Editing Rhododendron (section)

== Taxonomy ==
[[File:Ghorepani,_Tadapani,_as_well_as_Mulde_Hill.jpg|thumb|Rhododendrons (Guras) at [[Ghorepani|Ghorepani, Nepal]]]]
[[File:Manaslu-Circuit Rhododendron Forest.jpg|thumb|right|Rhododendron forest in [[Nepal]]]]
[[File:Rhododendron＆Vast ocean of clouds、コバノミツバツツジ＆篠山盆地雲海、盃ヶ岳4256293.JPG|thumb|right|Rhododendron in [[Japan]]]]
[[File:Garden with Rhododendrons.JPG|thumb|right|A garden with tall Rhododendrons in [[Lynnwood, Washington]]]]
[[File:Rhododendron (গুরাস)02.jpg|thumb|Rhododendron (গুরাস), [[Sandakphu|Sandakphu, West Bengal, India]]]]
[[File:Azalea -- Rhododendron.jpg|thumb|Azalea, Texas Gulf Coast]]
[[File:Azalea as a Bonsai (5155648449).jpg|thumb|[[Bonsai]]]]
''Rhododendron'' is the largest genus in the family [[Ericaceae]], with over 1,000 species,<ref name="KewNumberOfSpecies">{{cite web |title=Rhododendrons: Wakehurst's hidden treasure. Rhododendron facts |url=https://www.kew.org/read-and-watch/rhododendrons-at-wakehurst#:~:text=Over%201000%20species%20are%20known%20to%20exist |website=Royal Botanic Gardens Kew |publisher=Board of Trustees of the Royal Botanic Gardens, Kew |access-date=21 May 2022}}</ref><ref>{{cite web |url=http://events.berkeley.edu/index.php/calendar/sn/bot.html?event_ID=73328 |title=Sorting out the Genus Rhododendron: A Taxonomic Overview of this Large Complex of Species |website=University of California Botanical Garden at Berkeley |archive-url=https://web.archive.org/web/20140714150332/http://events.berkeley.edu/index.php/calendar/sn/bot.html?event_ID=73328 |archive-date=14 July 2014}}</ref> (though estimates vary from 850 to 1,200){{sfnp|Fayaz |2012 |page=3 |ref=Fayaz}}<ref name="RSCG-NumberOfSpecies">{{cite web |title=Rhododendron Species Conservation Group |url=https://rscg.org.uk/#:~:text=There%20are%20about%201%2C200%20species%20of%20rhododendron |website=Rhododendron Species Conservation Group |publisher=The Rhododendron Species Conservation Group |access-date=21 May 2022}}</ref> and is [[morphology (biology)|morphologically]] diverse. Consequently, the [[taxonomy (biology)|taxonomy]] has been historically complex.{{sfnp|Brown |Craven |Udovicic |Ladiges |2006 |ref=Brown2006}}

=== Early history ===
Although Rhododendrons had been known since the description of ''[[Rhododendron hirsutum]]'' by [[Charles de l'Écluse]] (Clusius) in the sixteenth century, and were known to classical writers (Magor 1990), and referred to as ''Chamaerhododendron'' (low-growing rose tree), the genus was first formally described by [[Carl Linnaeus|Linnaeus]] in his ''[[Species Plantarum]]'' in 1753.<ref name=LSPR/><ref>{{cite book |last=Linnaeus |first=C. |author-link=Carl Linnaeus |title=Genera Plantarum |edition=5th |page=185 |year=1754 |url=https://www.biodiversitylibrary.org/item/14678#page/1/mode/1up |publisher=Laurentii Salvii |location=Stockholm |access-date=1 October 2017}}</ref> He listed five species under ''Rhododendron'': [[Rhododendron ferrugineum|''R.''&nbsp;''ferrugineum'']] (the [[type species]]), ''[[Rhododendron dauricum|R.&nbsp;dauricum]]'', ''[[Rhododendron hirsutum|R.&nbsp;hirsutum]]'', ''R.&nbsp;chamaecistus'' (now ''[[Rhodothamnus chamaecistus]]'' (L.) Rchb.) and ''[[Rhododendron maximum|R.&nbsp;maximum]]''. At that time he considered the then known six species of ''[[Azalea]]''<ref name=LSPA>{{cite book |chapter-url=https://www.biodiversitylibrary.org/item/13829#page/162/mode/1up |last=Linnaeus |first=C. |author-link=Carl Linnaeus |title=Species Plantarum |chapter=Azalea |volume=Tomus I |page=150 |year=1753 |publisher=Laurentii Salvii |location=Stockholm |access-date=15 June 2014}}</ref> that he had described earlier in 1735 in his ''[[Systema Naturae]]'' as a separate genus.<ref name=WIAA>{{cite web |last=Andrews |first=Charles |url=http://azaleachapter.com/plant-info/plants/what-is-an-azalea |publisher=American Rhododendron Society (Azalea Chapter) |title=What is an Azalea?}}</ref><ref name=Spady>{{cite journal |url=http://scholar.lib.vt.edu/ejournals/JARS/v35n4/v35n4-spady.htm |first=Herbert A. |last=Spady |title=GROPING FOR GROUPING: Rhododendron Taxonomy |journal=The Quarterly Bulletin of the American Rhododendron Society |volume=35 |issue=4 |date=Fall 1981}}</ref>

