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FNA | Family List | FNA Vol. 8 | Ericaceae

23. Rhododendron Linnaeus, Sp. Pl. 1: 392. 1753; Gen. Pl. ed. 5, 185. 1754.

Azalea [Greek rhodon, rose and dendron, tree] Azalea [Greek rhodon, rose and dendron, tree]

Walter S. Judd, Kathleen A. Kron

Anthodendron Reichenbach; Azaleastrum (Planchon ex Maximowicz) Rydberg; Biltia Small; Hymenanthes Blume; Ledum Linnaeus; Rhodora Linnaeus; Tsusiophyllum Maximowicz; Tsutsusi Adanson

Subshrubs, shrubs, or trees, (rhizomatous or not). Stems erect (and/or creeping, prostrate in R. groenlandicum, R. lapponicum, R. tomentosum); twigs hairy or glabrous, (older twigs without peglike projections). Leaves persistent or deciduous, alternate; petiole present; blade coriaceous to membranous, (base cuneate, rounded, or obtuse), margins entire, (sometimes ciliate, plane to revolute, abaxial surface 1/3+ visible except sometimes in bud, surfaces hairy and/or scaled, midvein hairy or not, adaxial surface sometimes impressed; venation usually brochidodromous). ( Winter buds usually large, with imbricate scales; floral buds usually larger than vegetative buds.) Inflorescences terminal (axillary in R. albiflorum, short, corymbiform or rounded racemes (fasciculate racemes in R. canadense, R. lapponicum; fasciculate in R. albiflorum), 3-35-flowered (1-2-flowered in R. albiflorum); perulae brownish, scalelike, dry. ( Pedicels horizontal to erect (recurved); bracteoles 2, brownish, scalelike, basal.) Flowers bisexual, weakly bilaterally symmetric (radially symmetric in R. columbianum, R. groenlandicum, R. tomentosum, scented or not); sepals 5[-9], slightly connate; petals 5[-9], strongly to only slightly connate (sometimes nearly distinct), corolla deciduous, rotate to campanulate or funnelform; stamens 5-12[-20], included to long-exserted; (filaments usually unequal, usually unicellular-hairy, glabrous in R. vaseyi, glabrous or proximally unicellular-hairy in R. groenlandicum, R. lapponicum); anthers without awns, dehiscent by terminal pores; ovary 5[-18]-locular; style inserted in slight depression at ovary apex or smoothly intergrading, (usually curved, long, slender); stigma capitate. Fruits capsular, elongate, usually ovoid to cylindric, dehiscence usually basipetally septicidal (acropetally septicidal in R. columbianum, R. groenlandicum, R. tomentosum). Seeds (10-)100+, flattened-ellipsoidal to fusiform, often tailed, ± winged; testa smooth. x = 13.

Species ca. 1000 (25 in the flora): North America, Europe, Asia, Australia.

R. Good (1974) provided an excellent discussion of the phytogeography of Rhododendron, which occurs in arctic to tropical montane regions of North America, Europe, and Asia, extending southward to northeastern Australia. Its main centers of diversity are in the mountains of southern China and bordering countries, with a secondary center in New Guinea; eastern North America is a third center of diversity.

The species of Rhododendron are divided into subgenera (see references and discussion under various representative species). The subgenera are occasionally segregated; see, for example, H. F. Copeland (1943). The name Azalea Linnaeus often has been used for species with deciduous leaves; such use is misapplied.

Therorhodion has usually been included within Rhododendron, and these species are then placed in subg. Therorhodion (Maximowicz) A. Gray (W. R. Philipson and M. N. Philipson 1986). Here, Therorhodion is recognized as a genus distinct from Rhododendron (see K. A. Kron and W. S. Judd 1990; Kron 1997; Kron et al. 2002) because of its foliaceous perulae and base chromosome number of 12. Therorhodion probably is sister to the remaining genera of Rhodoreae (most of which belong within Rhododendron). Rhododendron is closely related to Menziesia, and the recognition of the latter may render Rhododendron paraphyletic (Kron 1997).

Numerous non-native species are grown for their beautiful flowers, and some of these, e.g., Rhododendron molle (Blume) G. Don, R. luteum Sweet, R. simsii Planchon, and R. obtusum (Lindley) Planchon, sometimes persist after cultivation. Probably, all species are poisonous due to the presence of andromedotoxin, a resinoid (M. D. McGee 1973).

