Rhododendron

For other uses, see Rhododendron (disambiguation).

Rhododendron
Rhododendron ferrugineum
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Ericales
Family:	Ericaceae
Subfamily:	Ericoideae
Tribe:	Rhodoreae
Genus:	Rhododendron L. Sp. Pl. i 392 (1753)
Type species
Rhododendron ferrugineum L.
Subgenera
Azaleastrum Choniastrum Hymenanthes Rhododendron Therorhodion - Former subgenera - Candidastrum Mumeazalea Pentanthera Tsutsusi

Rhododendron (from Ancient Greek ῥόδον rhódon "rose" and δένδρον déndron "tree") is a genus of 1,024 species of woody plants in the heath family (Ericaceae), either evergreen or deciduous, and found mainly in Asia, although it is also widespread throughout the Southern Highlands of the Appalachian Mountains of North America. It is the national flower of Nepal. Most species have showy 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

Further information: List of Rhododendron species

Description

Rhododendron is a genus characterised by shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm (3.9–39.4 in) tall, and the largest, R. protistum var. giganteum, reported to 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. 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 wardii var. puralbum

Rhododendron in Japan

A garden with tall Rhododendrons in Lynnwood, Washington

Rhododendron forest in Nepal

The Rhododendron genus is the largest of the genera in the Ericaceae family, with 1,024 species, though estimates vary from 850-1000 depending on the authority used, (Fayaz 2012) and is morphologically diverse. Consequently, the 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 Linnaeus in his Species Plantarum in 1753. He listed five species under Rhododendron (Rhododendron ferrugineum (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, if they seemed to differ significantly from the type species they were assigned to separate genera. For instance Rhodora for Rhododendron canadense (Linnaeus 1763), Vireya (Blume 1826) and Hymenanthes for Rhododendron metternichhii, now R. degronianum (1826). 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 (Don 1834) and the genus divided into eight sections. Of these Tsutsutsi (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 (1838), who described six sections, used slightly different numeration.

Rhododendron (গুরাস), Sandakphu, West Bengal, India

Rhododendrons(Guras or Buras) at Fakding,Nepal

As more species became available in the nineteenth century a better understanding of the characteristics necessary for the major divisions. Chief amongst these were Maximovicz's Rhododendreae Asiae Orientali (1870) and Planchon. Maximovicz used flower bud position and its relationship with leaf buds to create eight Sections. Bentham and Hooker (1876) used a similar scheme, but called the divisions Series. It was not until 1893 that 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 Balfour introduced the concept of grouping species into series, in The Species of Rhododendron (1930), referred to as the Balfourian system. That system continued up to modern times in Davidian's four volume The Rhododendron Species (1982-1995).

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 his Ein System der Gattung Rhododendron L. (1949), 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 (1980) in Edinburgh, placing more emphasis on the lepidote characteristics of the leaves united all of the lepidote species into subgenus Rhodendron, including four of Sleumer's (1980) subgenera (Rhododendron, Pseudoazalea, Pseudorhodorastrum, Rhodorastrum). Philipson & Philipson (1986) 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 (1990) moved the monotypic section Tsusiopsis together with the monotypic genus Tsusiophyllum into section Tsutsusi, while in the same year Kron & Judd reduced genus Ledum to a subsection of section Rhododendron. Then Judd & Kron (1995) moved two species (Rhododendron schlippenbachii, 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. For a comparison of the Sleuner and Chamberlain schemata see Table 1 of Goetsch (2005).

Phylogenetic analyses

Cladogram of genus Rhododendron (Goetsch et al. 2005)

A Rhododendron Choniastrum B Hymenanthes C Azaleastrum Therorhodion

The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who used matK sequencing, while Lian-Ming used ITS sequences to determine a cladistic analysis. They confirmed that the genus Rhodendron was monophyletic, with subgenus Therorhodion in the basal position, consistent with the matK studies. Following publication of the studies of Goetsch et al. with RPB2 (2005). 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..

The major finding of Goetsch and colleagues was that all species examined (except 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 to all other rhododendrons. The small polyphyletic subgenera Pentanthera and Azaleastrum were divided between two clades. The four sections of Pentanethra between clades B and C, with two each, while Azaleastrum had one section in each of A and C.

Thus subgenera Azaleastrum and Pentanethera needed to be dissassembled, 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 Rhodendron 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 Pentanethera, Tsutsusi, Candidastrum and Mumeazalea, while a new subgenus was created by elevating subgenus Azaleastrum section Choniastrum to subgenus rank.

Subgenus Pentanethera (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 Pentanethera (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 Pentanethera (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). (Table 1.)