Linnaeus' six species of ''Azalea'' were ''[[Rhododendron indicum|Azalea indica]]'', ''[[Rhododendron luteum|A.&nbsp;pontica]]'', ''[[Rhododendron calendulaceum|A.&nbsp;lutea]]'', ''[[Rhododendron viscosum|A.&nbsp;viscosa]]'', ''[[Rhododendron lapponicum|A.&nbsp;lapponica]]'' and ''A.&nbsp;procumbens'' (now ''[[Kalmia procumbens]]''), which he distinguished from ''Rhododendron'' by having five [[stamens]], as opposed to ten. As new species of what are now considered ''Rhododendron'' were discovered, they were assigned to separate genera if they seemed to differ significantly from the type species. For instance ''Rhodora'' (Linnaeus 1763) for ''[[Rhododendron canadense]]'', ''Vireya'' ([[Carl Ludwig Blume|Blume]] 1826)<ref>{{cite web |url=http://www.vireya.net/history.htm |first=Chris |last=Callard |title=The History of Vireya Rhododendron Culture |website=vireya.net |date=1998–2015 |access-date=28 September 2017}}</ref> and ''Hymenanthes'' (Blume 1826) for ''Rhododendron metternichii'', now [[Rhododendron degronianum|R. degronianum]]. Meanwhile, other botanists such as [[Richard Anthony Salisbury|Salisbury]] (1796)<ref>{{cite book |url=https://www.biodiversitylibrary.org/item/9522#page/293/mode/1up |last=Salisbury |first=R.A. |author-link=Richard Anthony Salisbury |title=Prodromus Stirpium in horto ad Chapel Allerton |date=1796 |page=286 |location=London |language=la}}</ref> and Tate (1831)<ref>{{cite book |chapter-url=http://www.british-history.ac.ok/vch/middx/vol12/pp150-155 |chapter=Economic history: Farm-gardening and market gardening |title=A History of the County of Middlesex: Volume 12, Chelsea |editor=Patricia E C Croot |location=London |date=2004 |pages=150–155 |publisher=British History Online}}</ref> began to question the distinction between ''Azalea'' and ''Rhododendron'', and finally in 1836, ''Azalea'' was incorporated into ''Rhododendron''<ref name=Don/> and the genus divided into eight sections. Of these ''Tsutsutsi'' (''[[Rhododendron subg. Tsutsuji|Tsutsusi]]''), ''[[Pentanthera]]'', ''[[Pogonanthum]]'', ''[[Ponticum]]'' and ''[[Rhodora]]'' are still used, the other sections being ''Lepipherum'', ''Booram'', and ''Chamaecistus''. This structure largely survived till recently (2004), following which the development of molecular phylogeny led to major {{Nowrap|re-examinations}} of traditional morphological classifications,<ref name=WIAA/><ref name=Spady/> although other authors such as Candolle, who described six sections,{{sfnp|Candolle |1838 |ref=Candolle}} 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''<ref name=Maximovicz/> and [[Jules Émile Planchon|Planchon]]. Maximovicz used flower bud position and its relationship with leaf buds to create eight "Sections".{{sfnp|JIN |DING |ZHANG |HONG |2010 |ref=JDZH}} [[Bentham and Hooker]] used a similar scheme, but called the divisions "Series".<ref name=BH/> It was not until 1893 that [[Bernhard Adalbert Emil Koehne|Koehne]] appreciated the significance of scaling and hence the separation of lepidote and elepidote species. The large number of species that were available by the early twentieth century prompted a new approach when [[Isaac Bayley Balfour|Balfour]] introduced the concept of grouping species into [[Series (botany)|series]]. ''The Species of Rhododendron''<ref>{{cite book |editor-last=Stevenson |editor-first=J.B. |title=The Species of Rhododendron |publisher=The Rhododendron Society |location=Edinburgh |date=1930}}</ref> referred to this series concept as the Balfourian system. That system continued up to modern times in Davidian's four volume ''The Rhododendron Species''.{{sfnp|Davidian |ref=Davidian}}

