JARS v37n1 - Tropical Rhododendrons

Tropical Rhododendrons
P. G. Valder, Sydney, Australia

When people talk about tropical rhododendrons, it is usually assumed they are talking about vireyas. Now I know that when numbers are quoted there is little chance of their being exactly right, but I have estimated that, out of some something like 1,000 species of Rhododendron , approximately one third are known to occur in the tropics. About four-fifths of these are vireyas, so it is not surprising that the word vireya tends to be synonymous with tropical rhododendron. However, what I want to do in this article is to draw your attention to those tropical rhododendrons which are not vireyas.

The southern limit of the genus Rhododendron excluding vireyas.
Fig. 1 The southern limit of the genus Rhododendron

It can be seen by looking at Figure 1 that tropical rhododendrons occur only in the Asian region. They extend into the southern hemisphere as far as the northern tip of Australia, the greatest concentration of species being found in New Guinea. Figure 2 shows that tropical rhododendrons other than vireyas are confined to the northern hemisphere, the southern-most representative being found in Sri Lanka, Sumatra and Malaysia.

The southern limit of the genus Rhododendron
Fig. 2. The southern limit of the genus Rhododendron
with the exception of the Section Vireya.

It is very hard to make an exact estimate of the number of rhododendron species in the tropics, since many of them occur close to the Tropic of Cancer and it is difficult, when relying on records in the literature, to be sure about some of them. For instance Rhododendrons of China (8) lists 21 species for Guangdong, 25 for Guangxi, and 130 for Yunnan, all provinces through which the tropic passes. It is clear that the bulk of the Yunnan species occur north of the tropic, but it is not at all easy to be certain about those of Guangxi and Guangdong. Likewise Taiwan's highest mountains lie astride the tropic and it is impossible to be sure which species known from this region occur in the tropics and which do not.
On the other hand, from a horticultural point of view, all this does not matter, since many frost-tender species occur north of the tropic and a number of cold-hardy species occur on high mountains to the south of it. Still one has to draw the line somewhere, as they say, and the Tropic of Cancer seems as good a place as any. So using the list of references given, I have made an attempt, the results of which are set out in Table 1.
The revision of Cullen and Chamberlain (2, 3) has been followed for the classification of the genus and for the species names in those groups already dealt with. This means that some of the names listed for azaleas and azaleastrums may undergo changes before long. The table does not include the 270 or more species of vireyas which occur in the tropics. I have not given authorities for the names used since these can be found in Cullen and Chamberlain (2, 3) and The Rhododendron Handbook 1980 (4).
This compilation of between 50 and 60 tropical rhododendrons should be of interest to those living in mild regions where most of the rhododendrons of the temperate zone are not too happy. Not, as I have mentioned already, that coming from the tropics means that they are all suited to mild or warm climates, since some of them come from high altitudes or places which are neither hot nor cold, since they are cloudy or misty for much of the time. Still the list might be of use to breeders and those interested in introducing previously untried species from southern China, Laos, Vietnam and Sumatra.

R. veitchianum flower
R. veitchianum , fine form in bloom
on Doi Suthep, N. Thailand in January 1975
photo by P. G. Valder

There has already been much breeding work with the frost-tender species of the subsection Maddenia and, when opportunities arise, it would be worth introducing species such as crenulatum , excellens , fleuryi , levinei and surasianum . The last mentioned is recorded from the same region of northern Thailand as veitchianum , but I could find no trace of it during a visit in 1975.
The subspecies and varieties of R. arboreum are all in cultivation, but introductions from their southernmost limits might include some useful variations. A form described as delavayi from northern Thailand appears somewhat different from delavayi as it occurs in Yunnan, for instance. As far as the breeding of 'ordinary' rhododendrons for warm climates is concerned, species such as wrayi , the southern-most forms of irroratum , formosanum , and simiarum might be crossed with hardy hybrids and low altitude forms of catawbiense . Also of interest in this regard is the species described as serotinum from Pu Bia in central Laos. It is described (9) as being a tree to 20 metres, common in evergreen forests at 2300-2700 metres altitude, and flowering in April. It is obviously not the same as the late-flowering plant grown under this name in cultivation. If, as has been stated, the corollas are up to 15.5 cm in diameter, it must be very striking and would certainly be worth introducing.

