Rhododendron

of Western Ukraine Rhododendron is a genus of shrubs and small to (rarely) large trees, the smallest species, R. cespitosum of New Guinea, growing to tall, and the largest, R. protistum var. giganteum, reported to tall. The leaves are spirally arranged; leaf size can range from to over , exceptionally 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. A recently discovered species in New Guinea has flowers up to six inches (fifteen centimeters) in width, the largest in the whole genus. The accompanying photograph shows it as having seven petals. 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. They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote). These scales, unique to subgenus Rhododendron, are modified hairs consisting of a polygonal scale attached by a stalk. Rhododendron are characterised by having inflorescences with scarious (dry) perulae, a chromosome number of x=13, fruit that has a septicidal capsule, an ovary that is superior (or nearly so), stamens that have no appendages, and agglutinate (clumped) pollen. == Taxonomy ==

Rhododendron is the largest genus in the family Ericaceae, with over 1,000 species, (though estimates vary from 850 to 1,200) and is morphologically diverse. Consequently, the taxonomy has been historically complex. It is currently the only genus accepted in the tribe Rhodoreae, though historically several subgroups have been given generic rank, including Azalea, Ledum and Menziesia. Early history Although Rhododendrons had been known since the description of Rhododendron hirsutum by Charles de l'Écluse (Clusius) in the sixteenth century, and were known to classical writers (Magor 1990), and referred to as Chamaerhododendron (low-growing rose tree), the genus was first formally described by Linnaeus in his Species Plantarum in 1753. He listed five species under Rhododendron: R. ferrugineum (the type species), R. dauricum, R. hirsutum, R. chamaecistus (now Rhodothamnus chamaecistus (L.) Rchb.) and R. maximum. At that time he considered the then known six species of Azalea that he had described earlier in 1735 in his Systema Naturae as a separate genus. Linnaeus' six species of Azalea were Azalea indica, A. pontica, A. lutea, A. viscosa, A. lapponica and A. procumbens (now Kalmia procumbens), which he distinguished from Rhododendron by having five stamens, as opposed to ten. As new species of what are now considered Rhododendron were discovered, they were assigned to separate genera if they seemed to differ significantly from the type species. For instance Rhodora (Linnaeus 1763) for Rhododendron canadense, Vireya (Blume 1826) and Hymenanthes (Blume 1826) for Rhododendron metternichii, now R. degronianum. Meanwhile, other botanists such as Salisbury (1796) and Tate (1831) began to question the distinction between Azalea and Rhododendron, and finally in 1836, Azalea was incorporated into Rhododendron referred to this series concept as the Balfourian system. That system continued up to modern times in Davidian's four volume The Rhododendron Species. Modern classification The next major attempt at classification was by Sleumer who from 1934 began incorporating the Balfourian series into the older hierarchical structure of subgenera and sections, according to the International Code of Botanical Nomenclature, culminating in 1949 with his "Ein System der Gattung Rhododendron" and subsequent refinements. In 1986 Philipson & Philipson raised two sections of subgenus Aleastrum (Mumeazalea, Candidastrum) to subgenera, while reducing genus Therorhodion to a subgenus of Rhododendron. In 1987 Spethmann, adding phytochemical features proposed a system with fifteen subgenera grouped into three 'chorus' subgenera. A number of closely related genera had been included together with Rhododendron in a former tribe, Rhodoreae. These have been progressively incorporated into Rhododendron. Chamberlain and Rae moved the monotypic section Tsusiopsis together with the monotypic genus Tsusiophyllum into section Tsutsusi, while Kron & Judd reduced genus Ledum to a subsection of section Rhododendron. Then Judd & Kron moved two species (R. schlippenbachii and R. quinquefolium) from section Brachybachii, subgenus Tsutsusi and two from section Rhodora, subgenus Pentanthera (R. albrechtii, R. pentaphyllum) into section Sciadorhodion, subgenus Pentanthera. Finally Chamberlain brought the various systems together in 1996, with 1,025 species divided into eight subgenera. Goetsch (2005) provides a comparison of the Sleumer and Chamberlain schemata (Table 1). The major finding of Goetsch and colleagues was that all species examined (except R. camtschaticum, subgenus Therorhodion) formed three major clades which they labelled ', , and ', with the subgenera Rhododendron and Hymenanthes as monophyletic groups nested within clades ' and ', respectively. By contrast subgenera Azaleastrum and Pentanthera were polyphyletic, while R. camtschaticum appeared as a sister to all other rhododendrons. The small polyphyletic subgenera Pentanthera and Azaleastrum were divided between two clades. The four sections of Pentanthera between clades ' and ', with two each, while Azaleastrum had one section in each of ' and '. Thus subgenera Azaleastrum and Pentanthera needed to be disassembled, and Rhododendron, Hymenanthes and Tsutsusi correspondingly expanded. In addition to the two separate genera included