=== Modern classification ===
The next major attempt at classification was by [[Sleumer]] who from 1934 began incorporating the Balfourian series into the older hierarchical structure of subgenera and sections, according to the [[International Code of Botanical Nomenclature]], culminating in 1949 with his "Ein System der Gattung ''Rhododendron''"<ref name=Sleumer49>{{cite journal |last=Sleumer |first=H |year=1949 |title=Ein System der Gattung ''Rhododendron'' L. |journal=Bot. Jahrb. Syst. |volume=74 |pages=511–553}}</ref> and subsequent refinements.<ref name=Sleumer66/><ref name=Sleumer80/> 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'').<ref name=Sleumer80/><ref>{{cite journal |first=J. |last=Cullen |title=A revision of Rhododendron I. Subgenus Rhododendron sections Rhododendron and Pogonanthum |journal=Notes from the Royal Botanic Garden Edinburgh |volume=39 |issue=1 |date=1980 |issn=0080-4274}}</ref> In 1986 Philipson & [[Melva Philipson|Philipson]] raised two sections of subgenus ''Aleastrum'' (''Mumeazalea'', ''Candidastrum'') to subgenera, while reducing genus ''Therorhodion'' to a subgenus of ''Rhododendron''.<ref>{{cite journal |last1=Philipson |first1=W.R. |last2=Philipson |first2=M.N. |year=1986 |title=A revision of Rhododendron III. Subgenera Azaleastrum, Mumeazalea, Candidastrum and Therorhodion |journal=Notes from the Royal Botanic Garden Edinburgh |volume=44 |pages=1–23 |issn=0080-4274}}</ref> In 1987 Spethmann, adding [[phytochemical]] features proposed a system with fifteen subgenera grouped into three 'chorus' subgenera.<ref>{{cite journal |last1=Spethmann |first1=W |year=1987 |title=A new infrageneric classification and phylogenetic trends in the genus ''Rhododendron'' (''Ericaceae'') |journal=Plant Systematics and Evolution |volume=157 |issue= 1–2|pages=9–31 |issn=1615-6110 |doi=10.1007/BF00939178|bibcode=1987PSyEv.157....9S |s2cid=35751500}}</ref>

A number of closely related genera had been included together with ''Rhododendron'' in a former tribe, Rhodoreae. These have been progressively incorporated into ''Rhododendron''.<ref>{{cite journal | last1 = Stevens | first1 = PF | year = 1971| title = A classification of the Ericaceae: subfamilies and tribes | journal = Botanical Journal of the Linnean Society | volume = 64 | issue = 1| pages = 1–53 | doi = 10.1111/j.1095-8339.1971.tb02133.x}}</ref> Chamberlain and Rae moved the [[monotypic]] section ''Tsusiopsis'' together with the monotypic genus ''Tsusiophyllum'' into section ''Tsutsusi'',{{sfnp|Chamberlain |Rae |1990 |ref=ChamRae1990}} while Kron & Judd reduced genus ''[[Ledum]]'' to a subsection of section ''Rhododendron''.<ref>{{cite journal | last1 = Kron | first1 = K. A. | last2 = Judd | first2 = W. S. | year = 1990 | title = Phylogenetic relationships within the Rhodoreae (Ericaceae) with specific comments on the placement of ''Ledum'' | journal = Systematic Botany | volume = 15 | issue = 1| pages = 57–68 | doi=10.2307/2419016| jstor = 2419016| bibcode = 1990SysBo..15...57K }}</ref> Then Judd & Kron moved two species ([[Rhododendron schlippenbachii|''R.''&nbsp;''schlippenbachii'']] and [[Rhododendron quinquefolium|''R.''&nbsp;''quinquefolium'']]) from section ''Brachybachii'', subgenus ''Tsutsusi'' and two from section ''Rhodora'', subgenus ''Pentanthera'' (''[[Rhododendron albrechtii|R.&nbsp;albrechtii]]'', ''[[Rhododendron pentaphyllum|R.&nbsp;pentaphyllum]]'') into section ''Sciadorhodion'', subgenus ''Pentanthera''.<ref>{{cite journal | last1 = Judd | first1 = W. S. | last2 = Kron | first2 = K. A. | year = 1995 | title = A revision of ''Rhododendron''. VI. Subgenus ''Pentanthera'' (sections ''Sciadorhodion'', ''Rhodora'', and ''Viscidula'') | journal = Edinburgh Journal of Botany | volume = 52 | issue = 1| pages = 1–54 | doi=10.1017/s0960428600001906| doi-access = free }}</ref> Finally Chamberlain brought the various systems together in 1996, with 1,025 species divided into eight subgenera. Goetsch (2005) provides a comparison of the Sleumer and Chamberlain schemata (Table&nbsp;1).{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}}{{sfnp|Brown |Craven |Udovicic |Ladiges |2006 |ref=Brown2006}}<ref name=Spady/><ref name=Chamberlain/>{{sfnp|Cullen |2005 |ref=Cullen2005}}

