In the 18th century, Swedish naturalist Carl Linnaeus placed azalea and rhododendron in separate genera, probably because their flowers had different numbers of stamens. Later taxonomists, recognizing the similarities between azalea and rhododendron, put them both in the genus Rhododendron. The tendency to see similarities has tempted Geoff Bryant, author of Rhododendrons and Azaleas, to maintain that azalea is merely a human construct that does not exist in nature. What humans call azaleas are really rhododendrons. This thesis ignores the fact that azaleas represent two of the eight subgenera of the genus Rhododendron. The two subgenera correspond to the two types of azaleas: deciduous azaleas, which are in the subgenus Pentanthera, and evergreen azaleas, which are in the subgenus Tsutsutai. Even these labels may be a bit misleading. To be sure, deciduous azaleas lose their leaves in autumn, but the phenomenon is more complicated for evergreen azaleas. In spring, evergreen azaleas issue forth bright green leaves, but in autumn shed them just as deciduous azaleas shed their leaves. In summer, evergreen azaleas put forth dark green leaves that grow in summer and autumn and tend to remain on the plant in winter. Exceptionally cold winters, however, may make evergreen azaleas shed all their leaves. This phenomenon has led some botanists to classify evergreen azaleas as “persistent-leaved” and to deny that a true evergreen azalea exists (Bryant 2001, 17). The gardener who wishes to retain the appearance of a true evergreen and who does not live in an exceptionally cold climate may prune the spring foliage so that all that will be left on the plant in winter are the dark green leaves that tend not to shed. Azaleas are ornamentals, chosen for their colors and ability to enhance a yard or garden.
The question of where azaleas originated has not been answered. One hypothesis, saying nothing about evergreen azaleas, holds that deciduous azaleas are native to North America, Europe, China, Japan, and Korea. Even if this is true, it says nothing about the origin of azalea. It seems possible that azaleas established themselves in both the Old and New Worlds before human habitation. The fact that azaleas are not native to Africa, the cradle of humanity, suggests that they spread to their current habitats millennia ago, when humans were yet confined to Africa. An Asian origin is possible.
If the question of origins cannot be answered, the origin of cultivation may be known. The earliest mention of azaleas comes from a Japanese poem, and the Japanese may have been the first to hybridize azaleas, possibly with rhododendrons. Since 759 CE, when azaleas were mentioned in verse, they have been a favorite of Japanese gardeners. Buddhist monks may have taken azaleas from Japan to China and other parts of Asia. If, however, azaleas are truly native to China, then these must have been secondary introductions, or the first human-assisted
migration of azaleas from Japan to China. From Asia, gardeners brought azaleas to Europe. Again, this may have been a secondary introduction. It is likely that of the azaleas that came from the gardens of China and Japan, some were hybrids. Azaleas readily hybridize with rhododendrons to yield 73 azaleodendron hybrids. Because the offspring of hybrids differ from the parents more starkly than do the progeny of pure types, many gardeners prefer true azaleas to azaleodendrons. As azaleas moved from Asia west to Europe, they also migrated east from North America to Europe. Europe, the meeting place for the disparate germplasms of Asia and the Americas, must have been the source of new varieties derived from Old and New World parents. From Virginia, the species Rhododendron viscosum was grown in England by 1680. In the 1870s, American botanist John Bartram may have introduced three species to Europe: Rhododendron perichymanoides, Rhododendron calendulaceum, and Rhododendron viscosum. The third must have been a secondary introduction. These three were hardier than the species the United Kingdom had and so gardeners bred hybrids to introduce the genes for hardiness into British germplasm.
Most evergreen azaleas may have originated in Japan, though only in the 20th century did Europeans import them. In 1918, British plant collector E. H. Wilson introduced 50 evergreen species to the United Kingdom, many of which later made their way to the United States. In 1892, botanist Charles S. Sargent at the Arnold Arboretum in Massachusetts introduced Rhododendron kaempferi, noted for its extreme hardiness, to the United States. The species has become a favorite of gardeners in the northeastern United States. Some hybrids of Rhododendron kaempferi can tolerate temperatures as low as -15°F.
