Rhododendron at Fakding, Nepal.jpg

''Rhododendron'' (; from Ancient Greek ''rhódon'' "rose" and ''déndron'' "tree") is a very large genus of about 1,024 species of woody plants in the heath family (Ericaceae). They can be either evergreen or deciduous. Most species are native to eastern Asia and the Himalayan region, but smaller numbers occur elsewhere in Asia, and in North America, Europe and Australia. It is the national flower of Nepal, the state flower of Washington and West Virginia in the United States, the state flower of Nagaland in India, the provincial flower of Jiangxi in China and the state tree of Sikkim and Uttarakhand in India. Most species have brightly colored flowers which bloom from late winter through to early summer.

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

Species

Description

''Rhododendron'' is a genus of shrubs and small to (rarely) large trees, the smallest species 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 Rhododendron sinogrande, ''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 (botany), section ''Rhododendron sect. Vireya, 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) Bud, perulae, a chromosome number of x=13, fruit that has a septicidal Capsule (botany), capsule, an Ovary (botany), ovary that is superior (or nearly so), stamens that have no appendages, and Agglutination (biology), agglutinate (clumped) pollen.

Taxonomy

''Rhododendron'' is the largest genus in the family Ericaceae, with over 1000 species, (though estimates vary from 850 to 1200) and is morphology (biology), morphologically diverse. Consequently, the taxonomy (biology), taxonomy has been historically complex.

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 Carl Linnaeus, Linnaeus in his Species Plantarum in 1753. He listed five species under ''Rhododendron'' (''Rhododendron ferrugineum'' (type species), ''Rhododendron dauricum, R. dauricum'', ''Rhododendron hirsutum, R. hirsutum'', ''R. chamaecistus'' (now ''Rhodothamnus chamaecistus'' (L.) Rchb.) and ''Rhododendron maximum, 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 ''Rhododendron indicum, Azalea indica'', ''Rhododendron luteum, A. pontica'', ''Rhododendron calendulaceum, A. lutea'', ''Rhododendron viscosum, A. viscosa'', ''Rhododendron lapponicum, 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'' (Carl Ludwig Blume, Blume 1826) and ''Hymenanthes'' (Blume 1826) for ''Rhododendron metternichii'', now Rhododendron degronianum, R. degronianum. Meanwhile, other botanists such as Richard Anthony Salisbury, Salisbury (1796) and Tate (1831) began to question the distinction between ''Azalea'' and ''Rhododendron'', and finally in 1836, ''Azalea'' was incorporated into ''Rhododendron'' and the genus divided into eight sections. Of these ''Tsutsutsi'' (''Rhododendron subg. Tsutsuji, Tsutsusi''), ''Pentanthera'', ''Pogonanthum'', ''Ponticum'' and ''Rhodora'' are still used, the other sections being ''Lepipherum'', ''Booram'', and ''Chamaecistus''. This structure largely survived till recently (2004), following which the development of molecular phylogeny led to major re-examinations of traditional morphological classifications, although other authors such as Candolle, who described six sections, used slightly different numeration.

Soon, as more species became available in the nineteenth century so did a better understanding of the characteristics necessary for the major divisions. Chief amongst these were Maximovicz's ''Rhododendreae Asiae Orientali'' and Jules Émile Planchon, Planchon. Maximovicz used flower bud position and its relationship with leaf buds to create eight "Sections". Bentham and Hooker used a similar scheme, but called the divisions "Series". It was not until 1893 that Bernhard Adalbert Emil Koehne, Koehne appreciated the significance of scaling and hence the separation of lepidote and elepidote species. The large number of species that were available by the early twentieth century prompted a new approach when Isaac Bayley Balfour, Balfour introduced the concept of grouping species into Series (botany), series. ''The Species of 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 era

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'' L.", and subsequent refinements. Most of the Balfourian series are represented by Sleumer as subsections, though some appear as sections or even subgenera. Sleumer based his system on the relationship of the flower buds to the leaf buds, habitat, flower structure, and whether the leaves were lepidote or non-lepidote. While Sleumer's work was widely accepted, many in the United States and the United Kingdom continued to use the simpler Balfourian system of the Edinburgh group.

