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Rhododendron ponticum
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Ericales
Family: Ericaceae
Genus: Rhododendron
Subgenera [1]

Rhododendron (from Ancient Greek ῥόδον rhódon "rose" and δένδρον déndron "tree")[2][3] is a genus of over 1,000 species of woody plants in the heath family, either evergreen or deciduous. Most species have showy flowers.

Azaleas make up two subgenera of Rhododendron. They are distinguished from "true" rhododendrons by having only five anthers per flower.


Rhododendron is a genus characterised by shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm (3.9–39 in) tall, and the largest, R. protistum var. giganteum, reported to 30 m (98 ft) tall.[4][5] 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 undersides 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 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.[6][7]


File:Rhododendron wardii var puralbum.jpg
Rhododendron wardii var. puralbum
File:Garden with Rhododendrons.JPG
A garden with tall Rhododendrons in Lynnwood, Washington

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


There are four large and four small subgenera:


See the List of Rhododendron species for a complete list of accepted species. Selected species include:


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 hybrids have fragrant flowers[8]—such as the Loderi hybrids, created by crossing R. fortunei and R. griffithianum.[9]


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,[10][11] including the merging of subgenus Hymenanthes into subgenus Pentanthera.



Species of the genus Rhododendron are native to Asia, North America, Europe and Australia. 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 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,Template:Why? the species in New Guinea are native to subalpine moist grasslands at around 3,000 metres above sea level in the Central Highlands.[12] 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.


A number of insects either target rhododendrons or will opportunistically attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will preferentially devour them.

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).


Major diseases include Phytophthora root rot, stem and twig fungal dieback; Ohio State University Extension provides information on maintaining health of rhododendrons.[13] Rhododendrons can easily be suffocated by other plants.


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.[14] 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.[14]

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 state and California). Large-scale commercial growing often selects for different characteristics that hobbyist growers might want, such as resistance to root rot when overwatered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.[15] 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 subclass of its economy.[16]

Horticultural divisions

Horticulturally, rhododendrons may be divided into the following groups:-[17]

  • Evergreen rhododendrons: the main default category
  • Vireya (Malesian) rhododendrons: epiphytic tender shrubs[18]
  • Azaleas (section of generally small-sized, small-leaved and small-flowered shrubs):
    • Deciduous hybrid azaleas:[19]
      • Ghent (Gandavense) hybrids - Belgian raised[20]
      • Knap Hill-Exbury hybrids - English raised[21]
      • Mollis hybrids - Dutch & Belgian raised[22]
      • New Zealand Ilam hybrids - derived from Knap Hill/Exbury hybrids
      • Occidentale hybrids - English raised
      • Rustica hybrids - sweet-scented, double-flowered
    • Evergreen hybrid azaleas:
      • Gable hybrids - raised by Joseph B. Gable in Pennsylvania, USA[23]
      • Glenn Dale hybrids - USA raised complex hybrids
      • Indian (Indica) hybrids - mostly of Belgian origin
      • Kaempferi hybrids - Dutch raised
      • Kurume hybrids - Japanese raised
      • Kyushu hybrids - very hardy Japanese azaleas (to -30°C)
      • Oldhamii hybrids - dwarf hybrids raised at Exbury, England
      • Satsuki hybrids - Japanese raised, originally for bonsai
      • Shammarello hybrids - raised in Northern Ohio, USA[24]
      • Vuyk (Vuykiana) hybrids - raised in Holland[25]
  • Azaleodendrons - semi-evergreen hybrids between deciduous azaleas and rhododendrons

Planting and care

File:Nova Zembla Rhododendron plants growing in NJ in April.jpg
Nova Zembla Rhododendrons growing in a nursery in New Jersey.

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 media such as composted pine bark.[26] Mulching and careful watering are important, especially before the plant is established.

A new calcium-tolerant stock of rhododendrons (patented as 'Inkarho') has been exhibited at the RHS Chelsea Flower Show in London (2011). Individual hybrids of rhododendrons have been grafted on to a rootstock on a single rhododendron plant that was found growing in a chalk quarry. The rootstock is able to grow in calcium-rich soil up to a pH of 7.5.[27][28]

Active substances


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 behaviour 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.[29] 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.[30] 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 R. ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution.[31] It was also mentioned in the third episode of Season 2 of BBC's Sherlock (TV series), and has been speculated to have been a part of Sherlock's fake death scheme.

