QBARS - v15n1 Rhododendrons

J. Deans
Member of the New Zealand Rhododendron Association
Proceedings of the Conference on Ornamental Trees and Shrubs for the Home Garden

The Rhododendron genus is one of the largest of all groups of trees and shrubs, containing as it does nearly 700 species alone, not including azaleas, which are botanically allied to them. Of this huge number roughly 500 kinds are to be found in the Sino-Himalayan mountain system. The great majority of these are to the north-east of India, which borders on Burma, China and Tibet. From this centre the genus extends in four directions. The largest of these is south through Malaya, the East Indies, the Philippines, New Guinea and to the northern tip of Australia, where one species, R. lochiae , is to be found at high altitudes in Queensland. There are about 200 species in this location but, coming from tropical regions, few of them are hardy in outdoor cultivation in temperate zones. The second extension is north eastwards through Asia, Formosa, Japan and across the ocean to North America, where azaleas have intercrossed with rhododendrons. Another branch flows westward along the Himalayan mountains and extends to Europe, where, although it goes at far south as Portugal, it decreases in number to only some six species. There is another large area of very cold country to the north of Tibet-China across Siberia to Sweden where the dwarf species R. lapponicum flourishes. This kind gives a good range of beautiful colored flowers in the warmer summer months. Incidentally, the greatest concentration of rhododendron species occurs in the regions of the Himalayas where the three great rivers from the central regions of Asia, the Salween, Mekong and Yangtze cut through the mountain ranges of the Chinese-Tibetan border. These three rivers run south, almost parallel, for hundreds of miles, with the monsoonal rains lessening towards the east where a different type of species grows.
As far as is known no rhododendron species has been discovered in Africa, or South and Central America, and they are missing from New Zealand and the other adjacent islands in the South Pacific.
Today, in many places in Britain can be found a concentration of these charming shrubs, including both species and hybrids, which could never be seen in a similar area in their wild state. The breeding of hybrids commenced more than a century ago when J. D. Hooker sent back seeds from the Himalayas to England. But it was not until the 20th century when the well known explorers, Wilson, Forrest and Kingdon-Ward, amongst others, did such wonderful work in seed collection in the eastern Himalayas that the hybridization of rhododendrons reached its maximum proportions.
This work has continued to the present day, when there are many thousands of these exotic plants available for those who wish to cultivate them.
A recently published book giving the names of most of the registered rhododendrons and azaleas put their numbers, not including species, at over 8000, of which a quarter are azaleas. When you realize that there are frequent colour variants in a rhododendron species, ranging from yellow to blood-red in some kinds, it is possible to visualize what a tremendous range the genus embraces.
The importation of seeds to England has taken place over a long period and the expeditions in search of new varieties have been to a large extent financed by private individuals and horticultural societies. On receipt of seed samples these people have grown them in different places and under varied conditions so that a fairly accurate idea is available of the kinds which suit many localities.
Rhododendrons are, as a rule, not difficult to grow; the larger leafed types generally require more shade, whereas the smaller leafed ones are normally found in exposed mountain regions which may be under snow for months of the year. These latter species are rather difficult to cultivate in New Zealand, and would probably be more at home where our sub-alpine scrub grows on the mountains. However, as there is some prejudice about introducing exotic plants among our native flora, due to the possibility of their spreading, it is not likely that they will be planted in these areas.
Rhododendrons do quite well in New Zealand, particularly in the hill country in the South Island where rainfall along the foothills is plentiful and well distributed, and the colder winters are sufficiently severe to keep pests in check. Frost, however, is a source of worry to South Island growers, as the incidence of frosts is variable and much damage can be done when they occur in late spring. This frost period catches the plants at flowering time and not only ruins the blooms but can cut back the new growth and so spoil the following year's flowering. In Canterbury dry spells also can do much harm and cause the death and retarding of growth of many plants. This setback can, however, be obviated to a certain extent where the plants can be irrigated.
The best example in Canterbury of rhododendron and azalea culture is at Ilam, where the late Mr. Edgar Stead, who was a great enthusiast, did much in the way of hybridizing both kinds. The results here were all the more commendable because the climate of Christchurch is not by any means a favorable one for rhododendrons, as the rainfall is inadequate and unevenly distributed over the seasons. However, irrigation equipment has been recently installed by the University of Canterbury who now maintain Ilam. Very good results were obtained when Mr. Stead used large quantities of sawdust as mulch for his plants. This was wisely put on in the autumn, thus allowing the rains to thoroughly penetrate the covering of sawdust. This point is important since sawdust applied in the spring is apt to form a mat, which can exclude the spring and summer rains which are so necessary for a surface rooting shrub. An acid soil is the most suitable medium for the growing of rhododendrons, but they will thrive in most ground that does not contain lime. Yet in China they are doing well on limestone ridges, but it is thought that there must be some ingredient in these which is not present in the usual lime formation.