R. wrayi
R. wrayi in bloom in the Cameron Highlands, Malaya
photo by P. G. Valder

The subsection Irrorata is the most tropical, if I can put it that way, of all the subsections of the subgenus Hymenanthes. The recent merging (1) of the names kontumense , ningyuenense and atjehense with that of irroratum tends to obscure the outlying occurrence of these forms from South Vietnam and northern Sumatra. They might be expected to behave very differently in cultivation to forms from further north. Another interesting species in this subsection is R. korthalsii from central Sumatra, which has been collected only once and not seen again (10).
Another unusual record is that of R. nhatrangense Dop from South Vietnam. It is known only in fruit (9) and shows characters in common with R. beesianum and R. dictyotum , now included in the subsection Taliensia. If R. nhatrangense is correctly placed in here, its locality lies far outside the area of the rest of the subsection.
Of the species of the subsection Maculifera from Taiwan, only R. morii is likely to be useful in mild climates, since R. pseudochrysanthum occurs only at high altitudes.
The best known of all tropical rhododendrons are the evergreen azaleas. In fact, up to the present, they are the only ones likely to be encountered in tropical gardens. However, the only truly tropical azaleas used in hybridizing so far are R. simsii and, to a much lesser extent, R. oldhamii , so there is much room for further work in this area. Even R. farrerae , of section Brachycalyx, might be used. Some of the forms occurring on Ma on Shan in the New Territories are most attractive.

R. farrerae
R. farrerae in bloom on Ma On Shan,
New Territories, Hong Kong.
photo by P. G. Valder

It is hardly an exaggeration to state that the subgenus Azaleastrum is tropical, since, if R. albiflorum and R. semibarbatum are excluded, its species are either tropical or occur at low altitudes. The section Euazaleastrum consists of R. ovatum , R. bachii , R. leptothrium and R. hongkongense , which are very similar and may eventually be lumped together, and R. vialii . Of these only R. hongkongense and R. vialii are known for certain to occur south of the Tropic of Cancer. R. hongkongense is a handsome plant which grows well in cultivation. R. vialii , with funnel-shaped red flowers, should be worth trying also. R. hongkongense has set seed when pollinated by evergreen and deciduous azaleas, but it remains to be seen whether the seedlings will thrive.

R. hongkongense
R. hongkongense collected
from Ma On Shan, New Territories,
Hong Kong
photo by P. G. Valder

The section Choniastrum, the old Stamineum series, contains some very beautiful plants with scented flowers. Amongst the tropical species R. westlandii is particularly handsome, with scented, lavender-pink, yellow blotched flowers up to 10 cm in diameter. There is a white form with flowers reminiscent of the deciduous azalea Persil. R. hancockii , from southeast Yunnan and Guangxi, is also white-flowered and, as described, has the largest flowers in the subgenus. R. ellipticum from Taiwan has medium-sized flowers in various shades of pink. It is late-flowering and can be very effective in the garden. It seems doubtful whether it is distinct from R. leiopodum . R. championiae has larger, thin-textured flowers which are white tinged with pink. The scales of its flower buds are extremely sticky and the leaves and young shoots are bristly.

R. championiae
R. championiae collected
from slopes of Victoria Peak
photo by P. G. Valder

R. ellipticum
R. ellipticum raised from
Dr. Creech's seeds from Taiwan
photo by P. G. Valder

As they occur in Malaya, R. moulmainense and R. klossii are very similar, with clusters of small, white, scented flowers. The form of R. moulmainense occuring on Doi Suthep in northern Thailand has larger flowers and was previously known as R. oxyphyllum . However, this species, along with several others including R. westlandii , has been included in R. moulmainense by Sleumer (7). Since, for horticultural purposes, R. westlandii is quite distinctive, I have kept it separate for the time being. R. taiense is another white-flowered species and the remainder are reported to be pale pink. I have already raised hybrids within the section Choniastrum and am awaiting their blooming. Members of the section also cross readily with evergreen azaleas, so the possibilities are quite exciting.

