This article is about the plant. For the place, see Rhododendron, Oregon. For the vessel, see M/V Rhododendron.

Rhododendron (from the Greek: rhodos, "rose"; dendron, "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 600 species. They may be either evergreen or deciduous, and most have showy flower displays. It includes the plants known to gardeners as azalea.

The species are organised by subgenus, section, subsection and series. These can be basically divided into four groups:

• Subgenus Rhododendron: small leaf or lepidotes (with scales on the underside of their leaves). Type: Rhododendron ferrugineum L.
• Subgenus Hymenanthes: large leaf or elepidotes (without scales on the underside of their leaves). Type: Rhododendron degronianum Carr.
• evergreen azaleas
• deciduous azaleas.

Rhododendron is a very widely distributed genus, but a major concentration of diversity occurs in the Sino-Himalayan mountains of southeast Asia from central Nepal and Sikkim east to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan and Taiwan. Several species occur in North America and a few in Europe. In addition, there are a significant number of tropical rhododendron species (subgenus Vireya), occuring from Nepal to northern Australia - with 55 known species in Borneo and 164 in New Guinea (Argent 2006).

Most species prefer acidic soil conditions. Some of the best known species are noted for their many clusters of large trumpet-shaped blooms and their glossy oval leaves. There are however alpine species with small flowers or with small leaves. Some species have leaves covered with scales or indumentum.

The Rhododendron is the national flower of Nepal. It is also the state flower of the US states of Washington and West Virginia, and the state of Sikkim in India.

There are over 25,000 cultivars of Rhododendron. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental stems.

Some species (e.g. Rhododendron ponticum in the United Kingdom) are invasive as introduced plants, spreading in woodland replacing the natural understory. It is difficult to eradicate, as its roots can make new shoots. Some species are poisonous to grazing animals. Some Rhododendrons have a toxin called grayanotoxin in their pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers.

Rhododendron species are used as food plants by the larvae of some Lepidoptera species - see list of Lepidoptera which feed on Rhododendron.

Image:rhododendron2.jpg Image:Wild Rhododendrons in Kashmir.jpg|Wild Rhododendrons in Kashmir by Edward Molyneux; painted before 1908

Bibliography

• Cox, Peter A. & Kenneth N.E. The Encyclopedia of Rhododendron Species. 1997. Glendoick Publishing. ISBN 095305330X.
• Dr. George Argent. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1902896610.

External links

• American Rhododendron Society website
• Description of damage caused by Rhododendrons in the UK
• Goetsch, L. A., Eckert, A. J. & Hall, B. D. (2005). The molecular systematics of Rhododendron (Ericaceae): A Phylogeny based upon RPB2 gene sequences. Sys. Bot. 30(3): 616-626.
• Summary of the proposed changes by Goetsch et al. to Rhododendron taxonomy
• Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.

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