HOME

TheInfoList



OR:

''Rhododendron'' (; from
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
''rhódon'' "rose" and ''déndron'' "tree") is a very large
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of about 1,024
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
of woody plants in the heath family (Ericaceae). They can be either
evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, whic ...
or
deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...
. Most species are native to eastern Asia and the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...
n region, but smaller numbers occur elsewhere in Asia, and in North America, Europe and Australia. It is the
national flower In a number of countries, plants have been chosen as symbols to represent specific geographic areas. Some countries have a country-wide floral emblem; others in addition have symbols representing subdivisions. Different processes have been used to ...
of
Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
, the state flower of
Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...
and
West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the ...
in the United States, the state flower of
Nagaland Nagaland () is a landlocked state in the northeastern region of India. It is bordered by the Indian states of Arunachal Pradesh to the north, Assam to the west, Manipur to the south and the Sagaing Region of Myanmar to the east. Its capital cit ...
in India, the provincial flower of
Jiangxi Jiangxi (; ; formerly romanized as Kiangsi or Chianghsi) is a landlocked province in the east of the People's Republic of China. Its major cities include Nanchang and Jiujiang. Spanning from the banks of the Yangtze river in the north int ...
in China and the
state tree This is a list of U.S. state, federal district, and territory trees, including official trees of the following of the states, of the federal district, and of the territories. Table See also * List of U.S. state, district, and territorial insign ...
of
Sikkim Sikkim (; ) is a state in Northeastern India. It borders the Tibet Autonomous Region of China in the north and northeast, Bhutan in the east, Province No. 1 of Nepal in the west and West Bengal in the south. Sikkim is also close to the Sil ...
and
Uttarakhand Uttarakhand ( , or ; , ), also known as Uttaranchal ( ; the official name until 2007), is a state in the northern part of India. It is often referred to as the "Devbhumi" (literally 'Land of the Gods') due to its religious significance and ...
in India. Most species have brightly colored flowers which bloom from late winter through to early summer.
Azalea Azaleas are flowering shrubs in the genus '' Rhododendron'', particularly the former sections '' Tsutsusi'' (evergreen) and '' Pentanthera'' (deciduous). Azaleas bloom in the spring (April and May in the temperate Northern Hemisphere, and Oct ...
s make up two subgenera of ''Rhododendron''. They are distinguished from "true" rhododendrons by having only five
anther The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filam ...
s per flower.


Species


Description

''Rhododendron'' is a genus of
shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from tree ...
s and small to (rarely) large
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
s, the smallest species growing to tall, and the largest, ''R. protistum'' var. ''giganteum'', reported to tall. The
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
are spirally arranged; leaf size can range from to over , exceptionally in ''R. sinogrande''. They may be either
evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, whic ...
or
deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...
. 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
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...
s. 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 An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed ...
with scarious (dry) perulae, a chromosome number of x=13,
fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
that has a septicidal capsule, an
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
that is superior (or nearly so), stamens that have no appendages, and agglutinate (clumped)
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametop ...
.


