Rhododendron (from Ancient Greek ῥόδον rhódon "rose" and δένδρον déndron "tree") is a genus of over 1000 species of woody plants in the heath family, either evergreen or deciduous. Most species have showy flowers.
The rhododendron is the national flower of Nepal.
Rhododendron is a genus characterized 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. giganteum, reported to over 30 m (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 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.
The species are organized by subgenus, section, subsection and series.
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.
See the List of Rhododendron species for a complete list of accepted species. Selected species include:
- Rhododendron arboreum – Tree Rhododendron
- Rhododendron arborescens – Sweet Azalea
- Rhododendron atlanticum – Dwarf 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 hippophaeoides
- Rhododendron hirsutum – Hairy Alpenrose
- Rhododendron keleticum
- 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 periclymenoides – Pinxterflower
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.
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.
Species of the genus Rhododendron are native to every continent of the world, except 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 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 subalpine moist grasslands at around 3000 metres above sea level in the Central Highlands. Relatively fewer species occur in North America and Europe.
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.
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.
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.
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. 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.
Horticulturally, rhododendrons may be divided into the following groups:-
- Evergreen rhododendrons: the main default category
- Vireya (Malesian) rhododendrons: epiphytic tender shrubs
- Azaleas (section of generally small-sized, small-leaved and small-flowered shrubs):
- Deciduous hybrid azaleas:
- Evergreen hybrid azaleas:
- Gable hybrids - raised by Joseph B. Gable in Pennsylvania, USA
- 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
- Vuyk (Vuykiana) hybrids - raised in Holland
- Azaleodendrons - semi-evergreen hybrids between deciduous azaleas and rhododendrons
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 media such as composted pine bark. 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.
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 R. ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution. 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.
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. Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB in rats.
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, and Shades of Grey.
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.
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