Table 1.: Taxonomic changes within genus Rhododendron
(Goetsch 2005)

Chamberlain (1996)				Goetsch
Genus	Subgenus	Section	Species	Subgenus	Section
Menziesa				Azaleastrum	Sciadorhodion
	Candidastrum
	Tsutsusi				Tsutsusi
	Mumeazaleum
	Pentanthera	Viscidula
		Pentanthera		Hymenanthes	Pentanethra
		Rhodora	R.canadense
			R.vaseyi	Azaleastrum	Sciadorhodion
	Azaleastrum	Choniastrum		Choniastrum

Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions.(Craven 2008) In 2011 the two species of Diplarche were also added to Rhododendron, incertae sedis.(Craven 2011) Similar findings were reported independently the following year by Brown et al.

Subdivision

This genus has been progressively subdivided into a hierarchy of subgenus, section, 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, 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 L.: Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, 462 species, type species: Rhododendron ferrugineum.
• Subgenus Hymenanthes (Blume) K.Koch: Large leaf or elepidotes (without scales). 1 section, 224 species, type Rhododendron degronianum.
• Subgenus Pentanthera (G. Don) Pojarkova: Deciduous azaleas. 4 sections, 23 species, type Rhododendron luteum.
• Subgenus Tsutsusi (Sweet) Pojarkova: Evergreen azaleas. 2 sections, 80 species, type Rhododendron indicum.
• Subgenus Azaleastrum Planch.: 2 sections, 16 species, type Rhododendron ovatum.
• Subgenus Candidastrum Franch. : 1 species, Rhododendron albiflorum.
• Subgenus Mumeazalea (Sleumer) W.R. Philipson & M.N. Philipson: 1 species, Rhododendron semibarbatum.
• Subgenus Therorhodion (Maxim.) A. Gray: 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.).

Table 2.: Morphological classification of Rhododendron
(Chamberlain 1996)

Inflorescence buds	Leaf scales	Leaf shoots	Leaves	Subgenus	Section
Terminal	Present			Rhododendron
	Absent	Previous year	Evergreen	Hymenanthes
			Deciduous	Pentanthera	Pentanthera
					Rhodora
					Viscidula
		Lowest leaves		Pentanthera	Sciadorhodion
				Tsutsusi
Lateral			Evergreen	Azaleastrum
			Deciduous	Candidastrum
				Mumeazalea
				Therorhodion

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

Clade A

• Subgenus Rhododendron L.: Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, about 400 species, type species: Rhododendron ferrugineum.
• Subgenus Choniastrum Franch. : 11 species

Clade B

• Subgenus Hymenanthes (Blume) K.Koch: Large leaf or elepidotes (without scales), including deciduous azaleas. 2 sections, about 140-225 species, type Rhododendron degronianum.

Clade C

• Subgenus Azaleastrum Planch.: Evergreen azaleas. 3 sections, about 120 species, type Rhododendron ovatum.

Sister taxon

• Subgenus Therorhodion (Maxim.) A. Gray: 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 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 G. Don (24 subsections)
  • (Pentanthera (G. Don) Pojarkova (2 subsections) new section, moved from subgenus Pentanthera)
• Subgenus Pentanthera (G. Don) Pojarkova (4 sections, 23 species) (Discontinued)
  • Pentanthera (G. Don) Pojarkova (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))

Distribution and habitat

Rhododendron fallacinum photographed in situ on Mount Kinabalu, Borneo

Species of the genus Rhododendron are widely distributed between latitudes 80°N and 20°S and are considered Alpine native plants from North America to Europe, Russia, and Asia, and from Greenland to Queensland, Australia and the Solomon Islands. The centres of diversification are in the Himalayas and Malaysia, with the greatest species diversity in the Sino-Himalayan region, Southwest China and northern Burma, from Uttarakhand, Nepal and Sikkim to northwestern Yunnan and western Sichuan and southeastern Tibet, and with other significant areas of diversity 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 Vereya 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 moist grasslands at around 3,000 metres above sea level in the Central Highlands. Subgenera Rhododendron and Hymenanthes, together with section Pentanethera of subgenus Pentanethera 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.

Ecology

Invasive species

Some species (e.g. Rhododendron ponticum in Ireland and the United Kingdom) are invasive as introduced plants, spreading in woodland areas replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

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 of some members of the order Lepidoptera (butterflies and moths) (See List of Lepidoptera that feed on rhododendrons).

Diseases

Main article: List of rhododendron diseases

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.

Cultivation

Rhododendron-Park Bremen, 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,(Craven 2008) while 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.

Cultivated evergreen variety shedding leaves in autumn

Commercial growing

Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States, most of them are located on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics 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. 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.