===Phylogeny===
{{cladogram
 |align=left
 |title= {{anchor|Clad1}}Cladogram of genus ''Rhododendron'' <br>(Goetsch ''et al.'' 2005)
 |cladogram={{clade
  |style=400px;
  |label1=
  |1={{clade
   |label1=
   |1={{clade
    |label1=
    |1={{clade
     |label1='''{{sc|A}}'''
     |1={{clade
        |1=[[Rhododendron subg. Rhododendron|''Rhododendron'']]
        |2=''[[Choniastrum]]''
        }}
     |label2='''{{sc|B}}'''
     |2=''[[Hymenanthes]]''
     }}
    |label2='''{{sc|C}}'''
    |2=''[[Azaleastrum]]''
    }}
   |2={{clade  |label1=  |1=''[[Therorhodion]]'' }}
   }}
 }}
}}
The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who used [[Maturase K|matK]] [[sequencing]]. Later Gao ''et al.'' (2002) used [[Internal transcribed spacer|ITS]] sequences<ref name=LianMing/> to determine a [[cladistic]] analysis. They confirmed that the genus ''Rhododendron'' was [[monophyletic]], with subgenus ''Therorhodion'' in the [[Basal (phylogenetics)|basal position]], consistent with the matK studies. Following publication of the studies of Goetsch ''et al.'' (2005) with [[RPB2]],{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}} 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.''.{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}}<ref name=FSRS>{{cite web |last1=Goetsch |first1=Loretta A.|last2=Eckert |first2=Andrew J. |last3=Hall |first3=Benjamin D. |date=2005 |url=http://www.flounder.ca/FraserSouth/Goetsch-Eckert-Hall.asp |archive-url=https://web.archive.org/web/20130227185832/http://www.flounder.ca/FraserSouth/Goetsch-Eckert-Hall.asp |archive-date=27 February 2013 |website=Fraser South Rhododendron Society |title=Excerpts from and comments on a paper published by: Loretta Goetsch, Andrew Eckert and Benjamin Hall, University of Washington}} 2005 Annual ARS Convention</ref>

The major finding of Goetsch and colleagues was that all species examined (except [[Rhododendron camtschaticum|''R.&nbsp;camtschaticum'']], subgenus ''Therorhodion'') formed three major [[clades]] which they labelled '''{{sc|A}}''', '''{{sc|B}}''', and '''{{sc|C}}''', with the subgenera ''Rhododendron'' and ''Hymenanthes''  as monophyletic groups nested within clades '''{{sc|A}}''' and '''{{sc|B}}''', respectively. By contrast subgenera ''Azaleastrum'' and ''Pentanthera'' were [[polyphyletic]], while ''R.&nbsp;camtschaticum'' appeared as a [[Sister group|sister]] to all other rhododendrons. The small polyphyletic subgenera ''Pentanthera'' and ''Azaleastrum'' were divided between two clades. The four sections of ''Pentanthera'' between clades '''{{sc|B}}''' and '''{{sc|C}}''', with two each, while ''Azaleastrum'' had one section in each of '''{{sc|A}}''' and '''{{sc|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 '''{{sc|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 '''{{sc|B}}''' (''Hymenanthes'') and '''{{sc|C}}''' (''Azaleastrum''), although the name was retained in section ''Pentanthera'' (14&nbsp;species) which was moved to subgenus ''Hymenanthes''. Of the remaining three sections, monotypic ''Viscidula'' was discontinued by moving [[Rhododendron nipponicum|''R.''&nbsp;''nipponicum'']] to ''Tsutsusi'' ('''{{sc|C}}'''), while ''Rhodora'' (2&nbsp;species) was itself polyphyletic and was broken up by moving [[Rhododendron canadense|''R.''&nbsp;''canadense'']] to section ''Pentanthera'' ('''{{sc|B}}''') and [[Rhododendron vaseyi|''R.''&nbsp;''vaseyi'']] to section ''Sciadorhodion'', which then became a new section of subgenus ''Azaleastrum''&nbsp;('''{{sc|C}}''').