Sun is desirable for most of the smaller species and many of the hardy hybrids to promote a good flowering. The bigger leafed varieties need more shade, but perhaps not quite so much as in their native habitat which is nearer the equator. One drawback to these tree varieties is that their leaves can be scorched if exposed to a hot sun. Rhododendrons are not generally tolerant of continuous strong winds, but the alpine varieties, which grow in altitudes of up to 16,000 feet in the Himalayas and Tibet, are quite at home under extremely cold and constant winds. In cultivation these are generally planted on banks, or with a diffused side light coming onto them. Good drainage is essential as, like most shrubs, they will not thrive under water-logged conditions. Colors in rhododendrons vary greatly, many newer hybrids having quite distinctive characteristics in size of blooms and brilliance of coloring. As plants increase in age it seems that species generally keep their shape and health rather better than hybrids, which are apt to become straggly and untidy. In these cases they can often be cut back, when new growth will take place and better blooms and foliage will result. In the tropics and lower areas of warm countries, rhododendrons will grow epiphytically on old mossy tree trunks. In some cases, under very wet conditions and where the foliage is very dense, they cannot be seen from the ground and only the fallen flower petals gives an indication that the plants are in bloom overhead. Some of these species would, no doubt, thrive well on the West Coast where rainfall is abundant and forest conditions are suitable. In Hawkes Bay one is growing on top of a tree fern and so far seems healthy.
It is impossible in a short talk like this to give a description of the many varied and beautiful rhododendrons which are now in cultivation. They range in size from miniatures, which in their native habitats form mats along the ground and spread over cliffs, to the giants of the race, like Rhododendron sinogrande which grows up to 40 ft. in height, and has leaves up to 3 ft. in length. These leaves are used by the natives in some instances to thatch their huts. R. giganteum is one which attains a height of 80 ft. but does not produce such large leaves as the previously mentioned one. It unfortunately flowers early in August in Canterbury and rarely escapes frost. It may however be more hardy in the warmer parts of the North Island, such as Pukeiti on the northern slopes of Mt. Egmont. Most people are not aware that there are numerous species which produce yellow flowers, which are very attractive when seen flowering in shady conditions, in contrast to the blood-red ones which are prettier when the sun is shining through them.
The flowering period has been much extended by the introduction of some new species which in favorable seasons may bloom twice a year. These, with other hybrids such as R. x 'Nobleanum' and 'Christmas Cheer', which normally flower in late autumn and through the winter, can give a small flowering all the year round. Flowers of these winter blooming sorts should be brought indoors, because of frost. The main flowering season, however, normally finishes in February. This year at Homebush we shall be rather short of flowers due to the very heavy flowering last season.
The propagation of rhododendrons can be achieved in several ways. The easiest is by layering which is usually done by bending a branch down, and covering it over with some inches of soil. At the same time the tip of the branch must be kept well turned up. This will insure that the plants come true and, if kept moist and clear of weeds, layers will be generally well rooted in two to three years, although some particular species may take considerably longer. Grafting is another method of raising them but in this instance unwanted shoots from the stock may eventuate which, unless checked, can take charge, and in some cases kill the graft. To obtain the greatest number of plants, seeding is the best method; outdoors seeds which fall on rotten logs or mossy banks will strike quite freely if kept moist. In shade houses also, seeds can be planted in boxes with satisfactory results, provided the growing medium is sterilized and a fine spray used to keep the soil in a damp but not waterlogged condition.
It is difficult to realize the size to which some rhododendrons will grow. There is one case in England where a single bush has attained a circumference of over 100 yards. This no doubt has been brought about by self layering. It shows, however, that these shrubs, to come to full maturity, require woodland conditions.
To conclude it can be safely said that many parts of New Zealand are eminently suitable for the culture of rhododendrons and azaleas. Of the cities Dunedin, with its mild showery weather in the summer and lack of hot drying winds or excessive winter cold, perhaps has the best climate for their cultivation, but there are many other localities along the Southern Alps where they are making a very fine showing. In Taranaki and other high altitude areas in the North Island there are many suitable gardens, but in others the climate is rather too hot and disease is liable to eventuate. On the northern slopes of Mt. Egmont, at Pukeiti, a large area of native bush has been donated by private persons. A trust has been formed and extensive planting has already taken place, which promises to become one of the show places in New Zealand.
The New Zealand Rhododendron Association, with its headquarters at Massey College, is doing good work by the propagation and distribution amongst its members of many of the finest kinds of these plants.
I have not attempted to describe the virtues of azaleas in their brief paper, but can say that they are a wonderful branch of the rhododendron genus due to the brilliant coloring of the flowers in the spring, and the leaf coloring in the autumn. Also, whilst there are a number of rather dingy rhododendrons it is very difficult to find a badly colored azalea. It is my conviction that the rhododendron genus, in all its branches, provides the most wonderful range of plants for beauty and variability in flower and form, of any shrubs in the world.


A member: Can Mr. Deans tell us if anything can be done to assist the recovery of rhododendrons damaged by hail?
Mr. Deans: I take it the plants are fairly large. In this case the best method is to cut them back. Rhododendrons will make good new growth quite readily.
A member: Are rhododendrons poisonous?
Mr. Deans: Yes, they are poisonous to some animals, but not to fallow deer. (Editor's note: D.S.I.R. Bulletin 99 states that rhododendron poisoning is not common in New Zealand, but does cause respiratory failure. Bulletin 161, Ministry of Agriculture (U.K.) states that milk from cows which have fed on rhododendrons is reduced in quantity, rather bitter and has a reddish coloration).
A member: What is the cause of moss on azaleas?
Mr. Deans: Except in wet conditions the occurrence of moss is usually a sign of ill health.