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From Wikipedia, the free encyclopedia

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

Source: RBG, Edinburgh

Rhododendron (from the Greek: ροδο, rodo, meaning "rose", and δενδρο, dendro, meaning "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 1000 species and most have showy flower displays. It includes the plants known to gardeners as azaleas. It is the national flower of Nepal & State flower of Uttrakhand ,India.

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. giganteum, reported to over 30 m tall.[1] 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 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 fallacinum photographed in-situ on Mount Kinabalu, Borneo

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 Himalayan mountains 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. 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.[2] Relatively fewer species occur in North America and Europe. Very few rhododendrons have been reported from South America (Bolivia and Ecuador) and Africa (Algeria) [3].


Rhododendron wardii var. puralbum

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.
  • 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.

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 [4][5], 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 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 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 garden with tall Rhododendrons in Lynnwood, Washington
Sample species

A sample hybrid:


Some species are poisonous to grazing animals. These 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. [6] 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.[7] 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 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.[8] 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.[8]


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.[9]

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.[10] 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 Extension provides information on maintaining health of rhododendronsRhododendrons can easily be suffocated by other plants.

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).

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,[11][12][13] Xiong et al. have found that the root of the plant is able to regulate kidney function by reducing the activity of NF-κB.[14]


Rhododendron ponticum is the state flower of Indian-administered Kashmir (Indian Controlled) 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 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 Mountains is the state flower of Washington.



  1. ^ Tree rhododendrons
  2. ^ Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
  3. ^ Global Biodiversity Information Facility. http://data.gbif.org
  4. ^ 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.
  5. ^ Summary of Goetsch-Eckert-Hall results
  6. ^ U S Food & Drug Administration Center for Food Safety & Applied Nutrition Food borne Pathogenic Microorganisms and Natural Toxins 1992 (Bad Bug Book)
  7. ^ Nurhayat Sütlüpmar, Afife Mat and Yurdagül Satganoglu Poisoning by toxic honey in Turkey. Archives of Toxicology. Volume 67, Number 2, pages 148-150, February, 1993
  8. ^ a b Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan
  9. ^ Peter A. Cox (1993). The Cultivation of Rhododendrons. B. T. Batsford, London ISBN 0-7134-5630-2 (pp80-1)
  10. ^ Soil information for planting rhododendrons
  11. ^ Erdemoglu, Nurgun et al. (2008). "Bioassay-guided isolation of anti-inflammatory and antinociceptive principles from a folk remedy, Rhododendron ponticum L. leaves". Journal of Ethnopharmacology (Academic journal) 119 (1, 2): 172–178. doi:10.1016/j.jep.2008.06.021.  
  12. ^ Prakash, T. et al. (2008). "Hepatoprotective activity of leaves of Rhododendron arboreum in CCl4 induced hepatotoxicity in rats". Journal of Medicinal Plants Research (Academic journal) 2 (11): 315 – 320. http://www.academicjournals.org/jmpr/abstracts/abstracts/abstrats2008/Nov/Prakash%20et%20al.htm. Retrieved 9 May 2009.  
  13. ^ 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): 86–89. http://www.ijp-online.com/article.asp?issn=0253-7613;year=1988;volume=20;issue=2;spage=86;epage=89;aulast=Agarwal;type=0. Retrieved 9 May 2009.  
  14. ^ Xiong, Jing et al. (2009). "The effect of root of rhododendron on the activation of NF-κ B in a chronic glomerulonephritis rat model". http://d.wanfangdata.com.cn/Periodical_njykdxxb-e200901015.aspx. Retrieved 9 May 2009.  