Taxonomy

''Rhododendron'' is the largest genus in the family Ericaceae, with over 1000 species, (though estimates vary from 850 to 1200) and is morphologically diverse. Consequently, the
taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
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 ' (Latin for "The Species of Plants") is a book by Carl Linnaeus, originally published in 1753, which lists every species of plant known at the time, classified into genera. It is the first work to consistently apply binomial names and was the ...
in 1753. He listed five species under ''Rhododendron'' (''
Rhododendron ferrugineum ''Rhododendron ferrugineum'', the alpenrose, snow-rose, or rusty-leaved alpenrose is an evergreen shrub that grows just above the tree line in the Alps, Pyrenees, Jura and northern Apennines, on acid soils. It is the type species for the genus ' ...
'' (
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specim ...
), '' 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 Azaleas are flowering shrubs in the genus '' Rhododendron'', particularly the former sections '' Tsutsusi'' (evergreen) and '' Pentanthera'' (deciduous). Azaleas bloom in the spring (April and May in the temperate Northern Hemisphere, and Oct ...
'' that he had described earlier in 1735 in his
Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial ...
as a separate genus. Linnaeus' six species of ''Azalea'' were '' Azalea indica'', '' A. pontica'', '' A. lutea'', '' A. viscosa'', '' A. lapponica'' and ''A. procumbens'' (now '' Kalmia procumbens''), which he distinguished from ''Rhododendron'' by having five stamens, as opposed to ten. As new species of what are now considered ''Rhododendron'' were discovered, they were assigned to separate genera if they seemed to differ significantly from the type species. For instance ''Rhodora'' (Linnaeus 1763) for ''
Rhododendron canadense ''Rhododendron canadense'', the rhodora or Canada rosebay, is a deciduous flowering shrub that is native to northeastern North America. Classification Today's botanists consider the rhodora to be a distant relative of the other North Americ ...
'', ''Vireya'' ( Blume 1826) and ''Hymenanthes'' (Blume 1826) for ''Rhododendron metternichii'', now R. degronianum. Meanwhile, other botanists such as
Salisbury Salisbury ( ) is a cathedral city in Wiltshire, England with a population of 41,820, at the confluence of the rivers Avon, Nadder and Bourne. The city is approximately from Southampton and from Bath. Salisbury is in the southeast of ...
(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'' ('' 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 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
Koehne The surname Koehne may refer to: History "Koehne" is the North German variant of the name "Kuehne". People * Bernhard Karl von Koehne (1817–1887), Russian heraldist and numismatist * Bernhard Adalbert Emil Koehne (1848–1918), German ...
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'' 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 & 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 Phytochemicals are chemical compounds produced by plants, generally to help them resist fungi, bacteria and plant virus infections, and also consumption by insects and other animals. The name comes . Some phytochemicals have been used as poison ...
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 In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
section ''Tsusiopsis'' together with the monotypic genus ''Tsusiophyllum'' into section ''Tsutsusi'', while Kron & Judd reduced genus ''
Ledum ''Ledum'' was a genus in the family Ericaceae, including 8 species of evergreen shrubs native to cool temperate and subarctic regions of the Northern Hemisphere and commonly known as Labrador tea. It is now recognised as a subsection of section R ...
'' to a subsection of section ''Rhododendron''. Then Judd & Kron moved two species (''
Rhododendron schlippenbachii ''Rhododendron schlippenbachii'', the royal azalea, is a species of ''Rhododendron'' native to the Korean Peninsula and adjacent regions of Manchuria (Liaoning, Nei Mongol), Japan, and the Russian Far East. It is the dominant understory shrub i ...
'', '' 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 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 matK
sequencing In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which suc ...
. Later Gao ''et al.'' (2002) used ITS sequences to determine a cladistic analysis. They confirmed that the genus ''Rhododendron'' was
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
, with subgenus ''Therorhodion'' in the 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 ''R. camtschaticum'', subgenus ''Therorhodion'') formed three major
clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English te ...
which they labelled A, B and C, with the subgenera ''Rhododendron'' and ''Hymenanthes'' nested within clades A and B as
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
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 ''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 ''Ledum'' was a genus in the family Ericaceae, including 8 species of evergreen shrubs native to cool temperate and subarctic regions of the Northern Hemisphere and commonly known as Labrador tea. It is now recognised as a subsection of section R ...
'', ''Tsusiophyllum''), Goetsch ''et al.''. added '' Menziesia'' (Clade C). Despite a degree of
paraphyly In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
, 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 ''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 n ...
'' to ''Tsutsusi'' (C), while ''Rhodora'' (2 species) was itself polyphyletic and was broken up by moving ''
Rhododendron canadense ''Rhododendron canadense'', the rhodora or Canada rosebay, is a deciduous flowering shrub that is native to northeastern North America. Classification Today's botanists consider the rhodora to be a distant relative of the other North Americ ...
'' 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 In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
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 In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between ...
) (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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
has been progressively subdivided into a hierarchy of
subgenus In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between ...
, section, subsection, and
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
.