Subgenus ''Tsutsusi'' ('''{{sc|C}}''') was reduced to section status retaining the name, and included in subgenus ''Azaleastrum''. Of the three minor subgenera, all in '''{{sc|C}}''', two were discontinued. The single species of [[monotypic]] subgenus ''Candidastrum'' ([[Rhododendron albiflorum|''R.''&nbsp;''albiflorum'']]) was moved to subgenus ''Azaleastrum'', section ''Sciadorhodion''. Similarly the single species in monotypic subgenus ''Mumeazalea'' ([[Rhododendron semibarbatum|''R.''&nbsp;''semibarbatum'']]) was placed in the new section ''Tsutsusi'', subgenus ''Azaleastrum''. Genus ''Menziesa'' (9&nbsp;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&nbsp;'''{{sc|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.{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}} (See schemata under [[#Subgenera|''Subgenera'']].)<ref name=FSRS/>

{| class="wikitable" style="width:500px;"
|+ <big>Taxonomic changes within genus ''Rhododendron''</big>
! style="border-right:solid black 2px; background: #ccf" colspan=4|Chamberlain (1996)||style="background: #ccf" colspan=2|Goetsch (2005)
|-
! style="background:#ccf" |Genus ||style="background:#ccf" |Subgenus ||style="background:#ccf" |Section ||style="border-right:solid black 2px; background:#ccf" |Species ||style="background:#ccf" |Subgenus ||style="background:#ccf" |Section
|-
| ''Menziesa'' ||   ||   ||style="border-right:solid black 2px"| 9&nbsp;species ||style="text-align:center;" rowspan=4|''Azaleastrum''<br/>'''{{sc|C}}'''||rowspan=4|''Sciadorhodion''
|-
| rowspan=10| ''Rhododendron'' || ''Candidastrum'' ||   ||style="border-right:solid black 2px"|''R.&nbsp;albiflorum''
|-
| rowspan=5| ''Pentanthera'' || ''Sciadorhodion'' ||style="border-right:solid black 2px"| 4&nbsp;species
|-
| rowspan=2| ''Rhodora'' ||style="border-right:solid black 2px"|''R.&nbsp;vaseyi''
|-
| style="border-right:solid black 2px"|''R.&nbsp;canadense'' ||style="text-align:center;" rowspan=2|''Hymenanthes''<br/>'''{{sc|B}}'''||rowspan=2|''Pentanthera''
|-
| ''Pentanthera'' ||style="border-right:solid black 2px"| 14&nbsp;species
|-
| ''Viscidula'' ||style="border-right:solid black 2px"| ''R.&nbsp;nipponicum'' ||style="text-align:center;" rowspan=4|''Azaleastrum''<br/>'''{{sc|C}}'''||rowspan=4|''Tsutsusi''
|-
| rowspan=2| ''Tsutsusi'' || ''Brachycalyx'' ||style="border-right:solid black 2px"| 15&nbsp;species
|-
| ''Tsutsusi'' ||style="border-right:solid black 2px"| 65&nbsp;species
|-
| ''Mumeazalea'' ||   ||style="border-right:solid black 2px"|''R.&nbsp;semibarbatum''
|-
| ''Azaleastrum'' || ''Choniastrum'' ||style="border-right:solid black 2px"| 11&nbsp;species ||style="text-align:center;"| ''Choniastrum''<br/>'''{{sc|A}}'''
|}

Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions.{{sfnp|Craven |Goetsch |Hall |Brown |2008 |ref=Craven2008}} In 2011 the two species of ''Diplarche'' were also added to ''Rhododendron'', ''[[incertae sedis]]''.{{sfnp|Craven|2011 |ref=Craven2011}}

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

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

In the later traditional classification, attributed to Chamberlain (1996), and as used by [[horticulturalists]] and the [[American Rhododendron Society]],{{sfnp|Genus Rhododendron Taxonomic Tree |ref=ARS2}} ''Rhododendron'' has eight subgenera based on [[Morphology (biology)|morphology]], namely the presence of scales (lepidote), [[deciduousness]] of leaves, and the floral and vegetative branching patterns, after Sleumer (1980).{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}}{{sfnp|Brown |Craven |Udovicic |Ladiges |2006 |ref=Brown2006}}<ref name=Sleumer80/> 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.<ref name="ARS1"/> The largest of these is subgenus ''Rhododendron'', containing nearly half of all known species and all of the lepidote species.

