Tree rhododendrons

Early introductions of rhododendrons were the result of plant hunting expeditions funded by the new wealth of the industrial revolution, competing fiercely with each other to fill their conservatories with exotic horticultural curiosities. This emerging wealthy class could afford estates of the scale necessary to accommodate species impractical for the smaller suburban garden, so size was no limit. As a result, many of the hybrids bred from these earlier introductions tended also to be impractically large.

One of a number of forms of Rhododendron arboreum	New shoots of the giant Rhododendron sinogrande	One of a number of forms of Rhododendron sinogrande

With the turn of the twentieth century and the rise of the middle classes, home gardening became more popular, and the thrust turned to the breeding of compact plants with showy, colourful flowers. This trend was spurred on by the discovery of low-growing species such as the strong red prostrate R. forrestii (George Forrest, Tibet, 1905) or R. degronianum ssp. yakushimanum. Japanese collector Nakai, Yakushima Island, 1921) and interest in the larger species started to wane. The frost tenderness of many early introductions, used to growing as sheltered understorey plants, also tended to restrict their success in the cooler northern hemisphere climates.

All the so-called 'tree' rhododendrons are forest dwellers growing at altitudes of between 2,500 and 3,000 m or more. Some species grow in dense, tangled forests where they are virtually the sole species. Others dominate the understorey beneath a canopy of taller evergreen or deciduous trees, through which frosts rarely penetrate. These may be birches, magnolias, maples, oaks or chestnuts, while at higher altitudes the conifers takes over - spruces, silver firs and larches.

The size these plants attain in the wild is, to some extent, the result of the abundant moisture available to them, particularly the monsoonal summer rains found in most of the growing areas. As spring growth commences over February and March, they are amply watered by melting snows. Thus, they live in an environment with a virtually continuous supply of abundant moisture. For this reason, every effort should be made in cultivation to protect them from the drying effects of hot sun or wind as well as frost, a woodland setting being ideal.

Many will take quite a few years to reach flowering, at least twenty or more, while at least fifty years is needed for a tree rhododendron to reach its optimum form. Many are worth growing for their splendid foliage alone, particularly the charming 'candlesticks' formed by the erect young leaf shoots in spring.

Larger species tend to be elepidotes, without the covering of fine scales over the foliage which assist the plant in transpiration. The Himalayan species R. nuttallii is an exception, a lepidote in the Maddenia subsection which reaches about 10 m in ideal conditions. It makes a good choice for Australian gardens, being quite heat and sun-tolerant, although it is inclined to wilt alarmingly when conditions become dangerously dry.

Rhododendron arboreum ssp. zeylanicum	Rhododendron magnificum	Rhododendron irroratum

Easily the best known of the tree rhododendrons is the obviously named R. arboreum, which is distributed widely across the Himalayas, China, Thailand, India, Myanmar and even as far south as the highlands of Sri Lanka. It has been recorded as reaching heights of up to 20 m, although it is more usually half that height or less, with glossy, deep green leaves bearing a white or fawn indumentum underneath.

The best known form of R. arboreum has tight, globular trusses of around twenty blood red flowers, borne from July to October, although other forms are various shades of white, pink or red. The red forms tend to be more tender. A number of local variations which were formerly classified as distinct species are now regarded as subspecies of R. arboreum, including R. delavayi (Abbé Delavay, West Yunnan, 1883) and R. zeylanicum.

R. arboreum ssp. zeylanicum reaches about 10 m in the wild, with distinctive spiral bark markings, and dark green bullate leaves with a thick brown indumentum. Its flowers are normally blood red, although there are pink forms, borne in October and November. It is native to Sri Lanka and Manipur in Assam.

Also from the Arborea subsection, R. lanigerum forms a shrub or small tree to about 6 in, very similar in form to the later-flowering R. niveum. It bears large, rounded trusses of purplish, bell-shaped flowers between August and October. Its delightful young growth is covered with a creamy-white tomentum. It was first collected by Kingdon-Ward in 1928 in the Delei Valley, Assam.

R. grande was discovered by Wight in Bhutan around 1847 and is found growing at between 1,700 and 3,600 m in eastern Nepal, Sikkim, Bhutan, southern Xizang and Arunachal Pradesh, where it has been recorded at up to 15 m. Its leaves, which follow distinctive pink bud-scales on new shoots, have a woolly, silvery or fawn undumentum, and can reach as long as 45 cm. Flower trusses, which bloom between August and October, consist of up to 25 cream, pale yellow, pink or deep rose flowers, spotted and with a purple blotch. The relatively early flowering and subsequent early leaf growth of this species demands a well-sheltered site as the foliage is very susceptible to wind and frost damage. While R. grande gives its name to the Grandia subsection, other close relatives are notable as the among the largest of rhododendrons.

