Rhododendron

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.[2][20] 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[21] that he had described earlier in 1735 in his Systema Naturae as a separate genus.[22][23]

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)[24] and Hymenanthes (Blume 1826) for Rhododendron metternichii, now R. degronianum. Meanwhile, other botanists such as Salisbury (1796)[25] and Tate (1831)[26] began to question the distinction between Azalea and Rhododendron, and finally in 1836, Azalea was incorporated into Rhododendron[27] 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,[22][23] although other authors such as Candolle, who described six sections,[28] 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[29] and Planchon. Maximovicz used flower bud position and its relationship with leaf buds to create eight "Sections".[30] Bentham and Hooker used a similar scheme, but called the divisions "Series".[31] 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. The Species of Rhododendron[32] referred to this series concept as the Balfourian system. That system continued up to modern times in Davidian's four volume The Rhododendron Species.[33]

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.",[34] and subsequent refinements.[35][36] 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).[36][37] In 1986 Philipson & Philipson raised two sections of subgenus Aleastrum (Mumeazalea, Candidastrum) to subgenera, while reducing genus Therorhodion to a subgenus of Rhododendron.[38] In 1987 Spethmann, adding phytochemical features proposed a system with fifteen subgenera grouped into three 'chorus' subgenera.[39]

A number of closely related genera had been included together with Rhododendron in a former tribe, Rhodoreae. These have been progressively incorporated into Rhododendron.[40] Chamberlain and Rae moved the monotypic section Tsusiopsis together with the monotypic genus Tsusiophyllum into section Tsutsusi,[41] while Kron & Judd reduced genus Ledum to a subsection of section Rhododendron.[42] Then Judd & Kron 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.[43] 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).[3][15][23][44][45]

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. Later Gao et al. (2002) used ITS sequences[46] to determine a cladistic analysis. They confirmed that the genus Rhododendron was monophyletic, with subgenus Therorhodion in the basal position, consistent with the matK studies. Following publication of the studies of Goetsch et al. (2005) with RPB2,[3] 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..[3][47]

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 as monophyletic groups nested within clades A and B, 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 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.[3] (See schemata under Subgenera) (Table 1.)[47]

Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions.[48] In 2011 the two species of Diplarche were also added to Rhododendron, incertae sedis.[49]

This genus has been progressively subdivided into a hierarchy of subgenus, section, subsection, and species.

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,[50] 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).[3][15][36] 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.[51] 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.[3]

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.).

Deciduous Rhododendron luteum in fall color

Evergreen azalea Rhododendron kaempferi

Evergreen azalea cultivar leaf color before shedding

Rhododendron after freezing rain

Following the cladistic analysis of Goetsch et al. (2005)[3] 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).

The larger subgenera are further subdivided into sections and subsections[51] 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)[3] and by Craven et al. (2008)[48] 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[50]

• 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[52])
• 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))

Rhododendron ponticum has become invasive in Ireland[53] and the United Kingdom.[54] 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.

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.

Major diseases include Phytophthora root rot, stem and twig fungal dieback.[55]

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.[56]^: 562

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.[58]

Horticulturally, rhododendrons may be divided into the following groups:[22][59]

• 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[60]
• 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:[61]
    • Exbury hybrids – derived from the Knap Hill hybrids, developed by Lionel de Rothschild at the Exbury Estate in England.[62][63]
    • Ghent (Gandavense) hybrids – Belgian raised[64]
    • Knap Hill hybrids – developed by Anthony Waterer at the Knap Hill Nursery in England.[62]
    • Mollis hybrids – Dutch and Belgian raised[65]
    • New Zealand Ilam hybrids – derived from Knap Hill/Exbury hybrids
    • Occidentale hybrids – English raised
    • Rustica Flore Pleno hybrids – sweet-scented, double-flowered[64]
  • Evergreen hybrid azaleas:
    • Gable hybrids – raised by Joseph B. Gable in Pennsylvania.[66]
    • 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[67]
    • Vuyk (Vuykiana) hybrids – raised in the Netherlands[68]
• Azaleodendrons – semi-evergreen hybrids between deciduous azaleas and rhododendrons

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.[69] 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.[70][71]

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[72]—such as the Loderi hybrids, created by crossing Rhododendron fortunei and R. griffithianum.[73] 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.[74]

Rhododendron species have long been used in traditional medicine.[75][76] 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 phenolic compounds and saponins the plant contains.[77] Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB in rats.[78]

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 BCE.[79] Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BCE during the Third Mithridatic War.[80] Later, it was recognized that honey resulting from these plants has a slightly hallucinogenic and laxative effect.[81] 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.[82] 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.[83] The effects of R. ponticum were mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution.[84] 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.