*Subgenus ''[[Rhododendron subg. Rhododendron|Rhododendron]]'' {{small|L.}}: Small leaf or [[lepidotes]] (scales on the underside of the leaves). 3 sections, 462 species, [[type species]]: ''R.&nbsp;ferrugineum''.
*Subgenus ''[[Rhododendron subg. Hymenanthes|Hymenanthes]]'' {{small|(Blume) K.Koch}}: Large leaf or [[elepidotes]] (without scales). 1 section, 224 species, type [[Rhododendron degronianum|''R.''&nbsp;''degronianum'']].
*Subgenus ''[[Rhododendron subg. Pentanthera|Pentanthera]]'' {{small|(G. Don) Pojarkova}}: Deciduous azaleas. 4 sections, 23 species, type [[Rhododendron luteum|''R.''&nbsp;''luteum'']].
*Subgenus ''[[Rhododendron sect. Tsutsusi|Tsutsusi]]'' {{small|(Sweet) Pojarkova}}: Evergreen azaleas. 2 sections, 80 species, type [[Rhododendron indicum|''R.''&nbsp;''indicum'']].
*Subgenus ''[[Rhododendron subg. Azaleastrum|Azaleastrum]]'' {{small|Planch.}}: 2 sections, 16 species, type ''[[Rhododendron ovatum|R. ovatum]]''.
*Subgenus ''Candidastrum'' {{small|[[Franch.]]}}: 1 species, ''[[Rhododendron albiflorum|R. albiflorum]]''.
*Subgenus ''Mumeazalea'' {{small|(Sleumer) W.R. Philipson & M.N. Philipson}}: 1 species, ''[[Rhododendron semibarbatum]]''.
*Subgenus ''Therorhodion'' {{small|(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.{{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}}

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.

[[File:Rhododendron luteum BotGardBln1105Autumn.JPG|thumb|right|Deciduous ''Rhododendron luteum'' in fall color]]
[[File:Rhododendron kaempferi 08.jpg|thumb|Evergreen azalea ''Rhododendron kaempferi'']]
[[File:2014-10-29 13 26 34 Evergreen Rhododendron foliage coloring before being shed in Ewing, New Jersey.JPG|thumb|right|Evergreen azalea cultivar leaf color before shedding]]
[[File:Rhododendron after freezing rain.jpg|thumb|right|Rhododendron after [[freezing rain]]]]
{| class="wikitable"
|+ <big>Morphological classification of ''Rhododendron'' </big><br>(Chamberlain 1996)<ref name=Chamberlain/>
! style="background:#ccf" |Inflorescence buds !!style="background:#ccf" |Leaf scales !!style="background:#ccf" |Leaf shoots !!style="border-right:solid black 2px; background:#ccf" |Leaves !!style="background:#ccf" |Subgenus !!style="background:#ccf" |Section
|-
|rowspan=7| Terminal ||colspan=3|Present ||style="border-left:solid black 2px"|''[[Rhododendron subg. Rhododendron|Rhododendron]]''
|-
| rowspan=6 |Absent||rowspan=4|Previous year||style="border-right:solid black 2px" |Evergreen||''[[Rhododendron subg. Hymenanthes|Hymenanthes]]''
|-
| rowspan=3 style="border-right:solid black 2px"|Deciduous||rowspan=3|''[[Rhododendron subg. Pentanthera|Pentanthera]]'' ||''Pentanthera''
|-
| ''Rhodora''
|-
| ''Viscidula''
|-
|rowspan=2 colspan=2|Lowest leaves|| style="border-left:solid black 2px"|''Pentanthera''||''Sciadorhodion''
|-
| style="border-left:solid black 2px"|''[[Rhododendron sect. Tsutsusi|Tsutsusi]]''
|-
|rowspan=4 colspan=3|Lateral|| style="border-right:solid black 2px"|Evergreen||''[[Rhododendron subg. Azaleastrum|Azaleastrum]]''
|-
| rowspan=3 style="border-right:solid black 2px"|Deciduous||''Candidastrum''
|-
| ''Mumeazalea''
|-
|''Therorhodion''
|}

Following the [[cladistic analysis]] of Goetsch ''et al.'' (2005){{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}} this scheme was simplified, based on the discovery of three major [[clade]]s (A, B, C) as follows.

