Rhododendrons and azaleas belong to the genus Rhododendron of the heath family (Ericaceae). The heath family includes the heaths and heathers, blueberries, mountain laurels and several other ornamental plant groups. Most members of this family require acid soil and good drainage. More than a thousand species have been described within the genus Rhododendron. These species are currently organized into subgenera, sections and subsections, with the species in each group having certain points of similarity to each other.

All azaleas are rhododendrons, with deciduous azaleas belonging to one subgenus (Pentanthera) and evergreen azaleas to another subgenus (Tsutsusi). . Many of our cultivated rhododendrons are derived from Asian species which grow in the foothills of the Himalayan mountains, western China, northern India, Burma and Assam. Others are derived from species native to Japan, Europe and eastern and western North America.

The American Rhododendron Society has published on line the Genus Rhododendron Taxonomic Tree.

The Royal Horticultural Society (RHS) is the International Cultivar Registration Authority for the genus Rhododendron. The International Rhododendron Register and Checklist, second edition, was published in November, 2004. Where the data is available the individual entries contain details of parentage, names of all those involved with the raising, naming, introduction and registration of each plant, as well as whatever descriptive information can be found.

Distribution

Species of the genus Rhododendron occur throughout moist areas of the Northern Hemisphere and into the Southern Hemisphere in southeastern Asia and northern Australasia. No species are native to South America and Africa. The highest species diversity is found in the Himalayas from Uttarakhand, Nepal and Sikkim to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan and Taiwan. Tropical rhododendron species range from southeast Asia to northern Australia, with 55 known species in Borneo and 164 in New Guinea. Relatively fewer species occur in North America and Europe.

Planting and care

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5-5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly-drained or alkaline soils, rhododendrons are often grown in raised beds using mediums such as composted pine bark. Mulching and careful watering are important, especially before the plant is established. A new calcium tolerant stock of Rhododendron (patented as "Inkarho") has been exhibited at the RHS Chelsea Flower Show in London (2011). Individual hybrids of rhododendrons have been grafted onto a root stock on a single rhododendron plant that was found growing in a chalk quarry. The root stock is able to grow in calcium rich soil up to a pH of 7.5.

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