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===Planting and care===
===Planting and care===
Like other [[Ericaceae|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 [[epiphyte]]s 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.<ref>[http://www.rhododendron.org/soil.htm Soil information for planting rhododendrons]</ref> Mulching and careful watering are important, especially before the plant is established.
Like other [[Ericaceae|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 [[epiphyte]]s 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.<ref>[http://www.rhododendron.org/soil.htm Soil information for planting rhododendrons]</ref> 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 <ref>[http://www.rhododendrons.co.uk/Category/41/lime-tolerant-rhododendrons.aspx]</ref>.


==Active substances==
==Active substances==

Revision as of 19:48, 23 May 2011

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

Subgenera [1]

Rhododendron (from Ancient Greek rhódon "rose", and déndron "tree")[2][3] is a genus of over 1000 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 (3.9–39.4 in) tall, and the largest, R. giganteum, reported to over 30 metres (98 ft) tall.[4] 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. [5][6]

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:

Species

A garden with tall Rhododendrons in Lynnwood, Washington

Species include:

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.

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,[7][8] 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.[9]

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.[10] 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.[11] 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.[12] 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.[12]

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.[13] 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.[14]

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.[15] 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 [16].

Active substances

Medicinal potential

It has been reported that the plant is of anti-inflammatory and hepatoprotective functions against related diseases, which is probably due to its anti-oxidant efficacy sourced from flavonoids, saponins and phenolic compounds,[17][18][19] Xiong et al. have found that the root of the plant is able to regulate kidney function by reducing the activity of NF-κB.[20]

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 have a slightly hallucinogenic and laxative effect.[21] 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.[22] 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 purposed way to arrange a fake execution.[23]

Culture

Symbolism

Rhododendron ponticum is the state flower of Indian-administered Kashmir and Pakistan Controlled Kashmir. Rhododendron arboreum (Lali Gurans) 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".

Footnotes

  1. ^ RBG, Edinburgh
  2. ^ Harper, Douglas. "rhododendron". Online Etymology Dictionary.
  3. ^ ῥόδον, δένδρον. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  4. ^ Wildeel.com, Tree rhododendrons
  5. ^ The Natural Communities of Virginia Classification of Ecological Community Groups (Version 2.3), Virginia Department of Conservation and Recreation, 2010
  6. ^ Schafale, M. P. and A. S. Weakley. 1990. Classification of the natural communities of North Carolina: third approximation. North Carolina Natural Heritage Program, North Carolina Division of Parks and Recreation.
  7. ^ 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. doi:10.1600/0363644054782170.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Summary of Goetsch-Eckert-Hall results
  9. ^ About Rhododendrons. Rhododendron Species Foundation.
  10. ^ Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
  11. ^ AG.ohio-state.edu
  12. ^ a b Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan
  13. ^ Peter A. Cox (1993). The Cultivation of Rhododendrons. B. T. Batsford, London ISBN 0-7134-5630-2 (pp80-1)
  14. ^ The-fuga-experience.com
  15. ^ Soil information for planting rhododendrons
  16. ^ [1]
  17. ^ Erdemoglu, Nurgun; Akkol, EK; Yesilada, E; Caliş, I (2008). "Bioassay-guided isolation of anti-inflammatory and antinociceptive principles from a folk remedy, Rhododendron ponticum L. leaves". Journal of Ethnopharmacology. 119 (1, 2): 172–8. doi:10.1016/j.jep.2008.06.021. PMID 18638535.
  18. ^ Prakash, T.; et al. (2008). "Hepatoprotective activity of leaves of Rhododendron arboreum in CCl4 induced hepatotoxicity in rats". Journal of Medicinal Plants Research. 2 (11): 315–20. {{cite journal}}: Explicit use of et al. in: |first= (help)
  19. ^ Agarwal, S.S. (1988). "Anti-inflammatory activity of flowers of Rhododendron arboreum (SMITH) in rat's hind paw oedema induced by various phlogistic agents". Indian Journal of Pharmacology. 20 (2): 86–9. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  20. ^ Xiong, Jing; et al. (2009). "The effect of root of rhododendron on the activation of NF-κ B in a chronic glomerulonephritis rat model". Retrieved 9 May 2009. {{cite web}}: Explicit use of et al. in: |first= (help) [dead link‍]
  21. ^ U S Food & Drug Administration Center for Food Safety & Applied Nutrition Food borne Pathogenic Microorganisms and Natural Toxins 1992 (Bad Bug Book)
  22. ^ Nurhayat Sütlüpmar, Afife Mat and Yurdagül Satganoglu (1993). "Poisoning by toxic honey in Turkey". Archives of Toxicology. 67 (2): 148–150. doi:10.1007/BF01973687. PMID 8481104. {{cite journal}}: Unknown parameter |month= ignored (help)
  23. ^ http://www.aschoonerofscience.com/?p=932

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.
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Rhododendron societies

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