judge both R. fallacinum and R. polyanthemum to be of very considerable garden interest. However, I dare predict that, in the next decade, the most valuable will prove to be R. orbiculatum and good forms of R. brookeanum . Certainly the former species has shown itself to be the easiest to manage in pot cultivation and the most free flowering.

Askew, G. P., (1964), "The mountain soils of the east ride of Mt. Kinabalu," Proc. Roy. Soc., B, 161, 15-174.
Meijer, W., (1963), "A botanical guide to the flora of Mount Kinabalu". The symposium on ecological.
research in humid tropics vegetation, Kuching, Sarawak
Sleumer, H., (n. d.), "An account of Rhododendron in Malesia", P. Noordhoff N. V., Groningen, The Netherlands.