State flower of Himachal Pradesh

In the language of flowers, the Rhododendron symbolizes danger and to beware.[85]

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 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.

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",

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,[86] and Shades of Grey.[87] 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, 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".

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.[88]^: 51 The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones.[88]^: 53 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.[89]

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 the UK the forerunner of the Rhododendron, Camellia and Magnolia Group (RCMG), The Rhododendron Society was founded in 1916.[90] while in Scotland species are being conserved by the Rhododendron Species Conservation Group.[91]

• A. P. de Candolle (1838). "Rhododendron". Prodromus systemati naturalis regni vegetabilis sive enumeratio contracta ordinum, generum specierumque plantarum huc usque cognitarum, juxta methodi naturalis normas digesta (in Latin). Vol. 7. Paris: Treuttel et Würtz. pp. 719–728. (also available online at Gallica)
• Sweet, Robert (1838). The British Flower Garden. The Two Series. Vol. I. Drawings by E.D. Smith. London: James Ridgway & Sons.
• Hooker, Joseph Dalton (1849). Hooker, William Jackson (ed.). 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 (2nd ed.). London: Reeve, Benham, and Reeve. doi:10.5962/bhl.title.11178.
• Luteyn, James Leonard & O'Brien, Mary E., eds. (1980). Contributions Toward a Classification of Rhododendron: Proceedings of the International Rhododendron Conference. International Rhododendron Conference (The New York Botanical Garden, May 15–17, 1978). New York: New York Botanical Garden Press. ISBN 978-0-89327-221-0.
• Davidian, H.H. (1982–1995). The Rhododendron Species. Portland, Oregon: Timber Press. In four volumes: Vol. I. Lepidotes ISBN 0-917304-71-3, Vol. II. Elepidotes. Arboreum-Lacteum ISBN 0-88192-109-2, Vol. III. Elepidotes Continued, Neriiflorum-Thomsonii, Azaleastrum and Camtschaticum ISBN 0-88192-168-8, Vol. IV. Azaleas ISBN 0-88192-311-7.
• Cox, Peter A. & Cox, Kenneth N. E. (1997). The Encyclopedia of Rhododendron Species. Glendoick Publishing. ISBN 978-0-9530533-0-8..
• Cullen, James (2005). Hardy Rhododendron Species: A Guide To Identification. Timber Press. ISBN 978-0881927238.
• Blazich, Frank A. & Rowe, D. Bradley (July 2008). "Rhododendron L., rhododendron and azalea" (PDF). In Bonner, Franklin T. & Karrfalt, Robert P. (eds.). The Woody Plant Seed Manual (PDF). Agr. Hdbk. Vol. 727. Washington, D.C.: U.S. Dept. Agr. For. Serv. pp. 943–951. Archived (PDF) from the original on 9 October 2022.

• Black, Michael (October 1969). "Historical Survey of Rhododendron Collecting With Emphasis on its Close Associations with Horticulture". The Quarterly Bulletin of the American Rhododendron Society. 23 (4).
• Magor, Walter (Fall 1990). "A History of Rhododendrons". Journal American Rhododendron Society. 44 (4).
• Goetsch, Loretta A.; Eckert, Andrew J.; Hall, Benjamin D. (July–September 2005). "The molecular systematics of Rhododendron (Ericaceae): a phylogeny based upon RPB2 gene sequences". Systematic Botany. 30 (3): 616–626. doi:10.1600/0363644054782170. S2CID 51949019.

• Wilson, EH; Rehder, A (1921). A Monograph of Azaleas; Rhododendron Subgenus Anthodendron. Publications of the Arnold Aboretum, no. 9. Cambridge: University Press.
• Creech, John L. (1955). "An Embryological Study in the Rhododendron Subgenus Anthodendron Endl". Botanical Gazette. 116 (3): 234–243. doi:10.1086/335866. JSTOR 2473343. S2CID 83861676.