Sleumer's system underwent many revisions by others, predominantly the Edinburgh group in their continuing Royal Botanic Garden Edinburgh notes. Cullen of the Edinburgh group, placing more emphasis on the lepidote characteristics of the leaves, united all of the lepidote species into subgenus ''Rhododendron'', including four of Sleumer's subgenera (''Rhododendron'', ''Pseudoazalea'', ''Pseudorhodorastrum'', ''Rhodorastrum''). In 1986 Philipson & Melva 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 (''Rhododendron schlippenbachii'', ''Rhododendron quinquefolium, R. quinquefolium'') from section ''Brachybachii'', subgenus ''Tsutsusi'' and two from section ''Rhodora'', subgenus ''Pentanthera'' (''Rhododendron albrechtii, R. albrechtii'', ''Rhododendron pentaphyllum, R. pentaphyllum'') into section ''Sciadorhodion'', subgenus ''Pentanthera''. Finally Chamberlain brought the various systems together in 1996, with 1,025 species divided into eight subgenera. For a comparison of the Sleumer and Chamberlain schemata see Table 1 of Goetsch (2005).

Phylogenetic analyses

The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who used Maturase K, matK sequencing. Later Gao ''et al.'' (2002) used Internal transcribed spacer, ITS sequences to determine a cladistic analysis. They confirmed that the genus ''Rhododendron'' was monophyletic, with subgenus ''Therorhodion'' in the Basal (phylogenetics), basal position, consistent with the matK studies. Following publication of the studies of Goetsch ''et al.'' (2005) with RPB2, there began an ongoing realignment of species and groups within the genus, based on evolutionary relationships. Their work was more supportive of Sleumer's original system than the later modifications introduced by Chamberlain ''et al.''. 2005 Annual ARS Convention

The major finding of Goetsch and colleagues was that all species examined (except Rhododendron camtschaticum, ''R. camtschaticum'', subgenus ''Therorhodion'') formed three major clades which they labelled A, B and C, with the subgenera ''Rhododendron'' and ''Hymenanthes'' nested within clades A and B as monophyletic groups respectively. By contrast subgenera ''Azaleastrum'' and ''Pentanthera'' were polyphyletic, while ''R. camtschaticum'' appeared as a Sister group, sister to all other rhododendrons. The small polyphyletic subgenera ''Pentanthera'' and ''Azaleastrum'' were divided between two clades. The four sections of ''Pentanthera'' between clades B and C, with two each, while ''Azaleastrum'' had one section in each of A and C.

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 C). 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 B (''Hymenanthes'') and C (''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 ''Rhododendron nipponicum'' to ''Tsutsusi'' (C), while ''Rhodora'' (2 species) was itself polyphyletic and was broken up by moving ''Rhododendron canadense'' to section ''Pentanthera'' (B) and ''Rhododendron vaseyi'' to section ''Sciadorhodion'', which then became a new section of subgenus ''Azaleastrum'' (C).

Subgenus ''Tsutsusi'' (C) was reduced to section status retaining the name, and included in subgenus ''Azaleastrum''. Of the three minor subgenera, all in C, two were discontinued. The single species of monotypic subgenus ''Candidastrum'' (''Rhododendron albiflorum'') was moved to subgenus ''Azaleastrum'', section ''Sciadorhodion''. Similarly the single species in monotypic subgenus ''Mumeazalea'' (''Rhododendron 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 A. 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, Subgenera) (Table 1.)

Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions. In 2011 the two species of ''Diplarche'' were also added to ''Rhododendron'', ''incertae sedis''.

Subdivision

This genus has been progressively subdivided into a hierarchy of subgenus, Section (botany), section, Subsection (botany), subsection, and species.

