name = "Rhododendron"
image_width = 250px
image_caption = "Rhododendron ponticum"
divisio = Magnoliophyta
genus = "Rhododendron"
genus_authority = L.
subdivision_ranks = Subgenera
Azaleas)----Source: [http://www.rbge.org.uk 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 1000 speciesand most have showy flower displays. It includes the plants known to gardeners as azaleas. It's the national flowerof Nepal.
The Rhododendron is a genus characterized by
shrubs and small to (rarely) large trees, the smallest species growing to 10-100 cm tall, and the largest, "R. arboreum", reported to 12 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 evergreenor 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.
Rhododendron is a very widely distributed genus, occurring throughout most of the
Northern Hemisphereexcept for dry areas, and extending into the Southern Hemispherein southeastern Asiaand northern Australasia. The highest species diversity is found in the Himalayan mountains from Uttarakhand, Nepaland Sikkimto Yunnanand Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japanand Taiwan. Rhododendron is the state tree of the state of Uttarakhandin India. In Korea, Rhododendron schlippenbachiiis particularly common (called 철쭉(choltjuk)). In addition, there are a significant number of tropical rhododendron species from southeast Asia to northern Australia, with 55 known species in Borneoand 164 in New Guinea. Argent, G. "Rhododendrons of subgenus Vireya". 2006. Royal Horticultural Society. ISBN 1-902896-61-0] Relatively fewer species occur in North Americaand Europe. Rhododendrons have not been found in South America or Africa.
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.] ".
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 Goetsch, L. A., Eckert, A. J. & Hall, B. D. (2005). [http://www.ingentaconnect.com/content/aspt/sb/2005/00000030/00000003/art00012 The molecular systematics of Rhododendron (Ericaceae): A Phylogeny based upon RPB2 gene sequences] . "Sys. Bot". 30(3): 616-626.] [ [http://www.flounder.ca/FraserSouth/Goetsch-Eckert-Hall.asp Summary of Goetsch-Eckert-Hall results] ] , 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 [http://www.rhs.org.uk/plants/registration_rhododendron.asp 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.
A sample hybrid:
*"Rhododendron" 'President Roosevelt'
Some species are poisonous to grazing animals. These Rhododendrons have a
toxincalled grayanotoxinin their pollenand nectar. People have been known to become ill from eating honeymade by bees feeding on rhododendron and azalea flowers. Xenophondescribed 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 hallucinogenicand laxativeeffect.Fact|date=July 2008 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, Turkeyduring the 1980s Nurhayat Sütlüpmar, Afife Mat and Yurdagül Satganoglu " [http://www.springerlink.com/content/w322484735r06208/ Poisoning by toxic honey in Turkey] ". Archives of Toxicology. Volume 67, Number 2, pages 148-150, February, 1993 ] . 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 landscapingin 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 Huxley, A., ed. (1992). "New RHS Dictionary of Gardening". Macmillan] . 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.
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 Statesmost of them are located on the west coast ( Oregon, Washingtonand 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. Peter A. Cox (1993). "The Cultivation of Rhododendrons". B. T. Batsford, London ISBN 0-7134-5630-2 (pp80-1) ]
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. [ [http://www.rhododendron.org/soil.htm Soil information for planting rhododendrons] ] Mulching and careful watering are important, especially before the plant is established.
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. Major diseases include "
Phytophthora" root rot, stem and twig fungal dieback; Ohio State University Extensionprovides [http://www.ag.ohio-state.edu/~ohioline/hyg-fact/3000/3043.html information on maintaining health of rhododendrons] Rhododendrons can easily be suffocated by other plants.
"Rhododendron" species are used as food plants by the
larvae of some members of the Order Lepidoptera( butterfliesand moths) (See List of Lepidoptera that feed on rhododendrons).
Rhododendron ponticum" is the national/ stateflower of Jammu and Kashmir( Indian Controlled) and Pakistan Controlled Kashmir" Rhododendron arboreum" (Lali Gurans) is the national flower of Nepal. " Rhododendron niveum" is the state tree of Sikkimin 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 Mountainsis the state flower of Washington.
*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.
List of plants poisonous to equines
*Chinese herbology 50 fundamental herbs
* [http://www.lahana.org/blog/Rhododendrons.htm Rhododendrons from Turkey, Anatolia]
* [http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=128386 Flora of China: "Rhododendron"]
* [http://www.countrysideinfo.co.uk/rhododen.htm Description of damage caused by Rhododendrons in the UK]
* [http://www.wfu.edu/~kronka/kakbek/rhododendrons.html Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.]
* [http://www.vireya.net/ Information on Vireyas]
* [http://www.rhodophiles.com/ Information on Rhododendrons by Marc Colombel, founder of the Société Bretonne du Rhododendron.]
* [http://rhodyman.net/rahome.html 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.]
* [http://rhododendron.org/ American Rhododendron Society]
* [http://www.rhs.org.uk/plants/plant_groups/rhododendrons.asp UK Royal Horticultural Society]
* [http://www.rhodygarden.org/ Rhododendron Species Foundation and Botanical Garden]
* [http://honeybee.helsinki.fi/users/avainola/rhodo/club.htm Société Finlandaise du Rhododendron]
* [http://www.eisa.net.au/~mirra/australi.html Australian Rhododendron Society]
* [http://www.rhodo.org/wissen.php?lang=gb German Rhododendron Society]
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