In the 1870s, amateur plant breeder Anthony Watener and his son crossed European imports with Chinese azaleas, naming the new hybrids after their nursery Knap Hill. Newer still are the Windsor hybrids that Sir Eric Savill bred in the Savill Gardens, England. In the 1980s, the University of Minnesota released the Northern Lights cultivars, renowned for their ability to withstand temperatures as low as -35°F. These varieties also do well in warm climates.
Climate and Geography
In the tropics, azaleas bloom year-round. The higher the latitude the briefer is the period of bloom. To compensate for this brevity, azaleas at high latitudes reward the gardener with abundant flowers. In central Florida, azaleas bloom between October and March. In San Francisco, California, the period is between mid- September and May. The Belgian and Glenn Dale hybrids flower from August through winter. In Washington, D.C., and Seattle, Washington, they flower from mid-April to July. The Southern Indian hybrids grow well in the American South. Rhododendron kaempferi and deciduous species may be grown as far north as Maine. Gardeners may grow deciduous azaleas almost anywhere in the United States. Evergreen azaleas do well in the eastern United States, whose climate is similar to their native Japan. Surprisingly, evergreens are not hardy enough for Canada. The Canadian gardener should choose a deciduous variety.
The Soil and Its Nutrients
Azaleas evolved as compact shrubs that formed the under story of forests. Tall trees shaded them part of the day, and long tree roots absorbed nutrients at depth, leaving nutrients to be absorbed in shallower soil. Azalea roots are accordingly short, seldom longer than two feet, though they may spread as widely underground as does the part of an azalea plant above ground. Within this two-foot zone, an azalea derives water and elements. Leaf litter from nearby trees decay in the soil, making it rich in organic matter and acidic. For this reason, the gardener should add abundant organic matter to the soil. Organic matter retains moisture in the soil to the benefit of azaleas, which do not tolerate drought, though deciduous azaleas are more drought tolerant than evergreen azaleas. Azaleas also do not tolerate waterlogged soil. Azalea roots need loose soil and will not penetrate clay. A plant that languishes after first thriving has likely had its roots hit the subsoil or a high water table. Both conditions prevent roots from absorbing enough nutrients. The gardener, aware of the importance of organic matter, should dig leaves, pine needles, straw, garden debris, or manure into the soil where he intends to plant an azalea. Peat, bark, and sawdust are also sources of organic matter, but they have few nutrients. The soil pH should be 4.5 to 5.5, though azaleas tolerate a pH as high as 6.5.
Azaleas sometimes have the misfortune of being grown in soils deficient in nitrogen, iron, or magnesium. A lack of nitrogen causes azaleas to grow slowly and to yellow old foliage. To correct this deficiency the gardener may add urea in a concentration of one ounce of urea to one-and-one-half gallons of water. Too potent a concentration of urea may burn azalea roots. The gardener may also supply nitrogen by adding ammonium sulfate or ammonium nitrate to the soil. Leaves that yellow while retaining green veins betray iron or magnesium deficiency. When the soil is alkaline, iron and magnesium may be unavailable for absorption. Azaleas grown in iron- or magnesium-deficient soil should benefit from the addition of one ounce each of iron sulfate and magnesium sulfate to one-and-one-half gallons of water, applying the solution to the soil. The sulfate ions in the mixture reduce soil alkalinity. Premature leaf shedding and lack of vigor may be traced to poor drainage.
Bryant, Geoff. Rhododendrons and Azaleas. Buffalo, NY: Firefly Books, 2001.
Fell, Derek, and Fred C. Galle. All about Azaleas, Camellias and Rhododendrons. San Ramon, CA: Ortho Books, 1995
Galle, Fred C. Azaleas. Portland, OR: Timber Press, 1987.
Reiley, H. Edward. Ortho’s All about Azaleas, Camellias and Rhododendrons. Des Moines, IA: Meredith Books, 2001.
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