Subgenera

Terminology from the Sleumer (1949) system is frequently found in older literature, with five subgenera and is as follows;
* Subgenus ''Lepidorrhodium'' Koehne: Lepidotes. 3 sections
* Subgenus ''Eurhododendron'' Maxim.: Elipidotes.
* Subgenus ''Pseudanthodendron'' Sleumer: Deciduous azaleas. 3 sections
* Subgenus ''Anthodendron'' Rehder & Wilson: Evergreen azaleas. 3 sections
* Subgenus ''Azaleastrum'' Planch.: 4 sections

In the later traditional classification, attributed to Chamberlain (1996), and as used by horticulturalists and the American Rhododendron Society, ''Rhododendron'' has eight subgenera based on Morphology (biology), morphology, namely the presence of scales (lepidote), deciduousness of leaves, and the floral and vegetative branching patterns, after Sleumer (1980). These consist of four large and four small subgenera. The first two subgenera (''Rhododendron'' and ''Hymenanthes'') represent the species commonly considered as 'Rhododendrons'. The next two smaller subgenera (''Pentanthera'' and ''Tsutsusi'') represent the 'Azaleas'. The remaining four subgenera contain very few species. The largest of these is subgenus ''Rhododendron'', containing nearly half of all known species and all of the lepidote species.

*Subgenus ''Rhododendron subg. Rhododendron, Rhododendron'' : Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, 462 species, type species: ''Rhododendron ferrugineum''.
*Subgenus ''Rhododendron subg. Hymenanthes, Hymenanthes'' : Large leaf or elepidotes (without scales). 1 section, 224 species, type ''Rhododendron degronianum''.
*Subgenus ''Rhododendron subg. Pentanthera, Pentanthera'' : Deciduous azaleas. 4 sections, 23 species, type ''Rhododendron luteum''.
*Subgenus ''Rhododendron sect. Tsutsusi, Tsutsusi'' : Evergreen azaleas. 2 sections, 80 species, type ''Rhododendron indicum''.
*Subgenus ''Rhododendron subg. Azaleastrum, Azaleastrum'' : 2 sections, 16 species, type ''Rhododendron ovatum''.
*Subgenus ''Candidastrum'' : 1 species, ''Rhododendron albiflorum''.
*Subgenus ''Mumeazalea'' : 1 species, ''Rhododendron semibarbatum''.
*Subgenus ''Therorhodion'' : 2 species (''Rhododendron camtschaticum'', ''Rhododendron redowskianun'').

For a comparison of the Sleumer and Chamberlain systems, see Goetsch ''et al.'' (2005) Table 1.

This division was based on a number of what were thought to be key morphological characteristics. These included the position of the inflorescence buds (terminal or lateral), whether lepidote or elepidote, deciduousness of leaves, and whether new foliage was derived from axils from previous year's shoots or the lowest scaly leaves (Table 2.).

Following the cladistic analysis of Goetsch ''et al.'' (2005) this scheme was simplified, based on the discovery of three major Clade, clades (A, B, C) as follows.

Clade A
* Subgenus ''Rhododendron subg. Rhododendron, Rhododendron'' : Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, about 400 species, type species: ''Rhododendron ferrugineum''.
* Subgenus ''Rhododendron subg. Choniastrum, Choniastrum'' : 11 species
Clade B
* Subgenus ''Rhododendron subg. Hymenanthes, Hymenanthes'' : Large leaf or elepidotes (without scales), including deciduous azaleas. 2 sections, about 140–225 species, type ''Rhododendron degronianum''.
Clade C
* Subgenus ''Rhododendron subg. Azaleastrum, Azaleastrum'' : Evergreen azaleas. 3 sections, about 120 species, type ''Rhododendron ovatum''.
Sister taxon
* Subgenus ''Therorhodion'' : 2 species (''Rhododendron camtschaticum'', ''Rhododendron redowskianun'').