'''Clade A'''
* Subgenus ''[[Rhododendron subg. Rhododendron|Rhododendron]]'' {{Small|L.}}: Small leaf or [[lepidotes]] (scales on the underside of the leaves). 3 sections, about 400 species, [[type species]]: ''R.''&nbsp;''ferrugineum''.
* Subgenus ''[[Rhododendron subg. Choniastrum|Choniastrum]]'' {{Small|Franch.}}: 11 species
'''Clade B'''
* Subgenus ''[[Rhododendron subg. Hymenanthes|Hymenanthes]]'' {{Small|(Blume) K.Koch}}: Large leaf or [[elepidotes]] (without scales), including deciduous azaleas. 2 sections, about 140–225 species, type ''[[Rhododendron degronianum|R. degronianum]]''.
'''Clade C'''
* Subgenus ''[[Rhododendron subg. Azaleastrum|Azaleastrum]]'' {{Small|Planch.}}: Evergreen azaleas. 3 sections, about 120 species, type ''[[Rhododendron ovatum]]''.
'''[[Sister taxon]]'''
* Subgenus ''Therorhodion'' {{Small|(Maxim.) A. Gray}}: 2 species (''[[Rhododendron camtschaticum|R. camtschaticum]]'' and ''[[Rhododendron redowskianun|R. redowskianun]]'').

==== Sections and subsections ====
The larger subgenera are further subdivided into sections and subsections<ref name="ARS1">{{cite web |url=http://www.rhododendron.org/v47n3p139.htm |title=Concerning the Origin and Distribution of Rhododendrons |last=Irving |first=E. |author2=R. Hebda |year=1993 |work=Journal of the American Rhododendron Society |publisher=American Rhododendron Society |access-date=4 November 2012}}</ref> 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){{sfnp|Goetsch |Eckert |Hall|2005 |ref=Goetsch}} and by Craven ''et al.'' (2008){{sfnp|Craven |Goetsch |Hall |Brown |2008 |ref=Craven2008}} 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{{sfnp|Genus Rhododendron Taxonomic Tree |ref=ARS2}}

*Subgenus ''Rhododendron'' L. (3 sections, 462 species: increased to five sections in 2008)
**(''Discovereya (Sleumer) Argent, raised from Vireya'')
**''[[Pogonathum]]'' Aitch. & Hemsl. (13 species; Himalaya and adjacent mountains)
**(''Pseudovireya (C.B.Clarke) Argent, raised from Vireya'')
**''[[Rhododendron (section)|Rhododendron]]'' L. (149 species in 25 subsections; temperate to subarctic Northern Hemisphere)
**''[[Rhododendron subgenus Vireya|Vireya]]'' (Blume) Copel.f. (300 species in 2 subsections; tropical southeast Asia, Australasia. At one time considered separate subgenus{{sfnp|Argent|2006 |ref=Argent2006}})
*Subgenus ''[[Rhododendron subg. Hymenanthes|Hymenanthes]]'' (Blume) K.Koch (1 section, 224 species) (''Increased to two sections'')
**''Ponticum'' {{small|G. Don}} (24 subsections)
**(''[[Rhododendron sect. Pentanthera|Pentanthera]]'' {{small|(G. Don) Pojarkova}} (2 subsections – new section, moved from subgenus ''Pentanthera'')
*Subgenus ''Pentanthera'' {{small|(G. Don) Pojarkova}} (4 sections, 23 species) (''Discontinued'')
**''[[Rhododendron sect. Pentanthera|Pentanthera]]'' {{small|(G. Don) Pojarkova}} (2 subsections – moved to subgenus ''Hymenanthes'')
**''[[Rhododendron sect. Rhodora|Rhodora]]'' (L.) G. Don (2 species; ''[[Rhododendron canadense]]'', ''[[Rhododendron vaseyi]]'') (''Discontinued, redistributed'')
**''[[Rhododendron sect. Sciadorhodion|Sciadorhodion]]'' Rehder & Wilson (4 species) (''Moved to subgenus Azaleastrum'')
**''[[Viscidula]]'' Matsum. & Nakai (1 species; ''[[Rhododendron nipponicum]]'') (''Discontinued, added to section Tsutsusi, subgenus Azaleastrum'')
*Subgenus ''Tsutsusi'' (Sweet) Pojarkova (2 sections, 80 species) (''Discontinued, reduced to section and moved to subgenus Azaleastrum'')
**''[[Brachycalyx]]'' Sweet (3 alliances, 15 species)
**''[[Tsutsusi]]'' (Sweet) Pojarkova (65 species)
*Subgenus ''Azaleastrum'' Planch. (2 sections, 16 species) (''Increased to three sections'')
**''Azaleastrum'' Planch. (5 species)
**(''Choniastrum'' Franch. (11 species) (''Raised to subgenus''))
**(''[[Sciadorhodion]] Rehder & Wilson (4 species) (Moved from subgenus Pentanthera''))
**''(Tsutsusi (Sweet) Pojarkova (reduced from subgenus))''
*Subgenus ''Candidastrum'' Franch. (1 species: ''[[Rhododendron albiflorum]]'') (''Discontinued, moved to section Sciadorhodion, subgenus Azaleastrum'')
*Subgenus ''Mumeazalea'' (Sleumer) W.R. Philipson & M.N. Philipson (1 species: ''[[Rhododendron semibarbatum]]'') (''Discontinued, moved to section Tsutsusi, subgenus Azaleastrum'')
*Subgenus ''Therorhodion'' A. Gray (2 species)
*(''Subgenus Choniastrum Franch. (11 species)'')

