Rhododendron

For other uses, see Rhododendron (disambiguation).

Rhododendron
Rhododendron ponticum
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Ericales
Family:	Ericaceae
Genus:	Rhododendron L.
Subgenera
Azaleastrum Candidastrum Hymenanthes Mumeazalea Pentanthera (Azaleas) Rhododendron Therorhodion Tsutsuji (Azaleas) Vireya

Rhododendron (from Ancient Greek ῥόδον rhódon "rose" and δένδρον déndron "tree") is a genus of over 1 000 species of woody plants in the heath family, most with showy flowers. It includes the plants known to gardeners as azaleas.

Description

Rhododendron is a genus characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10–100 centimetres (3.9–39 in) tall, and the largest, R. giganteum, reported to over 30 metres (98 ft) tall. The leaves are spirally arranged; leaf size can range from 1–2 cm (0.39–0.79 in) to over 50 cm (20 in), exceptionally 100 cm (39 in) in R. sinogrande. They may be either evergreen or deciduous. In some species the underside of the leaves is 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 section Vireya that often grow as epiphytes. Species in this genus may be part of the heath complex in oak-heath forests in eastern North America.

Taxonomy

Rhododendron wardii var. puralbum

Rhododendron in Japan

The species are organized by subgenus, section, subsection and series.

Subdivisions

Subgenera

There are 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 110 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.

Species

A garden with tall Rhododendrons in Lynnwood, Washington

Species include:

• Rhododendron arboreum – Tree Rhododendron
• Rhododendron arborescens – Sweet Azalea
• Rhododendron atlanticum – Dwarf Azalea
• Rhododendron boninense – a rare Japanese azalea
• Rhododendron calendulaceum – Flame Azalea
• Rhododendron canadense – Rhodora
• Rhododendron catawbiense – Catawba Rhododendron
• Rhododendron chapmanii – Chapman's Rhododendron
• Rhododendron ferrugineum – Alpenrose
• Rhododendron groenlandicum – Bog Labrador Tea
• Rhododendron hirsutum – Hairy Alpenrose
• Rhododendron lochiae – Australian Rhododendron
• Rhododendron luteum – Yellow or Honeysuckle Azalea
• Rhododendron macrophyllum – Pacific Coast Rhododendron
• Rhododendron maximum – Great or American Rhododendron
• Rhododendron moulmainense – Westland's Rhododendron
• Rhododendron mucronulatum - Korean Rhododendron (Korean Rosebay)
• Rhododendron nilgiricum
• Rhododendron occidentale
• Rhododendron oreotrephes
• Rhododendron periclymenoides – Pinxterflower
• Rhododendron ponticum
• Rhododendron schlippenbachii
• Rhododendron spinuliferum
• Rhododendron tomentosum

Hybrids

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.

• Rhododendron 'President Roosevelt'

Reclassification

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, including the merging of subgenus Hymenanthes into subgenus Pentanthera.

Ecology

Distribution

Rhododendron fallacinum photographed in-situ on Mount Kinabalu, Borneo

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. Interestingly, the species in New Guinea are native to sub-alpine moist grasslands at around 3000 metres in the Central Highlands. Relatively fewer species occur in North America and Europe.

Invasive species

Some species (e.g. Rhododendron ponticum in Ireland and 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.

Insects and 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.

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

Diseases

Main article: List of rhododendron diseases

Major diseases include Phytophthora root rot, stem and twig fungal dieback; Ohio State University Extension provides information on maintaining health of rhododendrons. Rhododendrons can easily be suffocated by other plants.

Cultivation

Rhododendron-Park Bremen, Germany

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. 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.

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 that hobbyist growers might want, such as resistance to root rot when over-watered, ability to be forced into budding early, ease of rooting or other propagation, and saleability. In the Indian state of Himachal Pradesh, Rhododendron flowers, have been used for some time to make popular fruit and flower wines. The industry is promoted by the state government with tax benefits, looking to promote this industry as a full fledged sub class of its economy.

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.

Active substances

Medicinal potential

Animal studies and in vitro research has identified possible anti-inflammatory and hepatoprotective activities which may be due to the anti-oxidant effects of flavonoids, saponins and phenolic compounds the plant contains. Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB.

Toxicology

Some species of rhododendron are poisonous to grazing animals because of 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 Rhododendron ponticum during the march of the Ten Thousand in 401 BC. Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BC during the Third Mithridatic War. Later, it was recognized that honey resulting from these plants has 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. 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. The effects of Rhododendron ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution.

Culture

Symbolism

Rhododendron ponticum is the state flower of Indian-administered Kashmir and Pakistan Controlled Kashmir. Rhododendron arboreum (Lali Guransh) is the national flower of Nepal. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron is also the state tree of the state of Uttarakhand, India.

Rhododendron catawbiense, the most widespread rhododendron of the Appalachian Mountains, is the state flower of West Virginia, and is in the Flag of West Virginia. Rhododendron macrophyllum, a widespread rhododendron of the Pacific Northwest, is the state flower of Washington.

Literature

In Joyce's Ulysses, rhododendrons play an important role in Leopold and Molly's early courtship: Molly remembers them in her soliloquy - "the sun shines for you he said the day we were lying among the rhododendrons on Howth head in the grey tweed suit and his straw hat the day I got him to propose to me". Jasper Fforde a British author, also uses rhododendron as a motif throughout many of his published books. See Thursday Next series, and Shades of Grey.

Culinary

In Nepal, the flower is considered edible and enjoyed for its sour taste. The pickled flower can last for months and the flower juice is also marketed. The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones.

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.

External links

• Rhododendrons from Turkey, Anatolia
• Flora of China: Rhododendron
• German Genebank Rhododendron
• Description of damage caused by Rhododendrons in the UK
• Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.
• Information on Vireyas
• Information+photos of hybrids and species
• Information on Rhododendrons by Marc Colombel, founder of the Société Bretonne du Rhododendron.
• Extensive information on rhododendron species: the history of their discovery, botanical details, toxicity, classification, cultural conditions, care for common problems, and suggestions for companion plants by Steve Henning.
• History of Rhododendrons
• Historical Survey of Rhododendron Collecting - With Emphasis on its Close Associations with Horticulture

Rhododendron societies

• American Rhododendron Society
• The Rhododendron, Camellia & Magnolia Group of the Royal Horticultural Society
• Rhododendron Species Foundation and Botanical Garden
• Société Finlandaise du Rhododendron
• Australian Rhododendron Society
• German Rhododendron Society
• New Zealand Rhododendron Association
• Danish Rhododendron Society