Sections and subsections

The larger subgenera are further subdivided into sections and subsections 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 (section), Rhododendron'' L. (149 species in 25 subsections; temperate to subarctic Northern Hemisphere)
**''Rhododendron subgenus Vireya, Vireya'' (Blume) Copel.f. (300 species in 2 subsections; tropical southeast Asia, Australasia. At one time considered separate subgenus)
*Subgenus ''Rhododendron subg. Hymenanthes, Hymenanthes'' (Blume) K.Koch (1 section, 224 species) (''Increased to two sections'')
**''Ponticum'' (24 subsections)
**(''Rhododendron sect. Pentanthera, Pentanthera'' (2 subsections – new section, moved from subgenus ''Pentanthera'')
*Subgenus ''Pentanthera'' (4 sections, 23 species) (''Discontinued'')
**''Rhododendron sect. Pentanthera, Pentanthera'' (2 subsections – moved to subgenus ''Hymenanthes'')
**''Rhododendron canadense, 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)'')

Distribution and habitat

Species of the genus ''Rhododendron'' are widely distributed between latitudes 80°N and 20°S and are native plant, native to areas from North America to Europe, Russia, and Asia, and from Greenland to Queensland, Australia and the Solomon Islands. The center of diversity, centres of diversity are in the Himalayas and Malaysia, 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 climate, tropical species within the ''Rhododendron sect. Vireya, Vireya'' section of subgenus ''Rhododendron'' occupy the Malay archipelago 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 zone, 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 Northeastern United States, Northeast, and the Appalachian Mountains.

Ecology

Invasive species

''Rhododendron ponticum'' has become invasive in Republic of Ireland, 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; see List of Lepidoptera that feed on rhododendrons.

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''.

Cultivation

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 wiktionary:subtemperate, subtemperate regions. Many species and cultivars are grown commercially for the Nursery (horticulture), nursery trade.

Rhododendrons can be propagated by air layering or stem cuttings. They can self-propagate by sending up shoots from the roots. Sometimes an attached branch that has drooped to the ground will root in damp mulch, and the resulting rooted plant then can be cut off the parent rhododendron.

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 ''Rhododendron arboreum'' and ''Rhododendron falconeri, R. falconeri'' eventually growing to a height of or more.

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 on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics than 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.

Horticultural divisions

Horticulturally, rhododendrons may be divided into the following groups:
* Evergreen rhododendrons - large group of evergreen shrubs that vary greatly in size. Most rhododendron flowers are bell-shaped and have 10 stamens.
* Rhododendron sect. Vireya, Vireya (Malesian) rhododendrons: epiphyte, epiphytic hardiness (plants), 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 (born 1882), Lionel de Rothschild at the Exbury Gardens, Exbury Estate in England.
*** Ghent (Gandavense) hybrids – Belgian raised
*** Knap Hill hybrids – developed by Anthony Waterer at the Knaphill, Knap Hill Nursery in England.
*** Mollis hybrids – Dutch and Belgian raised
*** New Zealand Ilam hybrids – derived from Knap Hill/Exbury hybrids
*** Occidentale hybrids – English raised
*** Rustica Flore Pleno hybrids – sweet-scented, double-flowered
** Evergreen hybrid azaleas:
*** Gable hybrids – raised by Joseph B. Gable in Pennsylvania.
*** 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 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 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 calcifuge, calcium-tolerant stock of rhododendrons (trademarked as 'Inkarho') has been exhibited at the Royal Horticultural Society, RHS Chelsea Flower Show in London (2011). Individual hybrids of rhododendrons have been grafting, 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 th
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 ''Rhododendron griffithianum, R. griffithianum''. Other examples include the PJM hybrids, formed from a cross between ''Rhododendron carolinianum'' and ''Rhododendron dauricum, R. dauricum'', and named after Peter J. Mezitt of Weston Nurseries, Massachusetts.

Uses

Pharmacology

''Rhododendron'' species have long been used in traditional medicine. Animal studies and ''in vitro'' research have identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of flavonoids or other natural phenol, 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.

Toxicology

Some species of rhododendron are poisonous to grazing animals because of a toxin called grayanotoxin in their pollen and nectar (plant), 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 Greeks, Greek soldiers after having consumed honey in a village surrounded by ''Rhododendron ponticum'' during the march of the Ten Thousand (Greek), Ten Thousand in 401 BCE. Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from ''Rhododendron'' deliberately left behind by Pontic Greeks, 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. The effects of ''R. ponticum'' were mentioned in the 2009 film ''Sherlock Holmes (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), Sherlock'', speculated to have been a part of Sherlock's fake death scheme.