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 Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...
and the American Rhododendron Society, ''Rhododendron'' has eight
subgenera In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between ...
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'' : Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, 462 species,
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specim ...
: ''
Rhododendron ferrugineum ''Rhododendron ferrugineum'', the alpenrose, snow-rose, or rusty-leaved alpenrose is an evergreen shrub that grows just above the tree line in the Alps, Pyrenees, Jura and northern Apennines, on acid soils. It is the type species for the genus ' ...
''. *Subgenus '' Hymenanthes'' : Large leaf or elepidotes (without scales). 1 section, 224 species, type '' Rhododendron degronianum''. *Subgenus '' Pentanthera'' : Deciduous azaleas. 4 sections, 23 species, type '' Rhododendron luteum''. *Subgenus '' Tsutsusi'' : Evergreen azaleas. 2 sections, 80 species, type ''
Rhododendron indicum ''Rhododendron indicum'' is an Azalea ''Rhododendron'' species native to Japan (S & W Honshu, Shikoku, Kyushu, Yakushima). Taxonomy It is the type species for the ''Tsutsusi'' section and subsection, and was the original ''Tsutsusi'' described by ...
''. *Subgenus '' 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'' (; 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 nat ...
'', '' 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 A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
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
clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English te ...
(A, B, C) as follows. Clade A * Subgenus '' Rhododendron'' : Small leaf or lepidotes (scales on the underside of the leaves). 3 sections, about 400 species,
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specim ...
: ''
Rhododendron ferrugineum ''Rhododendron ferrugineum'', the alpenrose, snow-rose, or rusty-leaved alpenrose is an evergreen shrub that grows just above the tree line in the Alps, Pyrenees, Jura and northern Apennines, on acid soils. It is the type species for the genus ' ...
''. * Subgenus '' Choniastrum'' : 11 species Clade B * Subgenus '' Hymenanthes'' : Large leaf or elepidotes (without scales), including deciduous azaleas. 2 sections, about 140–225 species, type '' Rhododendron degronianum''. Clade C * Subgenus '' Azaleastrum'' : Evergreen azaleas. 3 sections, about 120 species, type '' Rhododendron ovatum''.
Sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
* Subgenus ''Therorhodion'' : 2 species (''
Rhododendron camtschaticum ''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 nat ...
'', '' 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'' (24 subsections) **('' Pentanthera'' (2 subsections – new section, moved from subgenus ''Pentanthera'') *Subgenus ''Pentanthera'' (4 sections, 23 species) (''Discontinued'') **'' Pentanthera'' (2 subsections – moved to subgenus ''Hymenanthes'') **'' Rhodora'' (L.) G. Don (2 species; ''
Rhododendron canadense ''Rhododendron canadense'', the rhodora or Canada rosebay, is a deciduous flowering shrub that is native to northeastern North America. Classification Today's botanists consider the rhodora to be a distant relative of the other North Americ ...
'', '' Rhododendron vaseyi'') (''Discontinued, redistributed'') **'' Sciadorhodion'' Rehder & Wilson (4 species) (''Moved to subgenus Azaleastrum'') **'' Viscidula'' Matsum. & Nakai (1 species; ''
Rhododendron nipponicum ''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 n ...
'') (''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 ''Rhododendron'' subsection ''Brachycalyx'' is a subsection of the genus ''Rhododendron'', in section '' Tsutsusi'', subgenus '' Azaleastrum'', consisting of fifteen species of Azaleas from Asia. Description Leaves A leaf ( : le ...
'' 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 Native may refer to: People * Jus soli, citizenship by right of birth * Indigenous peoples, peoples with a set of specific rights based on their historical ties to a particular territory ** Native Americans (disambiguation) In arts and entert ...
to areas from North America to
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
,
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
, and
Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
, and from
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland ...
to
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...
,
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and the
Solomon Islands Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capit ...
. The centres of diversity are in the
Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...
and
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federal constitutional monarchy consists of thirteen states and three federal territories, separated by the South China Sea into two regions: Peninsular Malaysia and Borneo's East Mal ...
, with the greatest species diversity in the Sino-Himalayan region, Southwest China and northern
Burma Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John C. Wells, Joh ...
, from India –
Himachal Pradesh Himachal Pradesh (; ; "Snow-laden Mountain Province") is a state in the northern part of India. Situated in the Western Himalayas, it is one of the thirteen mountain states and is characterized by an extreme landscape featuring several pea ...
,
Uttarakhand Uttarakhand ( , or ; , ), also known as Uttaranchal ( ; the official name until 2007), is a state in the northern part of India. It is often referred to as the "Devbhumi" (literally 'Land of the Gods') due to its religious significance and ...
,
Sikkim Sikkim (; ) is a state in Northeastern India. It borders the Tibet Autonomous Region of China in the north and northeast, Bhutan in the east, Province No. 1 of Nepal in the west and West Bengal in the south. Sikkim is also close to the Sil ...
and
Nagaland Nagaland () is a landlocked state in the northeastern region of India. It is bordered by the Indian states of Arunachal Pradesh to the north, Assam to the west, Manipur to the south and the Sagaing Region of Myanmar to the east. Its capital cit ...
to
Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
, northwestern
Yunnan Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...
and western
Sichuan Sichuan (; zh, c=, labels=no, ; zh, p=Sìchuān; alternatively romanized as Szechuan or Szechwan; formerly also referred to as "West China" or "Western China" by Protestant missions) is a province in Southwest China occupying most of t ...
and southeastern
Tibet Tibet (; ''Böd''; ) is a region in East Asia, covering much of the Tibetan Plateau and spanning about . It is the traditional homeland of the Tibetan people. Also resident on the plateau are some other ethnic groups such as Monpa people, ...
. Other significant areas of diversity are in the mountains of
Korea Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republic ...
,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
and
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the no ...
. 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 The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
species within the '' Vireya'' section of subgenus ''Rhododendron'' occupy the
Malay archipelago The Malay Archipelago ( Indonesian/ Malay: , tgl, Kapuluang Malay) is the archipelago between mainland Indochina and Australia. It has also been called the " Malay world," " Nusantara", "East Indies", Indo-Australian Archipelago, Spices Arc ...
from their presumed Southeast Asian origin to Northern Australia, with 55 known species in
Borneo Borneo (; id, Kalimantan) is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and e ...
and 164 in
New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; id, Papua, or , historically ) is the world's second-largest island with an area of . Located in Oceania in the southwestern Pacific Ocean, the island is separated from Australia by the wide Torres ...
. The species in New Guinea are native to subalpine moist
grasslands A grassland is an area where the vegetation is dominated by grasses ( Poaceae). However, sedge (Cyperaceae) and rush ( Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur nat ...
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 East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea ...
(Japan, Korea, Taiwan,
East China East China () is a geographical and a loosely defined cultural region that covers the eastern coastal area of China. A concept abolished in 1978, for economical purposes the region was defined from 1949 to 1961 by the Chinese Central Governme ...
), but not in North America or Eurasia. In the United States, native ''Rhododendron'' mostly occur in lowland and montane forests in the
Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Thou ...
,
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
, the
Northeast The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A compass rose is primarily composed of four cardinal directions—north, east, south, and west—each sep ...
, and the
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. The ...
.