under Rhododendron by Chamberlain (Ledum, Tsusiophyllum), Goetsch et al.. added Menziesia (clade ''''). Despite a degree of paraphyly, the subgenus Rhododendron'' was otherwise untouched with regard to its three sections but four other subgenera were eliminated and one new subgenus created, leaving a total of five subgenera in all, from eight in Chamberlain's scheme. The discontinued subgenera are Pentanthera, Tsutsusi, Candidastrum and Mumeazalea, while a new subgenus was created by elevating subgenus Azaleastrum section Choniastrum to subgenus rank. Subgenus Pentanthera (deciduous azaleas) with its four sections was dismembered by eliminating two sections and redistributing the other two between the existing subgenera in clades '''' (Hymenanthes) and '' (Azaleastrum), although the name was retained in section Pentanthera (14 species) which was moved to subgenus Hymenanthes. Of the remaining three sections, monotypic Viscidula was discontinued by moving R. nipponicum to Tsutsusi (''), while Rhodora (2 species) was itself polyphyletic and was broken up by moving R. canadense to section Pentanthera ('') and R. vaseyi to section Sciadorhodion, which then became a new section of subgenus Azaleastrum (''''). Subgenus Tsutsusi ('''') was reduced to section status retaining the name, and included in subgenus Azaleastrum. Of the three minor subgenera, all in '', two were discontinued. The single species of monotypic subgenus Candidastrum (R. albiflorum) was moved to subgenus Azaleastrum, section Sciadorhodion. Similarly the single species in monotypic subgenus Mumeazalea (R. semibarbatum) was placed in the new section Tsutsusi, subgenus Azaleastrum. Genus Menziesa (9 species) was also added to section Sciadorhodion. The remaining small subgenus Therorhodion with its two species was left intact. Thus two subgenera, Hymenanthes and Azaleastrum were expanded at the expense of four subgenera that were eliminated, although Azaleastrum lost one section (Choniastrum) as a new subgenus, since it was a distinct subclade in ''. In all, Hymenanthes increased from one to two sections, while Azaleastrum, by losing one section and gaining two increased from two to three sections. (See schemata under Subgenera.) Some subgenera contain only a single section, and some sections only a single subsection. Shown here is the traditional classification, with species number after Chamberlain (1996), but this scheme is undergoing constant revision. Revisions by Goetsch et al. (2005) and by Craven et al. (2008) shown in (parenthetical italics''). Older ranks such as Series (groups of species) are no longer used but may be found in the literature, but the American Rhododendron Society still uses a similar device, called Alliances • Subgenus Rhododendron L. (3 sections, 462 species: increased to five sections in 2008) • (Discovereya (Sleumer) Argent, raised from Vireya) • Pogonathum Aitch. & Hemsl. (13 species; Himalaya and adjacent mountains) • (Pseudovireya (C.B.Clarke) Argent, raised from Vireya) • Rhododendron L. (149 species in 25 subsections; temperate to subarctic Northern Hemisphere) • Vireya (Blume) Copel.f. (300 species in 2 subsections; tropical southeast Asia, Australasia. At one time considered separate subgenus) • Subgenus Hymenanthes (Blume) K.Koch (1 section, 224 species) (Increased to two sections) • Ponticum (24 subsections) • (Pentanthera (2 subsections – new section, moved from subgenus Pentanthera) • Subgenus Pentanthera (4 sections, 23 species) (Discontinued) • Pentanthera (2 subsections – moved to subgenus Hymenanthes) • Rhodora (L.) G. Don (2 species; Rhododendron canadense, Rhododendron vaseyi) (Discontinued, redistributed) • Sciadorhodion Rehder & Wilson (4 species) (Moved to subgenus Azaleastrum) • Viscidula Matsum. & Nakai (1 species; Rhododendron nipponicum) (Discontinued, added to section Tsutsusi, subgenus Azaleastrum) • Subgenus Tsutsusi (Sweet) Pojarkova (2 sections, 80 species) (Discontinued, reduced to section and moved to subgenus Azaleastrum) • Brachycalyx Sweet (3 alliances, 15 species) • Tsutsusi (Sweet) Pojarkova (65 species) • Subgenus Azaleastrum Planch. (2 sections, 16 species) (Increased to three sections) • Azaleastrum Planch. (5 species) • (Choniastrum Franch. (11 species) (Raised to subgenus)) • (Sciadorhodion Rehder & Wilson (4 species) (Moved from subgenus Pentanthera)) • (Tsutsusi (Sweet) Pojarkova (reduced from subgenus)) • Subgenus Candidastrum Franch. (1 species: Rhododendron albiflorum) (Discontinued, moved to section Sciadorhodion, subgenus Azaleastrum) • Subgenus Mumeazalea (Sleumer) W.R. Philipson & M.N. Philipson (1 species: Rhododendron semibarbatum) (Discontinued, moved to section Tsutsusi, subgenus Azaleastrum) • Subgenus Therorhodion A. Gray (2 species) • (Subgenus Choniastrum Franch. (11 species)) The system used by the World Flora Online uses six subgenera, four of which are divided further: • subgenus Azaleastrum • section Azaleastrum • section Sciadorhodion • section Tsutsutsi • subgenus Choniastrum • subgenus Hymenanthes • section Pentanthera • section Ponticum • section Rhodora • subgenus Rhododendron • section Pogonanthum • section Rhododendron • subgenus Therorhodion • subgenus Vireya • section Albovireya • section Discovireya • section Hadranthe • section Malayovireya • section Pseudovireya • section Schistanthe • section Siphonovireya Species == Distribution and habitat ==

