This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Shrubs or trees with few exceptions evergreen. Flowers showy, funnel-shaped, or irregularly 5-lobed. Stamens usually 10, and declinate. Flower-buds clothed with leafy scales. Fruit capsular, splitting between the cells; seeds numerous. There are two or three North American species, several alpine and arctic in Europe and Asia; but they are found in the greatest numbers in the mountains of India. The name is of Greek derivation, signifying Rose-tree.
1. Rh. ferrugineum. Rose of the Alps. - A dwarf compact shrub about 2 feet high. Leaves oblong-lanceolate, glabrous above, rusty-scaly beneath. Flowers about 3/4 inch in diameter, rosy-red, in terminal clusters. From May to July.
2. Rh. hirsutum. Rose of the Alps. - Very much like the preceding, but the elliptical leaves are minutely toothed and ciliated, and furnished with resinous dots below.
3. Rh. ciliatum. - A very handsome and distinct species clothed with hispid hairs. Leaves oblong-lanceolate, ciliate scaly below, slightly coriaceous. Flowers large, campanulate, delicate rosy-pink and white. A Sikkim species of which, there are several fine varieties.
4. Rh. Caucasicum. - A small shrub about a yard high. Leaves obovate or lanceolate. Flowers campanulate, white within, rosy-pink outside, and spotted with green in the throat. This grows at a great elevation in the Caucasus Mountains, is perfectly hardy, and has produced several varieties superior in beauty to the typical form. The following are some of the best varieties, or perhaps, in some instances, hybrids, of this species: Prince Camille de Rohan, with large white undulated corollas finely spotted with brown; stramineum, clear pale yellow; pulchemmum, rose; and album, white.
5. Rh. Ponticum. - This is the common species of gardens, having, in the ordinary variety, pale purplish-violet spotted flowers. It is the hardiest of all the large-flowered ones, and less exacting in regard to soil and situation, and the one generally employed as a stock for grafting the tenderer kinds upon. In favourable situations it will attain a large size for a bush, occasionally a height of 20 feet with a corresponding spread of branches. There are white, scarlet, pink, and purplish violet varieties, variously spotted with yellow, green or brown, and also double-flowered ones. The most remarkable in the latter category is the variety called Vervaenum. This species is a native of Asia Minor and the Iberian peninsula, without any known intermediate stations.
Fig. 158. Rhododendron arboreum. (1/4 nat. size.)
6. Rh. arboreum (fig. 158). - This species is not quite hardy even in the most favoured localities of this country, but we give it a place here because it is sometimes planted, and because some of the varieties in cultivation are probably hybrids between this and other species. It forms naturally a small tree from 25 to 35 feet high, with thick leaves having revolute margins, glabrous above, and clothed with silvery scales beneath, and large clusters of beautiful scarlet flowers. Amongst the multitude of varieties and hybrids attributed to this species we may mention, altaclarense, a fine hardy scarlet, said to be a hybrid between this and Rh. Catawbiense. There are several wild forms referred here, as album, puniceum, roseum, cinna-mbmeum, etc. This species is a native of Nepal.
7. Rh. campanulatum. - A small shrub from 4 to 6 feet high. Leaves elliptical, glabrous above, pulverulent and fawn-coloured below. Flowers very large, campanulate, rose or white, spotted with purple at the base of the three upper lobes. This is likewise a native of Nepal, though perhaps rather hardier than the last. There are many varieties, flowering with us in March and April, and consequently often injured by the frosts.
8. Rh. maximum. - A small tree from 10 to 15 feet high. Its leaves are oblong-lanceolate, slightly revolute, glabrous above, slightly pulverulent and ferruginous beneath. Flowers in dense depressed terminal clusters of medium size, campanu-late, rose and white with a pale yellow blotch and purple-brown spots on the upper lobe, or wholly white. This is a very hardy species, a native of North America on the Atlantic coast from Carolina to Canada, and was introduced into Europe about the middle of the last century. The most familiar variety is album, whose flowers are of snowy whiteness. It has also furnished some good hybrids, amongst others Dona Maria, obtained in Belgium by crossing it with Rh. Ponticum. It is a superb and very hardy plant, equally desirable for the large size of its clusters and its pretty rosy flowers spotted with orange-red. The variety Prince Camille de Rohan, referred to above, is thought by some to be a hybrid between this and Rh. arboreum, or Rh. Caucasicum.