R. sinogrande grows at 2,100 to 4,300 m in East Arunachal Pradesh, Yunnan, upper Myanmar and southeast Xizang, and was discovered in 1912 by George Forrest in the Salwin-Kiu Chiang Divide, in southeastern Xizang. It forms a large shrub or medium-sized tree reaching about 12 m with certainly the largest, and possibly the most handsome of all rhododendron foliage.

The creamy bell-shaped flowers, borne in huge trusses around October, seem almost an afterthought to the huge, dark, leathery leaves which diminish in size as flowering age is reached. A more northerly form has soft yellow flowers. Like the other large-leaved species, R. sinogrande demands plenty of shelter but will tolerate a reasonably sunny position, although the largest foliage is borne on plants that are well shaded and growing in constantly moist soil.

Once classified as a form of R. sinogrande, the deep pink or rosy purple flowers of R. montroseanum are regarded as the finest of pinks in larger species. It reaches up to 15 in m the wild, with foliage up to 45 cm long, similar in form to both R. sinogrande and R. grande. A Kingdon-Ward discovery, it is found in the Tsangpo Gorge in southeastern Xizang at altitudes of 2,400 to 2,700 m, where it can constitute up to 50 per cent of the temperate rainforest in which it grows.

Also in the Grandia subsection, R. giganteum was discovered by George Forrest in 1919 in southwestern Yunnan. Perhaps the largest of all rhododendrons, it has been recorded at over 30 m in height, with a massive girth of trunk of almost 2.5 m at 1.5 m from ground level.

Some forms of R. barbatum grow into graceful small trees. As its name implies, twigs and leaf stalks are covered with stiff, bristly hairs. It has lovely smooth reddish bark, and, like others with smooth bark, tends to resent too much pruning. Its compact trusses of rich red flowers are borne around September.

R. macabeanum, again from the Grandia subsection, grows at 2,400 to 2,700 metres in in the Naga Hills,Assam, and Manipur in northern India, amongst birches or as compact forests on its own.

It was discovered by Sir George Watt in 1882. It reaches up to 15 m in the wild, and has leathery, dark green leaves reaching 30 cm, with a whitish woolly indumentum underneath. New shoots form a spectacular display of silvery 'candles' covered in distinctive bright red bud scales. Its huge trusses of up to twenty cream to bright or lemon yellow flowers are profusely borne in September to October. In comparison to other large-leaved species, it is relatively tolerant of wind, frost and drought, and thus makes an ideal garden plant.

R. magnificum, from around the Myanmar-Xlzang frontier at 1,500 to 2,400 metres, is very similar in form to R. prostitum, although it is recorded as being hardier and more floriferous. It was discovered by Kingdon-Ward in 1931 in the AdungValley, Myanmar, and can reach as much as 18 m in the wild, although it is always far smaller, typically to 6 m, in cultivation. It bears trusses of up to thirty floppy deep rose to crimson or reddish purple florets from August to October.

R. falconeri can be a spreading tree with several trunks reaching about 16 m, although it rarely reaches half that height. It has reddish-brown bark and its leaves which reach 30 cm or more in length have a rich brown indumentum. Young shoots have a temporary covering, of pale brown fur and its creamy yellow bell-shaped flowers flowers are borne in large rounded trusses in October and November. R. falconeri was first described by Sir Joseph Hooker in Sikkim in 1849, and is widely distributed across Nepal, Sikkim, Bhutan, western Arunachal Pradesh and Assam.

Also in the Falconera subsection, R. fictolacteum is a highly variable species, but particularly impressive is the large-leaved form from western China. It has bell-shaped white, cream or pink flowers with a purple blotch, borne in October-November. It was discovered by Abb& Delavay in 1886, growing in western Yunnan.

R. calophytum, from the Fortunea subsection, is distinguished by its distinctive and elegant long, narrow leaves. It reaches 15 m in its native western Szechuan. In September to October it carries large trusses of white or pink flowers marked by a reddish blotch.

These are by no means all of the rhododendrons which can qualify as tree species, but a representative selection. Fear of excessive height should not deter the growing of these potentially large plants in domestic gardens. In cultivation, they are very unlikely to attain proportions achieved in their natural environment, the larger broad-leaved species being understorey plants whose inclination is to grow tall and leggy to reach the scant light available.