Horticultural divisions

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:
    • Ghent (Gandavense) hybrids - Belgian raised
    • Knap Hill-Exbury hybrids - English raised
    • Mollis hybrids - Dutch & Belgian raised
    • New Zealand Ilam hybrids - derived from Knap Hill/Exbury hybrids
    • Occidentale hybrids - English raised
    • Rustica hybrids - sweet-scented, double-flowered
  • Evergreen hybrid azaleas:
    • Gable hybrids - raised by Joseph B. Gable in Pennsylvania, USA
    • 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 the Netherlands
• Azaleodendrons - semi-evergreen hybrids between deciduous azaleas and rhododendrons

Planting and care

Nova Zembla Rhododendrons growing in a nursery in New Jersey.

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5-5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly drained or alkaline soils, rhododendrons are often grown in raised beds using media such as composted pine bark. 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 R. fortunei and R. griffithianum. Other examples include the PJM hybrids, formed from a cross between Rhododendron carolinianum and Rhododendron 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 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.

Toxicology

Some species of rhododendron are poisonous to grazing animals because of a toxin called grayanotoxin in their pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behaviour of Greek soldiers after having consumed honey in a village surrounded by Rhododendron ponticum during the march of the Ten Thousand in 401 BC. Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BC during the Third Mithridatic War. Later, it was recognized that honey resulting from these plants has a slightly hallucinogenic and laxative effect. The suspect rhododendrons are Rhododendron ponticum and Rhododendron luteum (formerly Azalea pontica), both found in northern Asia Minor. A brief documented video of this occurring in the modern day involves a group of men in Nepal foraging for this affected honey can be found here: http://eupterrafoundation.com/hallucinogenic-honey. 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, and has been speculated to have been a part of Sherlock's fake death scheme.

Culture

Symbolism

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. Pink Rhododendron (Rhododendron campanulatum) is the State Flower of Himachal Pradesh, 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.

Literature

Leon Wyczółkowski, Pink Rhododendrons, 1903

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. Amongst the Zomi tribes in India and Myanmar, "Rhododendrons" called "Ngeisok" is used in a poetic manner to signify a lady.

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

Labrador tea

Main article: Labrador tea

Labrador tea is an herbal tea (not a true tea) made from three closely related species:

• Rhododendron tomentosum (Northern Labrador tea, previously Ledum palustre),
• Rhododendron groenlandicum, (Bog Labrador tea, previously Ledum groenlandicum or Ledum latifolium) and
• Rhododendron neoglandulosum, (Western Labrador tea, or trapper's tea, previously Ledum glandulosum).

See also

• List of Award of Garden Merit rhododendrons
• List of Rhododendron diseases
• List of Rhododendron species

Bibliography

Books and book chapters

• (also available online at Gallica)
• Sweet, Robert. The British Flower Garden (Series the Second) Volume I. Ridgway, London 1831
• Hooker, Joseph Dalton, Fitch, W. H.,Hooker, William Jackson. The rhododendrons of Sikkim-Himalaya : being an account, botanical and geographical, of the rhododendrons recently discovered in the mountains of eastern Himalaya, from drawings and descriptions made on the spot, during a government botanical mission to that country. Reeve, Benham, and Reeve, London, 1849-51.
• James Leonard Luteyn, Mary E. O'Brien. Contributions Toward a Classification of Rhododendron: Proceedings, International Rhododendron Conference, the New York Botanical Garden, May 15-17, 1978. New York Botanical Garden, American Rhododendron Society 1980. ISBN 0893272213
• 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.
• Cox, P. A. & Kenneth, N. E. The Encyclopedia of Rhododendron Species. 1997. Glendoick Publishing. ISBN 0-9530533-0-X.
• James Cullen. Hardy Rhododendron Species: A Guide To Identification. Timber Press, 2005. ISBN 0881927236
• Blazich, F.A. and D.B. Rowe. 2008. "Rhododendron L., rhododendron and azalea." p. 943-951. In: F.T. Bonner and R.P. Karrfalt (eds.). The woody plant seed manual. Agr. Hdbk 727. U.S. Dept. Agr. For. Serv., Washington, D.C.