9. Rh. Catawbiense. - This is another North American species, originally discovered by Mr. Fraser in the neighbourhood of the source of the Catawba river in North Carolina, and introduced early in the present century. It is a bushy shrub from 3 to 6 feet high with broadly oval flat glabrescent leaves, bright green beneath. The flowers are large, campanu-late, in compact rounded clusters, pink, deep rose, or with a tinge of violet and spotted on the superior lobe. The varieties are numerous, and, as well as those of the last, very desirable on account of their flowering in Summer, when there is no danger of the blooms being injured by frost.
10. Rh. chrysanthum. - A very dwarf evergreen species with linear-lanceolate leaves clustered at the ends of the branches. Leaves narrowed at the base into a long petiole, ferruginous below. Flowers yellow, broadly campanulate, in small terminal clusters; petioles long. A native of Siberia, flowering in Summer.
11. Rh. Dahuricum (fig. 159).- A small shrub from 3 to 4 feet high. Leaves deciduous, or persistent during a part of the winter only, oval-oblong, glabrous above in the adult stage, and rusty-tomentose beneath. Flowers solitary or few together, purple or violet, appearing in February before the new leaves are developed. This is a native of the northern regions of Asia, and perfectly hardy.
Fig. 159. Rhododendron Dahuricum. (1/4 nat. size.)
In addition to the above hardy or almost hardy species there are many others of more recent introduction, generally termed Sikkim Rhododendrons, which are even more strikingly beautiful both in foliage and inflorescence; but unfortunately they are tender, requiring protection in ordinary winters. Another objection to planting them in the open ground is the early flowering season of most of the species, and consequent greater liability to injury from frost. Some of these species have tubular corollas with a rotate limb, while others have them broadly expanded and of immense size, more resembling those of a Camellia. Though these are more suitable for the temperate house, we cannot omit to notice some of the better known species.
Rh. Windsori, a small shrub from Bootan, where it grows up to an elevation of 8,000 to 9,000 feet. Leaves coriaceous, obovate-lanceolate. Flowers from a deep purple to nearly white. - Rh. Campbelliae, a tree from 30 to 35 feet high in its native country, the Sikkim Himalayas. It is very near Rh. arboreum, differing in its more coriaceous leaves, which are cordate at the base and rusty beneath. The crimson spotted flowers are in dense clusters. - Rh. argenteum, a tree of about the same dimensions as the last, with noble foliage a foot or more in length, silvery beneath. The campanulate flowers are very large, rose or white with a purple blotch at the base. - Rh. Falconeri: this species is even finer than the last, if we only consider its large shining leaves, which rival those of the Magnolia grandiflora; but its white flowers are comparatively small. - Rh. Hodgsoni, a shrub of about half the stature of the preceding, but with equally large glabrous leaves. The campanulate nearly regular rose-coloured flowers are united in clusters of fifteen to twenty, and have eight rounded lobes and about sixteen stamens. - Rh. Nuttallii, a noble tree attaining a height of about 25 feet, with large coriaceous oval leaves shining and glabrous above and covered with brown scales beneath. But what renders this species remarkable is the enormous size of its gorgeous flowers, which are deeply cam-panulate and 6 or 7 inches in diameter, white tinged with rose and bright yellow in the centre. This is perhaps the grandest of all Rhododendrons. - Rh. Madddeni, a shrubby species about 6 feet high with erect slender branches, which, as well as the under side of the leaves, are clothed with a rusty tomentum. The flowers are pure white, tubular, from 4 to 5 inches deep, with a spreading limb resembling those of Liliurn cdndidum. To this list might be added many more from the same region; but we content ourselves with naming one more, the Rh. Blandfordiaflorum, remarkable for its tubular pendulous cinnabar and orange flowers. We must not forget to mention that several hardy Japanese species have recently been introduced: Rh. Fortunei and Rh. Metternichii, with oblong or obovate coriaceous leaves rusty-tomentose beneath, and corymbose heads of campanulate rose-coloured flowers.
Notwithstanding the great diversity and beauty of the wild forms of this genus, many horticulturists - notably in England - have crossed them, and thus obtained many new and distinct varieties, usually termed hybrids. For detailed descriptions of these we must refer our readers to the nursery catalogues of the principal growers.
Rhodothamnus Chamaecistus is a handsome alpine shrub from Switzerland, having oval serrate ciliate leaves and solitary rosy flowers. It is separated from Rhododendron on account of its rotate corolla and spreading stamens.
Rhodora Canadensis, syn. Rhododendron Rhodora, is a deciduous shrub often seen in gardens, with purple sweet-scented flowers appearing before the leaves in Spring.