Rhododendron

From Wikipedia, the free encyclopedia

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

iRhododendron

Rhododendron ponticum

Scientific classification

Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Magnoliopsida
Order:	Ericales
Family:	Ericaceae
Genus:	Rhododendron L.

Subgenera

Source: RBG, Edinburgh

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 800 species. and most have showy flower displays. It includes the plants known to gardeners as azalea.

The species are 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 trumpet-shaped flowers. There are however alpine species with small flowers or with small leaves.

1 Distribution
2 Diversity
3 Toxicology
4 Trivia
5 References
6 Rhododendron Societies

[edit] Distribution

Rhododendron 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 (Argent 2006). Relatively fewer species occur in North America and Europe.

[edit] Diversity

The species are organised 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: 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.

Image:Rhododendron decorum ssp diaprepes -1.jpg

Recent genetic investigations have caused an ongoing realignment of species and groups within the genus, and also have caused the old genera Ledum to be reclassified within subgenus Rhododendron. Further realignment within the subgenera is currently proposed [1], including the merging of subgenus Hymenanthes into subgenus Pentanthera. Most species prefer acidic soil conditions; some tropical Vireyas grow as epiphytes.

Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. 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.

Sample species

A sample hybrid:

Rhododendron 'President Roosevelt'

[edit] Toxicology

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. 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 eighties [2].

[edit] Trivia

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

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