Ecology


Invasive species

'' Rhododendron ponticum'' has become invasive in
Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...
and the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...
. 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
weevil Weevils are beetles belonging to the superfamily Curculionoidea, known for their elongated snouts. They are usually small, less than in length, and herbivorous. Approximately 97,000 species of weevils are known. They belong to several families, ...
s are major pests of rhododendrons, and many
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sy ...
s will preferentially devour them. ''Rhododendron'' species are used as food plants by the
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...
e ( caterpillars) of some
butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprises ...
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 plant Ornamental plants or garden plants are plants that are primarily grown for their beauty but also for qualities such as scent or how they shape physical space. Many flowering plants and garden varieties tend to be specially bred cultivars that ...
s in
landscaping Landscaping refers to any activity that modifies the visible features of an area of land, including the following: # Living elements, such as flora or fauna; or what is commonly called gardening, the art and craft of growing plants with a goal ...
in many parts of the world, including both
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
and subtemperate regions. Many species and cultivars are grown commercially for the 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 garden A woodland garden is a garden or section of a garden that includes large trees and is laid out so as to appear as more or less natural woodland, though it is often actually an artificial creation. Typically it includes plantings of flowering shr ...
s, 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 ''Rhododendron arboreum'', the tree rhododendron, is an evergreen shrub or small tree with a showy display of bright red flowers. It is found in Bhutan, China, India, Myanmar, Nepal, Sri Lanka, Pakistan and Thailand. It is the national flower of ...
'' and '' 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. * Vireya ( Malesian) rhododendrons: epiphytic tender shrubs * Azaleas – group of shrubs which have smaller and thinner leaves than evergreen rhododendrons. They are generally medium-sized shrubs with smaller funnel-shaped flowers that usually have 5 stamens: ** Deciduous hybrid azaleas: *** Exbury hybrids – derived from the Knap Hill hybrids, developed by Lionel de Rothschild at the Exbury Estate in England. *** Ghent (Gandavense) hybrids – Belgian raised *** Knap Hill hybrids – developed by Anthony Waterer at the Knap Hill Nursery in England. *** 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 Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
. *** 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 Bonsai ( ja, 盆栽, , tray planting, ) is the Japanese art of growing and training miniature trees in pots, developed from the traditional Chinese art form of '' penjing''. Unlike ''penjing'', which utilizes traditional techniques to produc ...
*** Shammarello hybrids – raised in northern
Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...
*** Vuyk (Vuykiana) hybrids – raised in the Netherlands * Azaleodendrons – semi-evergreen hybrids between deciduous azaleas and rhododendrons