photographed in situ'' on Mount Kinabalu, Borneo Species of the genus Rhododendron are widely distributed between latitudes 80°N and 20°S and are native to areas from North America to Europe, Russia, and Asia, and from Greenland to Queensland, Australia and the Solomon Islands. The centres of diversity are in the Himalayas and Maritime Southeast Asia, with the greatest species diversity in the Sino-Himalayan region, Southwest China and northern Burma, from India – Himachal Pradesh, Uttarakhand, Sikkim and Nagaland to Nepal, northwestern Yunnan and western Sichuan and southeastern Tibet. Other significant areas of diversity are in the mountains of Korea, Japan and Taiwan. More than 90% of Rhododendron sensu Chamberlain belong to the Asian subgenera Rhododendron, Hymenanthes and section Tsutsusi. Of the first two of these, the species are predominantly found in the area of the Himalayas and Southwest China (Sino-Himalayan Region). The 300 tropical species within the Vireya section of subgenus Rhododendron occupy the Maritime Southeast Asia from their presumed Southeast Asian origin to Northern Australia, with 55 known species in Borneo and 164 in New Guinea. The species in New Guinea are native to subalpine moist grasslands at around 3,000 metres above sea level in the Central Highlands. Subgenera Rhododendron and Hymenanthes, together with section Pentanthera of subgenus Pentanthera are also represented to a lesser degree in the Mountainous areas of North America and Western Eurasia. Subgenus Tsutsusi is found in the maritime regions of East Asia (Japan, Korea, Taiwan, East China), but not in North America or Eurasia. In the United States, native Rhododendron mostly occur in lowland and montane forests in the Pacific Northwest, California, the Northeast, and the Appalachian Mountains. ==Ecology==

Invasive species Rhododendron ponticum has become invasive in Ireland and the United Kingdom. It is an introduced species, spreading in woodland areas and replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots. Insects 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 (caterpillars) of some butterflies and moths. Diseases Major diseases include Phytophthora root rot, stem and twig fungal dieback. Rhododendron bud blast, a fungal condition that causes buds to turn brown and dry before they can open, is caused by the fungus Pycnostysanus azaleae, which may be brought to the plant by the rhododendron leafhopper, Graphocephala fennahi. == Conservation ==

In the UK the forerunner of the Rhododendron, Camellia and Magnolia Group (RCMG), The Rhododendron Society was founded in 1916. while in Scotland species are being conserved by the Rhododendron Species Conservation Group. == Cultivation ==

