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.
Besides the more tender Indian Azaleas (see Vol. II, p. 130) there are several hardy species and varieties. The natural species are practically confined to botanical collections, and include arborescens, 10-20 ft., deciduous, reddish flowers; calendulacea, 2-6 ft., deciduous, yellow, red and orange; amcena, 1-3 ft., rich crimson, a magnificent evergreen quite hardy in the London area; mollis (or sinensis), 3-4 ft., deciduous, orange red, and yellow, with almost innumerable forms with a great range of yellow, orange, salmon, rose, and intermediate shades of colour; one of the most popular forms being Anthony Koster; nudiflora, 3-4 ft., deciduous, pink or purple; occidentalis, from California, produces its white sweet-scented flowers in June and July; pontica (flava), pale yellow; rhombica, bright rose purple; Schlippenbachi, bright rose; Vaseyi, pale rose to white; and viscosa, white.
The Ghent, American, or Honeysuckle Azaleas have originated from nudiflora, calendulacea, and occidentalis, and are remarkable for their fragrance and hardiness, and for their brilliant colouring from May to July. There are numerous single- and double-flowered varieties, all easily grown in a compost of peat, loam, and leaf soil. Hybridizing operations between the best forms of the Ghent Azaleas, and A. mollis, have resulted in other lovely garden forms.