• Chamberlain, D.F. & Rae, S.J. (1990). "A revision of Rhododendron. IV. Subgenus Tsutsusi". Edinburgh Journal of Botany. 47 (2): 89–200. doi:10.1017/S096042860000319X.
• Powell, E. Ann; Kron, Kathleen A (2004). "Molecular systematics of Rhododendron subgenus Tsutsusi (Rhodoreae, Ericoideae, Ericaceae)". Botany 2004. Abstract ID:147.
• Kron, K.A. & Powell, E.A. (March 2009). "Molecular systematics of Rhododendron subgenus Tsutsusi (Rhodoreae, Ericoideae, Ericaceae)". Edinburgh Journal of Botany. 66 (1): 81–95. doi:10.1017/S0960428609005071.
• ZHANG Yue-Jiao; JIN Xiao-Feng; DING Bing-Yang; ZHU Jing-Ping (March 2009). "Pollen morphology of Rhododendron subgen. Tsutsusi and its systematic implications". Journal of Systematics and Evolution. 47 (2): 123–138. doi:10.1111/j.1759-6831.2009.00011.x. S2CID 86594487.
• JIN Xiao-Feng; DING Bing-Yang; ZHANG Yue-Jiao; HONG De-Yuan (2010). "A Taxonomic Revision Of Rhododendron subg. Tsutsusi sect. Brachycalyx (Ericaceae)". Annals of the Missouri Botanical Garden. 97 (2): 163–190. doi:10.3417/2007139. S2CID 86507576.

• Sleumer, Hermann Otto (1966). An account of rhododendron in Malesia. Groningen: P. Noordhoff.. A reprint from Flora Malesiana ser. I, vol. 6, part 4. Pages 473 through 674.
• Leach, David G. (Winter 1978). "The Discovery of the Malaysian Rhododendrons". The Quarterly Bulletin of the American Rhododendron Society. 32 (1).
• Argent, G. (2006). Rhododendrons of subgenus Vireya. Royal Horticultural Society. ISBN 978-1-902896-61-8.
• Brown, Gillian K.; Craven, Lyn A.; Udovicic, Frank; Ladiges, Pauline Y. (August 2006). "Phylogenetic relationships of Rhododendron section Vireya (Ericaceae) inferred from the ITS nrDNA region" (PDF). Australian Systematic Botany. 19 (4): 329–342. doi:10.1071/SB05019. Archived from the original on 15 July 2014.
• Hall, B.D.; Craven, L.A. & Goetsch, L.A. (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 Species. 1: 91–97. Yearbook of the Rhododendron Species Foundation, Federal Way, WA.
• Craven, L.A.; Goetsch, L.A.; Hall, B.D.; Brown, G.K. (2008). "Classification of the Vireya group of Rhododendron (Ericaceae)". Blumea - Biodiversity, Evolution and Biogeography of Plants. 53 (2): 435–442. doi:10.3767/000651908X608070.
• Goetsch, L.A.; Craven, L.A.; Hall, B.D. (2011). "Major speciation accompanied the dispersal of Vireya Rhododendrons (Ericaceae, Rhododendron sect. Schistanthe) through the Malayan archipelago: Evidence from nuclear gene sequences". Taxon. 60 (4): 1015–1028. doi:10.1002/tax.604006.
• Adams, Peter (Fall 2012). "Evolution, Adaptive Radiation and Vireya Rhododendrons - Part I" (PDF). Journal American Rhododendron Society: 201–203. Archived from the original (PDF) on 20 August 2016. Retrieved 18 June 2014.
• Adams, Peter (Spring 2013). "Evolution, Adaptive Radiation and Vireya Rhododendrons – Part II" (PDF). Journal American Rhododendron Society: 74–76. Archived from the original (PDF) on 20 August 2016. Retrieved 18 June 2014.
• Fayaz, A. (2012). Biodiversity of the Vireya group of Rhododendron L. (Ericaceae) collections in New Zealand and their potential contribution to international conservation (PhD). Turitea, New Zealand: Massey University. Retrieved 26 September 2017.

• Craven, L.A. (April 2011). "Diplarche and Menziesia transferred to Rhododendron (Ericaceae)". Blumea - Biodiversity, Evolution and Biogeography of Plants. 56 (1): 33–35. doi:10.3767/000651911X568594.

• USDA Plants Database: Rhododendron
• ITIS Report: Rhododendron
• eFloras.org
  • Flora of North America: Rhododendron
  • Flora of China: Rhododendron
  • Annotated Checklist of the Flowering Plants of Nepal: Rhododendron

• American Rhododendron Society
  • The Quarterly Bulletin of the American Rhododendron Society 1947–1981
  • Journal of the American Rhododendron Society (JARS) 1982–
  • "Genus Rhododendron Taxonomic Tree". American Rhododendron Society. Information Source: Cox, Peter A. & Cox, Kenneth N. E. (1997). The Encyclopedia of Rhododendron Species. Glendoick Publishing. ISBN 978-0-9530533-0-8..
• 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

• Royal Botanic Garden Edinburgh: Rhododendrons at the four Gardens Archived 29 May 2018 at the Wayback Machine
• National Rhododendron Gardens Melbourne Australia