Planting and care

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5–5.5; some tropical Vireyas and a few other rhododendron species grow as
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...
s 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 The RHS Chelsea Flower Show, formally known as the ''Great Spring Show'',Phil Clayton, ''The Great Temple Show'' in ''The Garden'' 2008, p.452, The Royal Horticultural Society is a garden show held for five days in May by the Royal Horticultural ...
in London (2011). Individual hybrids of rhododendrons have been
grafted Grafting or graftage is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion () while the lower part is called the rootstock. The succ ...
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
cultivar A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture ...
s of Rhododendron in th
International Rhododendron Registry
held by the
Royal Horticultural Society The Royal Horticultural Society (RHS), founded in 1804 as the Horticultural Society of London, is the UK's leading gardening charity. The RHS promotes horticulture through its five gardens at Wisley (Surrey), Hyde Hall (Essex), Harlow Carr (Nor ...
. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental bark or stems. Some hybrids have fragrant flowers—such as the Loderi hybrids, created by crossing '' Rhododendron fortunei'' and '' R. griffithianum''. Other examples include the PJM hybrids, formed from a cross between ''
Rhododendron carolinianum ''Rhododendron minus'' var. ''minus'', the Carolina azalea or Carolina rhododendron, is a rhododendron species native to the mountains of North Carolina, South Carolina, Tennessee, and Northeast Georgia. It is commonly known as ''Rhododendron c ...
'' and '' 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 ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology a ...
'' research have identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of
flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...
s or other phenolic compounds and
saponin Saponins (Latin "sapon", soap + "-in", one of), also selectively referred to as triterpene glycosides, are bitter-tasting usually toxic plant-derived organic chemicals that have a foamy quality when agitated in water. They are widely distributed ...
s 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 A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...
called grayanotoxin in their
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametop ...
and
nectar Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualist ...
. People have been known to become ill from eating
honey Honey is a sweet and viscous substance made by several bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of plants (primar ...
made by
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s feeding on rhododendron and
azalea Azaleas are flowering shrubs in the genus '' Rhododendron'', particularly the former sections '' Tsutsusi'' (evergreen) and '' Pentanthera'' (deciduous). Azaleas bloom in the spring (April and May in the temperate Northern Hemisphere, and Oct ...
flowers.
Xenophon Xenophon of Athens (; grc, Ξενοφῶν ; – probably 355 or 354 BC) was a Greek military leader, philosopher, and historian, born in Athens. At the age of 30, Xenophon was elected commander of one of the biggest Greek mercenary armies o ...
described the odd behaviour of Greek soldiers after having consumed honey in a village surrounded by '' Rhododendron ponticum'' during the march of the Ten Thousand in 401 BCE.
Pompey Gnaeus Pompeius Magnus (; 29 September 106 BC – 28 September 48 BC), known in English as Pompey or Pompey the Great, was a leading Roman general and statesman. He played a significant role in the transformation of ...
'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 The Third Mithridatic War (73–63 BC), the last and longest of the three Mithridatic Wars, was fought between Mithridates VI of Pontus and the Roman Republic. Both sides were joined by a great number of allies dragging the entire east of th ...
. 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 Anatolia, tr, Anadolu Yarımadası), and the Anatolian plateau, also known as Asia Minor, is a large peninsula in Western Asia and the westernmost protrusion of the Asian continent. It constitutes the major part of modern-day Turkey. The re ...
. Eleven similar cases during the 1980s have been documented in
Istanbul ) , postal_code_type = Postal code , postal_code = 34000 to 34990 , area_code = +90 212 (European side) +90 216 (Asian side) , registration_plate = 34 , blank_name_sec2 = GeoTLD , blank_i ...
,
Turkey Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a transcontinental country located mainly on the Anatolian Peninsula in Western Asia, with a small portion on the Balkan Peninsula ...
. 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 Sherlock Holmes () is a fictional detective created by British author Arthur Conan Doyle. Referring to himself as a " consulting detective" in the stories, Holmes is known for his proficiency with observation, deduction, forensic science and ...
'' as a proposed way to arrange a
fake execution A mock execution is a stratagem in which a victim is deliberately but falsely made to feel that their execution or that of another person is imminent or is taking place. The subject is made to believe that they are being led to their own execution ...
. It was also mentioned in the third episode of Season 2 of BBC's '' Sherlock'', speculated to have been a part of Sherlock's fake death scheme.


