Rhododendron

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", and dendron, "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 1000 species and most have showy flower displays. It includes the plants known to gardeners as azaleas.

The Rhododendron is an interesting genus characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10-20 cm tall, and the largest, R. arboreum, reported to 50 m tall. The leaves are spirally arranged; leaf size can range from 1-2 cm to over 50 cm, exceptionally 100 cm in R. sinogrande. They may be either evergreen or deciduous. In some species the underside of the leaves are covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as subgenus Vireya that often grow as epiphytes.

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Distribution

Rhododendron is the national flower of Nepal. It is mostly found in Himalayan countries such as Nepal. It is a very widely distributed genus, occurring throughout most of the Northern Hemisphere except for dry areas, and extending into the Southern Hemisphere in southeastern Asia and northern Australasia. The highest species diversity is found in the Sino-Himalayan mountains 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. In addition, there are a significant number of tropical rhododendron species from southeast Asia to northern Australia, with 55 known species in Borneo and 164 in New Guinea ^[1]. Relatively fewer species occur in North America and Europe. Rhododendrons have not been found in South America or Africa.

Classification

  The species are organized by subgenus, section, subsection and series. These are currently divided into four large and four small subgenera:

• Subgenus Rhododendron L.: small leaf or lepidotes (with scales on the underside of their leaves); several hundred species, type: Rhododendron ferrugineum. The tropical rhododendrons (sect. Vireya, about 300 species) are usually included as a section in this subgenus, but sometimes split off as a ninth subgenus.
• Subgenus Hymenanthes (Blume) K.Koch: large leaf or elepidotes (without scales on the underside of their leaves); about 140 species, type: Rhododendron degronianum.
• Subgenus Pentanthera G.Don: deciduous azaleas; about 25 species, type Rhododendron luteum.
• Subgenus Tsutsusi: evergreen azaleas, about 15 species; type Rhododendron indicum.
• Subgenus Azaleastrum Planch.: five species; type Rhododendron ovatum.
• Subgenus Candidastrum (Sleumer) Philipson & Philipson: one species; Rhododendron albiflorum.
• Subgenus Mumeazalea: one species, Rhododendron semibarbatum.
• Subgenus Therorhodion: one species, Rhododendron camtschaticum.

Recent genetic investigations have caused an ongoing realignment of species and groups within the genus, and also have caused the old genus Ledum to be reclassified within subgenus Rhododendron. Further realignment within the subgenera is currently proposed ^[2][3], including the merging of subgenus Hymenanthes into subgenus Pentanthera.

Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. There are over 28,000 cultivars of Rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental bark or stems.

Some species (e.g. Rhododendron ponticum in the United Kingdom) are invasive as introduced plants, spreading in woodland areas replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

Sample species

• Rhododendron atlanticum
• Rhododendron canadense
• Rhododendron catawbiense
• Rhododendron chapmanii
• Rhododendron ferrugineum
• Rhododendron groenlandicum
• Rhododendron lochiae
• Rhododendron luteum
• Rhododendron macrophyllum
• Rhododendron maximum
• Rhododendron moulmainense
• Rhododendron occidentale
• Rhododendron ponticum
• Rhododendron schlippenbachii
• Rhododendron spinuliferum
• Rhododendron tomentosum

A sample hybrid:

• Rhododendron 'President Roosevelt'

Toxicology

Some species are poisonous to grazing animals. These 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. Xenophon described the odd behavior of Greek soldiers after having consumed honey in a village surrounded by rhododendrons. Later, it was recognized that honey resulting from these plants have a slightly hallucinogenic and laxative effect. The suspect rhododendrons are Rhododendron ponticum and Rhododendron luteum (formerly Azalea pontica), both found in northern Asia Minor. Eleven similar cases have been documented in Istanbul, Turkey during the 1980s ^[4]. Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage.

Use in landscaping

Both species and hybrid rhododendrons (including azaleas) are used extensively as ornamental plants in landscaping in many parts of the world, and many species and cultivars are grown commercially for the nursery trade. Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen ^[5]. Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings and woodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such as R. arboreum and R. falconeri eventually growing to 10-15 m or more tall ^[5].

Commercial growing

Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States most of them are located on the west coast (Oregon, Washington and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might, such as resistance to root rot when over-watered, ability to be forced into budding early, ease of rooting or other propagation, and saleability. ^[6]

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.^[7]. Mulching and careful watering are important, especially before the plant is established.

Insects and diseases

Main article: List of rhododendron diseases

There are a number of insects that either target rhododendrons or will opportunistically attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will attack rhododendrons. Major diseases include Phytophthora root rot, stem and twig fungal dieback; Ohio State University Extension provides information on maintaining health of rhododendrons.

Rhododendron species are used as food plants by the larvae of some members of the Order Lepidoptera (See List of Lepidoptera that feed on rhododendrons).

Symbolism

Rhododendron arboreum (Lali Gurans) is the national flower of Nepal. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron catawbiense, the predominant Rhododendron in the Appalachian Mountains, is the state flower of West Virginia, and is in the Flag of West Virginia. Rhododendron macrophyllum, the predominant rhododendron on the Pacific Coast and in the Cascade Mountains is the state flower of Washington.

Gallery

References

• Cox, P. A. & Kenneth, N. E. The Encyclopedia of Rhododendron Species. 1997. Glendoick Publishing. ISBN 0-9530533-0-X.
• Davidian, H. H. The Rhododendron Species. In four volumes from 1982-1995. Timber Press. ISBN 0-917304-71-3, ISBN 0-88192-109-2, ISBN 0-88192-168-8, ISBN 0-88192-311-7.

Rhododendron Societies

• American Rhododendron Society
• UK Royal Horticultural Society
• Rhododendron Species Foundation and Botanical Garden
• Société Bretonne du Rhododendron
• Société Finlandaise du Rhododendron
• Australian Rhododendron Society
• German Rhododendron Society