Culture

Symbolism

In the language of flowers, the Rhododendron symbolizes danger and to beware.

''Rhododendron arboreum'' (''lali guransh'') is the national flower of Nepal. ''R. ponticum'' is the Floral emblem, state flower of Jammu and Kashmir (state), Indian-administered Kashmir and Azad Kashmir, Pakistan-controlled 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.

Literature

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",
In James Joyce, Joyce's ''Ulysses (novel), 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 1: The Road to High Saffron, Shades of Grey''.
Amongst the Zomi tribes in India and Myanmar, "Rhododendrons" called "Ngeisok" is used in a poetic manner to signify a lady.

In Daphne Du Maurier's novel ''Rebecca (novel), Rebecca'', the character of Rebecca is associated with "blood red" rhododendrons throughout the novel, perhaps due to the toxic roots of the plant mirroring the poisonous character of Rebecca. On the other hand, azaleas (a type of rhododendron) represent the second Mrs. De Winter.

In the young adult novel ''Walk Two Moons'' by Sharon Creech, the protagonist, Sal, and her friend, Phoebe, construct an elaborate story in which they suspect Phoebe's neighbor of murdering her husband and burying his body beneath a rhododendron in her yard. Chapter 14 of the book is entitled "The Rhododendron".

Culinary

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 (drink), squash called burans (named after the flower) in the hilly regions of Uttarakhand. It is admired for its distinctive flavor and color.

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'')

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.

See also

* List of Award of Garden Merit rhododendrons
* List of Rhododendron diseases
* List of Rhododendron species
* List of Sections in Subgenus Rhododendron

References

Bibliography

Books and book chapters

* (also available online a
Gallica

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* In four volumes: Vol. I. ''Lepidotes'' , Vol. II. ''Elepidotes. Arboreum-Lacteum'' , Vol. III. ''Elepidotes Continued, Neriiflorum-Thomsonii, Azaleastrum and Camtschaticum'' , Vol. IV. ''Azaleas'' .
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Articles

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Subdivisions

Azaleas

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Tsutsusi

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Vireya

* . A reprint from Flora Malesiana ser. I, vol. 6, part 4. Pages 473 through 674.
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* Yearbook of the Rhododendron Species Foundation, Federal Way, WA.
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Separate genera

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Additional resources

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

External links

History of Rhododendron Discovery & Culture













* [http://www.vireya.net/ Information on Vireyas]

Information+photos of hybrids and species

Information on Rhododendrons by Marc Colombel, founder of the Société Bretonne du Rhododendron.


* [https://web.archive.org/web/20090130005655/http://www.rhodo.citymax.com/page/page/627471.htm History of Rhododendrons]
* [http://www.rododendron.cz/fotogalerie-rododendronu/pruhonicky-park-2011/ Rhododendron in botanical garden Pruhonice-Czech republic ]

Databases

* USDA Plants Database
''Rhododendron''
* ITIS Report
''Rhododendron''

''eFloras.org''
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Flora of North America: Rhododendron
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Flora of China: Rhododendron
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Annotated Checklist of the Flowering Plants of Nepal: Rhododendron

Societies

American Rhododendron Society
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The Quarterly Bulletin of the American Rhododendron Society 1947–1981
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Journal of the American Rhododendron Society (JARS) 1982–
** Information Source: .

The Rhododendron, Camellia & Magnolia Group of the Royal Horticultural Society

Rhododendron Species Foundation and Botanical Garden



Australian Rhododendron Society

German Rhododendron Society

New Zealand Rhododendron Association

Danish Rhododendron Society

Fraser South Rhododendron Society

Botanical gardens

Royal Botanic Garden Edinburgh: Rhododendrons at the four Gardens


National Rhododendron Gardens Melbourne Australia

{{Authority control

Rhododendron,
Ericaceae genera
Plants used in traditional Chinese medicine
National symbols of Nepal
Taxa named by Carl Linnaeus
Extant Ypresian first appearances