See also


  • 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

Rhododendron societies

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

RHODODENDRON. Classical writers, such ' as Dioscorides and Pliny, seem, from what can, be ascertained, to have called the oleander (Nerium Oleander) by this name, but in modern usage it is applied to a large genus of shrubs and trees belonging to the order of heaths (Ericaceae). No adequate distinction can be drawn between this genus and Azalea- the proposed marks of distinction, however applicable in particular cases, breaking down when tested more generally. The rhododendrons are trees or shrubs, never herbs, with simple, evergreen or deciduous leaves, and flowers in terminal clusters surrounded in the bud by bud-scales but not as a rule by true leaves. The flowers are remarkable for the frequent absence or reduced condition of the calyx. The funnelor bell-shaped corolla, on the other hand, with its five or more lobes, is usually conspicuous, and in some species so much so as to render these plants greatly prized in gardens. The free stamens are usually ten, with slender filaments and anthers opening by pores at the top. The ovary is fiveor manycelled, ripening into a long woody pod which splits from top to bottom by a number of valves, which break away from the central placenta and liberate a large number of small branlike seeds provided with a membranous wing-like appendage at each end. The species are for the most part natives of the mountainous regions of the northern hemisphere, extending as far south as the Malay Archipelago and New Guinea, but not hitherto found in South America or Australia. None are natives of Britain. They vary greatly in stature, some of the alpine species being mere pygmies with minute leaves and tiny blossoms, while some of the Himalayan (-species are moderate-sized trees with superb flowers. ,v1 So/ire-rare epiphytal, growing on the branches of other trees, but not deriving their sustenance from them. The varieties grown in gardens are mostly grafted on the Pontic species (R. ponticum) and the Virginian R. catawbiense. The common Pontic variety is excellent for game-covert, from its hardiness, the shelter it affords, and the fact that hares and rabbits rarely eat it. Variety of colour has been infused by crossing or hybridizing the species first named, or their derivatives, with some of the more gorgeously coloured Himalayan-American varieties. In many instances this has been done without sacrifice of hardihood.

Some of the finest hybrids for the open air, especially in favoured spots, are altaclerense (scarlet); Harrisi (rosy crimson); Kewense (rose); Luscombei (rose-pink); Manglesi (white); nobleanum (crimson), one of the first to flower after Christmas; praecox (rosepurple); and Shilsoni (crimson). There are almost countless colour variations of these, but one of the most exquisite of late years is that known as Pink Pearl, with large clear rosy-pink blossoms of great purity. What are termed greenhouse rhododendrons are derivatives from certain Malayan and Javanese species, and are consequently much more tender. They are characterized by the possession of a cylindrical (not funnel-shaped) flower-tube and other marks of distinction. The foliage of rhododendrons contains much tannin, and has been used medicinally. Whether the honey mentioned by Xenophon as poisonous was really derived from plants of this genus as alleged is still an open question.


The hardy evergreen kinds are readily propagated by seed, by layers, and by grafting. Grafting is resorted to only for the propagation of the rarer and more tender kinds. Loamy soil containing a large quantity of peat or vegetable humus is essential, the roots of all the species investigated being associated with a fungus partner (mycorhiza). An excess of lime or chalk in the soil proves fatal to rhododendrons and their allies sooner or later - a fact overlooked by many amateurs. The hardy deciduous kinds are valuable for forcing, and withstand cold-storage treatment well. The tender Malayan and Javanese species thrive in warm greenhouse temperature, but are difficult to cultivate where the water is very alkaline.

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also rhododendron



Rhododendron repens


From Ancient Greek ῥόδον (rhodon), rose) + δένδρον (dendron), tree).

Proper noun

Wikipedia has an article on:



  1. (botany) a botanical name at the rank of genus.
Wikispecies has information on:


See also

  • Azaleastrum
  • Candidastrum
  • Hymenanthes
  • Mumeazalea
  • Pentanthera
  • Therorhodion
  • Tsutsusi




Rhododendron m.

