RHODODENDRON. Classical writers, such as Dioscorides and Pliny, seem, from what can be ascertained, to have called the oleander (Nerium Oleander) by this name, but in modern usage it is applied to a large genus of shrubs and trees belonging to the order of heaths (Ericaceae). No adequate distinction can be drawn between this genus and Azalea (q.v.) the proposed marks of distinction, however applicable in particular cases, breaking down when tested more generally. The rhododendrons are trees or shrubs, never herbs, with simple, evergreen or deciduous leaves, and flowers in terminal clusters surrounded in the bud by bud-scales but not as a rule by true leaves. The flowers are remarkable for the frequent absence or reduced condition of the calyx. The funnel- or bell-shaped corolla, on the other hand, with its five or more lobes, is usually conspicuous, and in some species so much so as to render these plants greatly prized in gardens. The free stamens are usually ten, with slender filaments and anthers opening by pores at the top. The ovary is five- or manycelled, ripening into a long woody pod which splits from top to bottom by a number of valves, which break away from the central placenta and liberate a large number of small branlike seeds provided with a membranous wing-like appendage at each end. The species are for the most part natives of the mountainous regions of the northern hemisphere, extending as far south as the Malay Archipelago and New Guinea, but not hitherto found in South America or Australia. None are natives of Britain. They vary greatly in stature, some of the alpine species being mere pygmies with minute leaves and tiny blossoms, while some of the Himalayan species are moderate-sized trees with superb flowers. Some are epiphytal, growing on the branches of other trees, but not deriving their sustenance from them. The varieties grown in gardens are mostly grafted on the Pontic species (R. ponticum) and the Virginian R. catawbiense. The common Pontic variety is excellent for game-covert, from its hardiness, the shelter it affords, and the fact that hares and rabbits rarely eat it. Variety of colour has been infused by crossing or hybridizing the species first named, or their derivatives, with some of the more gorgeously coloured Himalayan-American varieties. In many instances this has been done without sacrifice of hardihood.
Some of the finest hybrids for the open air, especially in favoured spots, are altaclerense (scarlet); Harrisi (rosy crimson); Kewense (rose); Luscombei (rose-pink); Mangiest (white); nobleanum (crimson), one of the first to flower after Christmas; praecox ( rosepurple) ; and Shilsoni (crimson). There are almost countless colour variations of these, but one of the most exquisite of late years is that known as Pink Pearl, with large clear rosy-pink blossoms of great purity. What are termed greenhouse rhododendrons are derivatives from certain Malayan and Javanese species, and are consequently much more tender. They are characterized by the possession of a cylindrical (not funnel-shaped) flower-tube and other marks of distinction. The foliage of rhododendrons contains much tannin, and has been used medicinally. Whether the honey mentioned by Xenophon as poisonous was really derived from plants of this genus as alleged is still an open question.
Cultivation. The hardy evergreen kinds are readily propagated by seed, by layers, and by grafting. Grafting is resorted to only for the propagation of the rarer and more tender kinds. Loamy soil containing a large quantity of peat or vegetable humus is essential, the roots of all the species investigated being associated with a fungus partner (mycorhiza). An excess of lime or chalk in the soil proves fatal to rhododendrons and their allies sooner or later a fact overlooked by many amateurs. The hardy deciduous kinds are valuable for forcing, and withstand cold-storage treatment well. The tender "Malayan and Javanese species thrive in warm greenhouse temperature, but are difficult to cultivate where the water is very alkaline.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)