R. moulmainense
R. moulmainense a form
from Doi Suthep N. Thailand
photo by P. G. Valder

R. westlandii
R. westlandii in bloom on
Ma On Shan, New Territories, Hong Kong
photo by P. G. Valder

1. Cheng, Chow (c. 1974). Formosan Native Rhododendrons. Chow Cheng Orchid Gardens, Taichung, Taiwan.
2.  Cullen, J. and Chamberlain, D.F. (1978), A preliminary synopsis of the genus Rhododendron . Notes Roy. Bot. Gard. Edinburgh 36: 105-126.
3. _________(1979). A preliminary synopsis of the genus Rhododendron: II. Notes Roy. Bot. Gard. Edinburgh 37 327-328.
4.  Leslie, A. (compiler). (1980). The Rhododendron Handbook. Royal Horticultural Society, London.
5.  Patrick, J.J.R. and Hsu, C.C. (1970). Taiwan Rhododendron. A preliminary report. The Rhododendron and Camellia Yearbook 1971, Royal Horticultural Society, London, pp. 20-27.
6.  Patrick, J. (1972). More on Taiwan Rhododendron. Quar. Bull. Amer. Rhod. Soc.26(2), 127-129.
7.  Patrick, J. (1975). Field investigation of a new Taiwan rhododendron. Quar. Bull. Amer. Rhod. Soc. 29 (1), 20-25.
8.  Rhododendrons of China (1980). Translated from Inconographia Cormophytorum Sinicorum, Vol. III (1974) by Judy Young and Lu-sheng Chong. American Rhododendron Society and the Rhododendron Species Foundation. Binford and Mort, Portland, Oregon.
9.  Sleumer, H. (1958). The genus Rhododendron L. in Indochina and Siam. Blumea, Suppl. IV, 39-59.
10.  Sleumer, H. (1966). An account of Rhododendron in Malesia. P. Noordhoff Ltd. Groningen, Netherlands.
11.  Stevenson, J.B. (ed.)(193O). The Species of Rhododendron. The Rhododendron Society, London.
12.  Wilson, E.H. (1924). The rhododendrons of the Bonin and Liukiu Islands and Formosa. Rhododendron Society Notes II (5), 228-240.
13.  Wilson, E.H. (1935). The rhododendrons of Eastern China. Rhododendron Society Notes III (1), 18-28.
14.  Wilson, E.H. and Rehder, A. (1921). A monograph of Azaleas. Harvard University, Cambridge, Massachusetts.

Table 1 Tropical Rhododendrons Other Than Vireyas
Classification Species Recorded Distribution
South of the Tropic of Cancer
Subgenus Rhododendron
Section Rhododendron
Subsection Maddenia
Dalhousiae Group excellens
nuttallii aff.
Guangdong, Guangxi
N. Vietnam
Maddenii Group maddenii subsp. crassum N. Vietnam
Ciliicalyx Group burmanicum
S. Vietnam
N. & S. Vietnam, Laos, Thailand
S. Vietnam
Burma, Laos, Thailand
Subgenus Hymenanthes
Section Hymenanthes
Subsection Arborea arboreum subsp. delavayi
var. delavayi
var. peramoenum
subsp. nilagiricum
subsp. zeylanicum
Burma, S. Vietnam, Thailand
Yunnan, N. Vietnam
S. India
Sri Lanka
Subsection Argyrophylla formosanum
simiarum subsp. simiarum
Hong Kong, Guangdong, Guangxi
Subsection Falconera falconeri aff.
N. Vietnam
Subsection Fortunea serotinum Laos
Subsection Irrorata excelsum
irroratum subsp. irroratum subsp. kontumense subsp. pogonostylum
tanastylum var. tanastylum
S. Vietnam
Yunnan, S. Vietnam
S. Vietnam, N. Sumatra
N. Vietnam
Subsection Maculifera morii
Subsection Taliensia nhatrangense S. Vietnam
Subgenus Azaleastrum
Section Choniastrum cavaleriei


N. Vietnam
Hong Kong, Guangdong, Guangxi
Yunnan, Guangxi
Guangdong, Guangxi
Laos, Kampuchea, Hainan, Guangxi, Malaya
Yunnan, N. Vietnam, S. Vietnam, Kampuchea, Burma, Thailand, Malaya
Hong Kong, Guangdong
Section Euazaleastrum hongkongense
Hong Kong, Guangdong
Yunnan, Laos/S. Vietnam border
Subgenus Anthodendron
Section Brachycalyx farrerae Hong Kong, Guangdong, Guangxi
Section Tsutsutsi annamense
Laos, S. Vietnam
Hainan, N. Vietnam
Hainan, N. Vietnam
Yunnan, Thailand
Guangdong, Guangxi
N. & S. Vietnam
Taiwan, Laos, N. Vietnam, Thailand, Luzon