The system used by the [[World Flora Online]] {{As of|2023|December|lc=yes}} uses six subgenera, four of which are divided further:<ref name=WFO_4000033027>{{cite web |author=WFO |date=December 2023 |title=''Rhododendron'' L.. |work=World Flora Online |url=https://list.worldfloraonline.org/wfo-4000033027 |access-date=2024-02-22}}</ref>
*[[Rhododendron subg. Azaleastrum|subgenus ''Azaleastrum'']] {{small|Planch. ex K.Koch}}<ref name=WFO_1000040525>{{cite web |author=WFO |date=December 2023 |title=''Rhododendron'' subg. ''Azaleastrum'' Planch. ex K.Koch |work=World Flora Online |url=https://list.worldfloraonline.org/wfo-1000040525 |access-date=2024-02-22}}</ref>
**[[Rhododendron sect. Azaleastrum|section ''Azaleastrum'']] {{small|Planch. ex Maxim.}}
**[[Rhododendron sect. Sciadorhodion|section ''Sciadorhodion'']] {{small|Rehder & E.H.Wilson}}
**[[Rhododendron sect. Tsutsutsi|section ''Tsutsutsi'']] {{small|(Sweet) Pojark.}}
*[[Rhododendron subg. Choniastrum|subgenus ''Choniastrum'']] {{small|Franch.}}
*[[Rhododendron subg. Hymenanthes|subgenus ''Hymenanthes'']] {{small|(Blume) K.Koch}}<ref name=WFO_1000040538>{{cite web |author=WFO |date=December 2023 |title=''Rhododendron'' subg. ''Hymenanthes'' (Blume) K.Koch |work=World Flora Online |url=https://list.worldfloraonline.org/wfo-1000040538 |access-date=2024-02-22}}</ref>
**[[Rhododendron sect. Pentanthera|section ''Pentanthera'']] {{small|G.Don}}
**[[Rhododendron sect. Ponticum|section ''Ponticum'']] {{small|G.Don}}
**[[Rhododendron sect. Rhodora|section ''Rhodora'']] {{small|(L.) G.Don}}
*[[Rhododendron subg. Rhododendron|subgenus ''Rhododendron'']] {{small|L.}}<ref name=WFO_3500001788>{{cite web |author=WFO |date=December 2023 |title=''Rhododendron'' subg. ''Rhododendron'' L.. |work=World Flora Online |url=https://list.worldfloraonline.org/wfo-3500001788 |access-date=2024-02-22}}</ref>
**[[Rhododendron sect. Pogonanthum|section ''Pogonanthum'']] {{small|G.Don}}
**[[Rhododendron sect. Rhododendron|section ''Rhododendron'']] {{small|}}
*[[Rhododendron subg. Therorhodion|subgenus ''Therorhodion'']] {{small|(Maxim) Rehder}}
*[[Rhododendron subg. Vireya|subgenus ''Vireya'']] {{small|Clarke}}<ref name=WFO_3500001320>{{cite web |author=WFO |date=December 2023 |title=''Rhododendron'' subg. ''Vireya'' L.. |work=World Flora Online |url=https://list.worldfloraonline.org/wfo-3500001320 |access-date=2024-02-22}}</ref>
**[[Rhododendron sect. Albovireya|section ''Albovireya'']] {{small|(Sleumer) Argent}}
**[[Rhododendron sect. Discovireya|section ''Discovireya'']] {{small|(Sleumer) Argent}}
**[[Rhododendron sect. Hadranthe|section ''Hadranthe'']] {{small|Schltr.}}
**[[Rhododendron sect. Malayovireya|section ''Malayovireya'']] {{small|(Sleumer) Argent}}
**[[Rhododendron sect. Pseudovireya|section ''Pseudovireya'']] {{small|(Clarke) Sleumer}}
**[[Rhododendron sect. Schistanthe|section ''Schistanthe'']] {{small|Schltr.}}
**[[Rhododendron sect. Siphonovireya|section ''Siphonovireya'']] {{small|Argent}}

=== Species ===
{{further|List of Rhododendron species}}