This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The trade in Rhododendrons is extensive, and of late years many fine garden forms have been raised. Even the commonest kinds are gorgeous when carrying their clusters of tubular or bell-shaped flowers; but those with large blooms and clear and distinct colours are naturally most appreciated, especially if they are also hardy enough to stand our ordinary winters. The now common R. ponticum, from Asia Minor, has proved itself so valuable, not only as stock for the grafting of the choicer varieties, but as a game-covert plant, that it is raised in enormous numbers every year from seeds. Other kinds are largely used in the same way, and all possess the great advantage that they are immune from the attacks of rabbits. In the most favoured parts of the kingdom, like Devonshire and Cornwall, many parts of Ireland, and the south-west of Scotland many of the lovely Himalayan Rhododendrons flourish as if they were indigenous. Some valuable hybrids have been raised from these.
The old idea that Rhododendrons would flourish in a peaty soil only, and in no other, no longer holds good. Nurserymen now grow them in loam as well as in peat; but the main point is to refrain from adding lime to the soil in any form, and not to plant on a limestone soil.
Amongst the Himalayan Rhododendrons hardy in the milder parts are the following: arboreum, white, rose, or crimson, March to May; Aucklandi, pure white tinted with pink; barbatum, crimson; campanu-latum, pale lilac, spotted purple; ciliatum, reddish purple; Dalhousice, white, tinged rose; Falconeri, white; fulgens, blood red; grande, white; Hookeri, deep red; niveum, yellowish and lilac, spotted crimson; Thomsoni, blood red; and others.
Some very fine hybrids have been raised from some of these species and others, the best known being Harrisi (arboreum x Thomsoni); Kewense (Aucklandi x Hookeri); Luscombei (Fortunei x Thomsoni); Nobleanum (arboreum x caucasicum), which produces its crimson flowers in January and February; praecox (ciliatum x dauricum); Shilsoni (barbatum X Thomsoni); Wilsoni (ciliatum x glaucum); and several others.
The best of the North American Rhododendrons include albiflorum, creamy white; californicum, rose purple; catawbiense, sulphur yellow, with a white variety (Cunningham's White) which is largely used as a stock; maximum, pale rose or white, spotted red and yellow; and punc-tatum, rose. To these may be added R. Rhodora (better known as Rhodora canadensis), a deciduous shrub, 3 ft. high, with sweet-scented purple flowers.
Amongst European species are caucasicum, 3 ft., rose, white inside, spotted with green; ferrugineum, the Alpine Rose, 1 ft., scarlet or rose red, with several varieties. Closely related is R. hirsutum, 1-2 ft., pale red or scarlet; Smimowi, 3-6 ft., crimson purple.
Apart from the Sikkim Rhododendrons, Asia, including China and Japan, has also supplied some good kinds, such as Anthopogon, 1-2 ft., sulphur yellow; Collettianum, 8-10 ft., white; dauricum, 3 ft., rose, January to March; Mettemichi, rose; primulinum, 1 1/2 ft., pale yellow, &c; racemosum, 1 ft., pinkish white; yedoense, rose pink; and yunnanense, 4 ft., white or pale lilac.
LILACS FORCED INTO BLOOM IN THE DARK.
As to garden hybrid Rhododendrons, several thousands of kinds have now been raised, one of the first being altaclerense (catawbiense x ponti-cum), with brilliant scarlet blooms, in 1835. The range of colour is great, but ranges between pure white or blush white, pink and rose, purple, magenta, and claret, up to deep crimson and scarlet. Some three or four hundred varieties have received fancy names, and the reader is referred to the catalogues of specialists for these. Some names are to be found in all when the varieties are universally popular, and this rule applies with particular force to the exquisite "Pink Pearl", which has become so famous during recent years. Fuller details as to species, varieties, hybrids, etc, will be found in the Practical Guide to Garden Plants (Longmans), and in Mr. W. Watson's book on Rhododendrons and Azaleas.