Culture


Symbolism

In the language of flowers, the Rhododendron symbolizes danger and to beware. ''
Rhododendron arboreum ''Rhododendron arboreum'', the tree rhododendron, is an evergreen shrub or small tree with a showy display of bright red flowers. It is found in Bhutan, China, India, Myanmar, Nepal, Sri Lanka, Pakistan and Thailand. It is the national flower of ...
'' (''lali guransh'') is the national flower of
Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
. ''R. ponticum'' is the state flower of Indian-administered Kashmir and Pakistan-controlled Kashmir. '' Rhododendron niveum'' is the state tree of
Sikkim Sikkim (; ) is a state in Northeastern India. It borders the Tibet Autonomous Region of China in the north and northeast, Bhutan in the east, Province No. 1 of Nepal in the west and West Bengal in the south. Sikkim is also close to the Sil ...
in
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
. Rhododendron arboreum is also the state tree of the state of
Uttarakhand Uttarakhand ( , or ; , ), also known as Uttaranchal ( ; the official name until 2007), is a state in the northern part of India. It is often referred to as the "Devbhumi" (literally 'Land of the Gods') due to its religious significance and ...
, India. Pink Rhododendron (''
Rhododendron campanulatum ''Rhododendron campanulatum'', the bell-flowered rhododendron or bell rhododendron, is a species of flowering plant in the family Ericaceae, native to the Himalayas and Tibet. It is the state flower of Himachal Pradesh Himachal Pradesh (; ; ...
'') is the state flower of
Himachal Pradesh Himachal Pradesh (; ; "Snow-laden Mountain Province") is a state in the northern part of India. Situated in the Western Himalayas, it is one of the thirteen mountain states and is characterized by an extreme landscape featuring several pea ...
, India. Rhododendron is also the provincial flower of Jiangxi, China and the state flower of
Nagaland Nagaland () is a landlocked state in the northeastern region of India. It is bordered by the Indian states of Arunachal Pradesh to the north, Assam to the west, Manipur to the south and the Sagaing Region of Myanmar to the east. Its capital cit ...
, the 16th state of the Indian Union. '' Rhododendron maximum'', the most widespread rhododendron of the
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. The ...
, is the state flower of the US state of
West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the ...
, and is in the
Flag of West Virginia The flag of West Virginia is the official flag of the U.S. State of West Virginia and was officially adopted by the West Virginia Legislature on March 7, 1929. The present flag consists of a pure white field bordered by a blue stripe with the co ...
. '' Rhododendron macrophyllum'', a widespread rhododendron of the
Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Thou ...
, is the state flower of the US state of
Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...
.


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 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 Howth ( ; ; non, Hǫfuð) is an affluent peninsular village and outer suburb of Dublin, Ireland. The district as a whole occupies the greater part of the peninsula of Howth Head, which forms the northern boundary of Dublin Bay, and include ...
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 Variations of gray or grey include achromatic grayscale shades, which lie exactly between white and black, and nearby colors with low colorfulness. A selection of a number of these various colors is shown below. Chart of computer web color ...
''. Amongst the Zomi tribes in
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
and
Myanmar Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John Wells explai ...
, "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 A young adult is generally a person in the years following adolescence. Definitions and opinions on what qualifies as a young adult vary, with works such as Erik Erikson's stages of human development significantly influencing the definition of ...
novel ''
Walk Two Moons ''Walk Two Moons'' is a novel written by Sharon Creech, published by HarperCollins in 1994 and winner of the 1995 Newbery Medal. The novel was originally intended as a follow-up to Creech's previous novel ''Absolutely Normal Chaos''; but, the ide ...
'' 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 Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
, 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 Uttarakhand ( , or ; , ), also known as Uttaranchal ( ; the official name until 2007), is a state in the northern part of India. It is often referred to as the "Devbhumi" (literally 'Land of the Gods') due to its religious significance and ...
. 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
* * * * In four volumes: Vol. I. ''Lepidotes'' , Vol. II. ''Elepidotes. Arboreum-Lacteum'' , Vol. III. ''Elepidotes Continued, Neriiflorum-Thomsonii, Azaleastrum and Camtschaticum'' , Vol. IV. ''Azaleas'' . * . * *


Articles

* * *


Subdivisions


Azaleas

* *


Tsutsusi

* * * * *


Vireya

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


Separate genera

*


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''
*
Flora of North America: Rhododendron
*
Flora of China: Rhododendron
*
Annotated Checklist of the Flowering Plants of Nepal: Rhododendron


Societies


American Rhododendron Society
*
The Quarterly Bulletin of the American Rhododendron Society 1947–1981
*
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