Additional pharmacology

Animal studies and in vitro research has identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of flavonoids or other phenolic compounds and saponins the plant contains.[32][33][34] Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB in rats.[35]



Rhododendron arboreum (lali guransh) is the national flower of Nepal. R. ponticum is the state flower of Indian-administered Kashmir and Pakistan-controlled Kashmir. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron is also the state tree of the state of Uttarakhand, India.

Rhododendron maximum, 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.


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,[36] and Shades of Grey.[37]


The rhododendron is the national flower of Nepal, where 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. The juice of rhododendron flower is used to make a squash called burans(named after the flower)in the hilly regions of Uttarakhand. It is admired for its distinctive flavor and color.[citation needed]

See also


  1. "RBG, Edinburgh". Retrieved 2013-02-26. 
  2. Template:OEtymD
  3. Template:LSJ, Template:LSJ
  4. [1], Big Tree Rhododendron
  5. [2] Rhododendron protistum var. giganteum
  6. The Natural Communities of Virginia Classification of Ecological Community Groups (Version 2.5), Virginia Department of Conservation and Recreation, 2012
  7. 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.
  9. Davidian, H.H., "Rhododendron Species, Vol. II: Elepidotes, Part 1 Arboreum-Lacteum" Timber Press, Inc., Portland, Oregon, plate 44 & 45
  10. 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. 
  12. Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
  13. "". Retrieved 2013-02-26. 
  14. 14.0 14.1 Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan
  15. Peter A. Cox (1993). The Cultivation of Rhododendrons. B. T. Batsford, London ISBN 0-7134-5630-2 (pp80-1)
  16.[dead link]
  17. RHS A-Z encyclopedia of garden plants. United Kingdom: Dorling Kindersley. 2008. p. 1136. ISBN 1405332964. 
  18. "Vireya Rhododendrons - Welcome". Retrieved 2013-02-26. 
  19. "Deciduous Azaleas". Retrieved 2013-02-26. 
  20. "JARS v38n3 - Rescuing the Ghent and Rustica Flore Pleno Azaleas". 2012-04-03. Retrieved 2013-02-26. 
  21. "Don Hyatt's Knap Hill Azalea Page". Retrieved 2013-02-26. 
  22. "Quarterly Bulletin of the ARS - Vol 14 No 1, Living, Mollis Azaleas". 1960-01-01. Retrieved 2013-02-26. 
  23. "The Rhododendron Legacy of Joe Gable". Retrieved 2013-02-26. 
  24. "Quarterly Bulletin of the ARS - Vol 9 No 4, Baldsiefen, Shammarello's Wonderland". 1955-10-04. Retrieved 2013-02-26. 
  25. "Quarterly Bulletin of the ARS - Vol 33 No 1, Nosal, The Vuykiana Azaleas". 1979-01-01. Retrieved 2013-02-26. 
  26. "Soil information for planting rhododendrons". Retrieved 2013-02-26. 
  27. "Buy Lime Tolerant Inkarho Rhododendrons Online". Retrieved 2013-02-26. 
  28. Gardening. "The new lime-tolerant rhododendrons". Telegraph. Retrieved 2013-02-26. 
  29. U S Food & Drug Administration Center for Food Safety & Applied Nutrition Food borne Pathogenic Microorganisms and Natural Toxins 1992 (Bad Bug Book)
  30. Nurhayat Sütlüpmar, Afife Mat and Yurdagül Satganoglu (February 1993). "Poisoning by toxic honey in Turkey". Archives of Toxicology 67 (2): 148–150. doi:10.1007/BF01973687. PMID 8481104. 
  31. "Rhododendron Poison – Truth behind the science of Sherlock Holmes". A Schooner of Science. 2009-12-27. Retrieved 2013-02-26. 
  32. 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. 
  33. 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. 
  34. Agarwal, S.S.; Sharma Kalpana (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.;year=1988;volume=20;issue=2;spage=86;epage=89;aulast=Agarwal;type=0. 
  35. Xiong, Jing; Zhu, Zhonghua; Liu, Jianshe; Wang, Yang (2009). "The effect of root of rhododendron on the activation of NF-κ B in a chronic glomerulonephritis rat model". Journal of Nanjing Medical University 23: 73. doi:10.1016/S1007-4376(09)60031-9. 
  36. Fforde, Jasper (26 July 2005). Something rotten. ISBN 978-0-14-303541-1. 
  37. Jasper Fforde. "Shades of Grey - An Interview with Jasper Fforde about Shades of Grey". Retrieved 2013-02-26. 


External links

Rhododendron societies