, Germany Both species and hybrid rhododendrons (including azaleas) are used extensively as ornamental plants in landscaping in many parts of the world, including both temperate and subtemperate regions. Many species and cultivars are grown commercially for the nursery trade. Rhododendrons can be propagated by air layering or stem cuttings. 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 Rhododendron arboreum and R. falconeri eventually growing to a height of or more. • Evergreen rhododendrons - large group of evergreen shrubs that vary greatly in size. Most rhododendron flowers are bell-shaped and have 10 stamens. • Vireya (Malesian) rhododendrons: epiphytic tender shrubs • Azaleas – group of shrubs which have smaller and thinner leaves than evergreen rhododendrons. They are generally medium-sized shrubs with smaller funnel-shaped flowers that usually have 5 stamens: • Deciduous hybrid azaleas: • Exbury hybrids – derived from the Knap Hill hybrids, developed by Lionel de Rothschild at the Exbury Estate in England. • Ghent (Gandavense) hybrids – Belgian raised • Knap Hill hybrids – developed by Anthony Waterer at the Knap Hill Nursery in England. • New Zealand Ilam hybrids – derived from Knap Hill/Exbury hybrids • Occidentale hybrids – English raised • Rustica Flore Pleno hybrids – sweet-scented, double-flowered • Glenn Dale hybrids – US 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 • Vuyk (Vuykiana) hybrids – raised in the Netherlands • 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 (trademarked 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. 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. Some hybrids have fragrant flowers—such as the Loderi hybrids, created by crossing Rhododendron fortunei and R. griffithianum. Other examples include the PJM hybrids, formed from a cross between Rhododendron carolinianum and R. dauricum, and named after Peter J. Mezitt of Weston Nurseries, Massachusetts. == Toxicity ==

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 mad 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 BCE. Pompey's soldiers reportedly suffered lethal casualties following the consumption of mad honey that was deliberately left behind by Pontic forces in 67 BCE 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 during the 1980s have been documented in Istanbul, Turkey. 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. Rhododendron, including its stems, leaves and flowers, contains toxins that, if ingested by a cat's stomach, can cause seizures and even coma and death. ==Uses==

Rhododendron species have long been used in traditional medicine. In Nepal, the flower is considered edible and consumed 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. Labrador tea The herbal tea called Labrador tea (not a true tea) is made from one of three closely related species: • Rhododendron tomentosum (Northern Labrador tea, previously Ledum palustre) • Rhododendron groenlandicum, (Bog Labrador tea, previously Ledum groenlandicum or Ledum latifolium) • Rhododendron neoglandulosum, (Western Labrador tea, or trapper's tea, previously Ledum glandulosum) ==In culture==

In the language of flowers, the rhododendron symbolizes danger and to beware. In Uttarakhand, in north India, the Buransh flower is deeply embedded in local culture, playing a significant role in festivals like Holi and weddings, where it is used in garlands and decorations to bless attendees. The flower is also utilized in making a healthful, antioxidant-rich juice that is popular during local festivities and summer months. Additionally, Buransh flowers are incorporated into local arts and crafts, where they are used to make colourful necklaces and jewelry, symbolizing the spiritual and physical prosperity of the community. Rhododendron arboreum (Nepali: लालीगुराँस, laligurans) is the national flower of Nepal. R. ponticum is the state flower of Indian-administered Jammu and Kashmir and Pakistan-administered Azad Kashmir. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron arboreum is also the state tree of the state of Uttarakhand, India. Pink rhododendron (Rhododendron campanulatum) is the state flower of Himachal Pradesh, India. Rhododendron is also the provincial flower of Jiangxi, China and the state flower of Nagaland, the 16th state of the Indian Union. Rhododendron maximum, the most widespread rhododendron of the Appalachian Mountains, is the state flower of the US state 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 the US state of Washington. Amongst the Zomi tribes in India and Myanmar, "Rhododendrons" called "Ngeisok" is used in a poetic manner to signify a lady., Pink Rhododendrons, 1903 In media The nineteenth-century American poet and essayist Ralph Waldo Emerson in 1834 wrote a poem titled "The Rhodora, On Being Asked, Whence Is the Flower". Rhododendrons play a role and are soliloquized in James Joyce's Ulysses. The flowers are referenced throughout Daphne Du Maurier's novel Rebecca (1938) and in Sharon Creech's young adult novel Walk Two Moons (1994). British author Jasper Fforde also uses rhododendron as a motif throughout many of his books, e.g. the Thursday Next series and Shades of Grey (2009). The effects of R. ponticum were 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, speculated to have been a part of Sherlock's fake death scheme. ==See also==

Records of the Rhododendron Society of America reside at the Albert and Shirley Small Special Collections Library at the University of Virginia. ==External links==