Articles

• Walter Magor. A History of Rhododendrons. JARS Volume 44, Number 4, Fall 1990
• GAO Lian-Ming, LI De-Zhu, ZHANG Chang-Qin, YANG Jun-Bo. Infrageneric and Sectional Relationships in the Genus Rhododendron (Ericaceae) Inferred from ITS Sequence Data. Am Botanica Sinica 2002, 44(11) 1351-1356

Subdivisions

Azaleas

• Wilson EH, Rehder A. A MONOGRAPH OF AZALEAS RHODODENDRON SUBGENUS ANTHODENDRON. THE UNIVERSITY PRESS, CAMBRIDGE April 15 1921. PUBLICATIONS OF THE ARNOLD ARBORETUM, No. 9
• John L. Creech. An Embryological Study in the Rhododendron Subgenus Anthodendron Endl. Botanical Gazette Vol. 116, No. 3 (Mar., 1955), pp. 234-243

Tsutsusi

• Powell, E. Ann, Kron, Kathleen A. Molecular systematics of Rhododendron subgenus Tsutsusi (Rhodoreae, Ericoideae, Ericaceae). Botany 2004: 147
• K. A. Kron and E. A. Powell. MOLECULAR SYSTEMATICS OF RHODODENDRON SUBGENUS TSUTSUSI (RHODOREAE, ERICOIDEAE, ERICACEAE) Edinburgh Journal of Botany Volume 66 Issue 1 March 2009, pp 81–95
• Yue-Jiao ZHANG, Xiao-Feng JIN, Bing-Yang DING and Jing-Ping ZHU. Pollen morphology of Rhododendron subgen. Tsutsusi and its systematic implications. Journal of Systematics and Evolution. Volume 47, Issue 2, pages 123–138, March 2009
• Jin Xiao-Feng, Ding Bing-Yang, Zhang Yue-Jiao, and Hong De-Yuan. A Taxonomic Revision Of Rhododendron subg. Tsutsusi sect. Brachycalyx (Ericaceae). Annals of the Missouri Botanical Garden 97(2):163-190. 2010 DOI:10.3417/2007139

Vireya

• Sleumer, Hermann Otto. An account of rhododendron in Malesia. P. Noordhoff, Groningen 1966
• David G. Leach. The Discovery of the Malaysian Rhododendrons. JARS Volume 32, Number 1 Winter 1978
• Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
• Gillian K. Brown, Lyn A. Craven, Frank Udovicic and Pauline Y. Ladiges. Phylogenetic relationships of Rhododendron section Vireya (Ericaceae) inferred from the ITS nrDNA region. Australian Systematic Botany 19, 329–342. August 2006. 10.1071/SB05019
• Hall, B.D., L.A. Craven & L.A. Goetsch. 2006. The taxonomy of subsection Pseudovireya – two distinctly different taxa within subsection Pseudovireya and their relation to the rooting of section Vireya within subgenus Rhododendron. Rhododendron Sp. 1: 72, 91–97
• Peter Adams. Evolution, Adaptive Radiation and Vireya Rhododendrons - Part I. JARS FALL 2012 pp. 201-3
• Peter Adams. Evolution, Adaptive Radiation and Vireya Rhododendrons - Part I. JARS SPRING 2013 pp. 74-76
• Biodiversity of the Vireya group of Rhododendron L. (Ericaceae) collections in New Zealand and their potential contribution to international conservation. Fayaz A. PhD Thesis, Department of Plant Science, Massey University, New Zealand. 2012

Separate genera

• Craven, L.A. Diplarche and Menziesia transferred to Rhododendron (Ericaceae) Blumea Volume 56, Number 1, April 2011, pp. 33–35

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
• Rhododendrons from Turkey, Anatolia
• German Genebank Rhododendron
• Description of damage caused by Rhododendrons in the UK
• Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.
• Information on Vireyas
• Information+photos of hybrids and species
• Information on Rhododendrons by Marc Colombel, founder of the Société Bretonne du Rhododendron.
• Extensive information on rhododendron species: the history of their discovery, botanical details, toxicity, classification, cultural conditions, care for common problems, and suggestions for companion plants by Steve Henning.
• History of Rhododendrons
• Historical Survey of Rhododendron Collecting - With Emphasis on its Close Associations with Horticulture
• Rhododendron in botanical garden Pruhonice-Czech republic

Databases

• USDA: Rhododendron
• ITIS: Rhododendron

Flora

• Flora of China: Rhododendron
• Flora of North America: Rhododendron

Rhododendron societies

• American Rhododendron Society
  • Journal of the American Rhododendron Society (JARS) 1947-1996
• The Rhododendron, Camellia & Magnolia Group of the Royal Horticultural Society
• Rhododendron Species Foundation and Botanical Garden
• Société Finlandaise du Rhododendron
• Australian Rhododendron Society
• German Rhododendron Society
• New Zealand Rhododendron Association
• Danish Rhododendron Society
• Fraser South Rhododendron Society

Botanical gardens

• Royal Botanic Garden Edinburgh: Rhododendron Publications