  1. rhododendron (flowering shrub in the genus Rhododendron)

This German entry was created from the translations listed at rhododendron. It may be less reliable than other entries, and may be missing parts of speech or additional senses. Please also see Rhododendron in the German Wiktionary. This notice will be removed when the entry is checked. (more information) December 2009


Up to date as of January 23, 2010

From Wikispecies


Classification System: APG II (down to family level)

Main Page
Cladus: Eukaryota
Regnum: Plantae
Cladus: Angiospermae
Cladus: Eudicots
Cladus: core eudicots
Cladus: Asterids
Ordo: Unassigned Asterids
Ordo: Ericales
Familia: Ericaceae
Subfamilia: Ericoideae
Tribus: Rhodoreae
Genus: Rhododendron
Subgenera: R. subg.  Azuleastrum - R. subg.  Candidastrum - R. subg.  Hymenanthes - R. subg.  Mumeazalea - R. subg.  Pentanthera - R. subg.  Rhododendron - R. subg.  Tsutsusi - R. subg. Vireya - Unassigned Rhododendron


Rhododendron L.

Species Overview

R. aberconwayi - R. acuminatum - R. adamsii - R. adenogynum - R. aganniphum - R. agastum - R. alabamense - R. albertsenianum - R. albiflorum - R. albrechtii - R. alutaceum - R. amagianum - R. ambiguum - R. anagalliflorum - R. annae - R. anthopogon - R. anthosphaerum - R. aperantum - R. araiophyllum - R. arborescens - R. arboreum - R. argyrophyllum - R. arizelum - R. atlanticum - R. atropurpureum - R. augustinii - R. aureum - R. auriculatum - R. aurigeranum - R. auritum - R. austrinum - R. baileyi - R. bainbridgeanum - R. balfourianum - R. barbatum - R. basilicum - R. beesianum - R. beyerinckianum - R. boothii - R. brachyanthum - R. brachycarpum - R. brookeanum - R. bureavii - R. burmanicum - R. calendulaceum - R. callimorphum - R. calophytum - R. calostrotum - R. camelliiflorum - R. campanulatum - R. campylocarpum - R. campylogynum - R. canadense - R. canescens - R. capitatum - R. carneum - R. carringtoniae - R. catacosmum - R. catawbiense - R. caucasicum - R. cephalanthum - R. cerasinum - R. chamaethomsonii - R. charitopes - R. christianae - R. christii - R. chrysodoron - R. chrysolepis - R. ciliatum - R. ciliicalyx - R. cinnabarinum - R. citriniflorum - R. citrinum - R. clementinae - R. commonae - R. concinnum - R. coriaceum - R. coryanum - R. crinigerum - R. cruttwellii - R. cuffeanum - R. culminicola - R. cumberlandense - R. cuneatum - R. dalhousiae - R. dasypetalum - R. dauricum - R. davidsonianum - R. decandrum - R. decorum - R. degronianum - R. dendricola - R. dichroanthum - R. dielsianum - R. dilatatum - R. diversipilosum - R. eclecteum - R. edgarianum - R. edgeworthii - R. elliottii - R. erastum - R. ericoides - R. eriocarpum - R. eudoxum - R. exasperatum - R. facetum - R. falconeri - R. fallacinum - R. farrerae - R. fastigiatum - R. ferrugineum - R. flammeum - R. floccigerum - R. formosanum - R. formosum - R. forrestii - R. fortunei - R. fulgens - R. fulvum - R. galactinum - R. gaultheriifolium - R. genestierianum - R. glaucophyllum - R. glischrum - R. gracilentum - R. grande - R. griersonianum - R. griffithianum - R. groenlandicum - R. habrotrichum - R. haematodes - R. heliolepis - R. hemitrichotum - R. herzogii - R. hippophaeoides - R. hirsutum - R. hirtipes - R. hodgsonii - R.hongkongense - R. hookeri - R. hylaeum - R. hyperythrum - R. hypoleucum - R. impeditum - R. inconspicuum - R. indicum - R. insculptum - R. insigne - R. intricatum - R. irroratum - R. jasminiflorum - R. javanicum - R. jenestierianum - R. johnstoneanum - R. kaempferi - R. kanehirae - R. kasoense - R. kawakamii - R. keiskei - R. keysii - R. kiusianum - R. kiyosumense - R. komiyamae - R. konori - R. kyawi - R. lacteum - R. laetum - R. lanatum - R. lanigerum - R. lapponicum - R. lasiostylum - R. ledebourii - R. lepidostylum - R. lepidotum - R. leptanthum - R. leptocarpum - R. leptothrium - R. leucaspis - R. lindaueanum - R. lindleyi - R. lochiae - R. longiflorum - R. longistylum - R. lowii - R. lowndesii - R. ludlowii - R. lukiangense - R. luteosquamatum - R. lutescens - R. luteum - R. macabeanum - R. macgregoriae - R. macrocarpum - R. macrophyllum - R. macrosepalum - R. maddenii - R. magnificum - R. makinoi - R. mariesii - R. martinianum - R. maximum - R. maxwellii - R. mayebarae - R. megacalyx - R. megeratum - R. micranthum - R. microphyton - R. mimetes - R. minus - R. molle - R. mollicomum - R. morii - R. moulmainense - R. moupinense - R. mucronatum - R. mucronulatum - R. multicolor - R. multinervium - R. nakaharae - R. neoglandulosum - R. neriiflorum - R. nipponicum - R. nivale - R. niveum - R. nudipes - R. nummatum - R. nuttallii - R. obtusum - R. occidentale - R. oldhamii - R. orbiculare - R. oreodoxa - R. oreotrephes - R. ovatum - R. pachypodum - R. pemakoense - R. pentaphyllum - R. periclymenoides - R. phaeochitum - R. phaeochrysum - R. planetum - R. pleianthum - R. pocophorum - R. polycladum - R. ponticum - R. praestans - R. praevernum - R. primuliflorum - R. prinophyllum - R. pronum - R. proteoides - R. protistum - R. pruniflorum - R. prunifolium - R. przewalskii - R. pseudochrysanthum - R. pubescens - R. pumilum - R. purdomii - R. quinquefolium - R. racemosum - R. rarum - R. recurvoides - R. reticulatum - R. retusum - R. rex - R. rigidum - R. roxieanum - R. rubiginosum - R. rubropilosum - R. rufescens - R. rufohirtum - R. rufum - R. rugosum - R. rupicola - R. russatum - R. saluenense - R. sanctum - R. sanguineum - R. sargentianum - R. scabridibracteum - R. scabrum - R. schlippenbachii - R. searsiae - R. seinghkuense - R. selense - R. semibarbatum - R. serpyllifolium - R. setosum - R. sichotense - R. sidereum - R. siderophyllum - R. simiarum - R. simsii - R. sinogrande - R. smirnowii - R. souliei - R. sperabile - R. sphaeroblastum - R. spinuliferum - R. stewartianum - R. strigillosum - R. subarcticum - R. subsessile - R. subulatum - R. superbum - R. sutchuenense - R. taggianum - R. taliense - R. tanastylum - R. tashiroi - R. telmateium - R. temenium - R. tephropeplum - R. thomsonii - R. tolmachevii - R. tomentosum - R. tosaense - R. traillianum - R. trichocladum - R. trichostomum - R. triflorum - R. triumphans - R. tschonoskii - R. tsusiophyllum - R. ungernii - R. uniflorum - R. uvarifolium - R. vaccinioides - R. valentinianum - R. vaseyi - R. veitchianum - R. vernicosum - R. vesiculiferum - R. vidalii - R. virgatum - R. viscosum - R. vitis-idaea - R. wallichii - R. wardii - R. wasonii - R. weyrichii - R. wightii - R. williamsianum - R. womersleyi - R. yedoense - R. yelliotii - R. yungningense - R. yunnanense - R. zaleucum - R. zoelleri

Vernacular names

Česky: Rododendron; Pěnišník
Deutsch: Rhododendren
Galego: Rododendro
日本語: ツツジ属
Türkçe: Ormangülü
中文: 杜鵑花屬
Wikimedia Commons For more multimedia, look at Category:Rhododendron on Wikimedia Commons.

Simple English

Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Ericales
Family: Ericaceae
Genus: Rhododendron

Rhododendron is a large genus of flowering plants.

Most of them display their flowers quite aptly. What gardeners know as Azalea is in fact a kind of rhododendron. Many people have rhododendrons in their garden or flower pot. It is the state flower of Kashmir.


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