SELECTED REFERENCES Chamberlain, D. F. 1982. A revision of Rhododendron II. Subgenus Hymenanthes. Notes Roy. Bot. Gard. Edinburgh 39: 209-486. Chamberlain, D. F. et al. 1996. The Genus Rhododendron: Its Classification and Synonymy. Oxford. Chamberlain, D. F. and S. J. Rae. 1990. A revision of Rhododendron IV. Subgenus Tsutsusi. Edinburgh J. Bot. 47: 89-200. Cullen, J. 1980. A revision of Rhododendron. 1. Subgenus Rhododendron sections Rhododendron and Pogonanthum. Notes Roy. Bot. Gard. Edinburgh 39: 1-207. Goetsch, L., A. J. Eckert, and B. D. Hall. 2005. The molecular systematics of Rhododendron (Ericaceae): A phylogeny based upon RPB2 gene sequences. Syst. Bot. 30: 616-626. Harmaja, H. 1991. Taxonomic notes on Rhododendron subsection Ledum (Ledum, Ericaceae), with a key to its species. Ann. Bot. Fenn. 28: 171-173. Judd, W. S. and K. A. Kron. 1995. A revision of Rhododendron VI. Subgenus Pentanthera (sections Sciadorhodion, Rhodora, and Viscidula). Edinburgh J. Bot. 52: 1-54. Kron, K. A. 1993. A revision of Rhododendron section Pentanthera. Edinburgh J. Bot. 50: 249-364. Kurashige, Y. et al. 2001. Sectional relationships in the genus Rhododendron (Ericaceae): Evidence from matK and trnK intron sequences. Pl. Syst. Evol. 228: 1-14. Philipson, W. R. and M. N. Philipson. 1986. A revision of Rhododendron III. Subgenera Azaleastrum, Mumeazalea, Candidastrum and Therorhodion. Notes Roy. Bot. Gard. Edinburgh 44: 1-23. Sleumer, H. 1980. A system of the genus Rhododendron L. In: J. L. Luteyn and M. E. O’Brien, eds. 1980. Contributions toward a Classification of Rhododendron: Proceedings, International Rhododendron Conference...May 15-17, 1978.... Bronx. Pp. 1-18.


1 Leaves persistent, blade margins entire   (2)
+ Leaves deciduous; leaf blade margins serrulate (serrate), or entire or undulate   (9)
       
2 (1) Leaf blade surfaces eglandular-hairy (hairs branched) and/or stipitate-glandular-hairy abaxially, hairs usually quickly deciduous to becoming matted and obscure, but not lepidote   (3)
+ Leaf blade surfaces with glandular-peltate scales (lepidote) abaxially, scales sometimes obscured by various eglandular hairs   (5)
       
3 (2) Pedicels glabrous; petioles glabrous.   6 Rhododendron macrophyllum
+ Pedicels multicellular, stipitate-glandular- or eglandular-hairy; petioles multicellular-hairy (hairs ± branched), often glabrescent   (4)
       
4 (3) Calyx lobes 2-6 mm; pedicels and ovaries stipitate-glandular-hairy; leaf blades: apex acuminate to acute, length/width ratio (2.4-)2.6-5.5(-8), abaxial surface smooth to slightly roughened.   7 Rhododendron maximum
+ Calyx lobes 0.5-1.7 mm; pedicels and ovaries multicellular eglandular-hairy (i.e., hirsute); leaf blades: apex rounded/mucronate to obtuse or acute, length/width ratio (1.3-)1.5-3(-3.5); abaxial surface minutely and obscurely papillose.   8 Rhododendron catawbiense
       
5 (2) Corollas white or pale to dark pink to rose or purple, petals connate 3/4+ their lengths; capsules basipetally dehiscent; leaf scales broad-rimmed   (6)
+ Corollas white to cream, petals slightly connate basally; capsules acropetally dehiscent; leaf scales not broadly rimmed   (7)
       
6 (5) Shrubs to 3(-5) m; leaf blades (1-)5-8(-13) × (1-)2-3.5(-5.5) cm; corollas 15-37 mm, tube 8-22 mm, upper lobe usually green-spotted; stamens 10; capsules 6-14 mm.   4 Rhododendron minus
+ Shrubs 0.5(-0.7) m; leaf blades 0.4-2(-2.5) × 0.2-0.7(-0.9) cm; corollas (6.5-) 7.5-14(-15) mm, tube 1.5-6.5 mm, upper lobe not spotted; stamens 5-10; capsules 4-7 mm.   5 Rhododendron lapponicum
       
7 (5) Twigs unicellular-hairy, papillate, and with flattened, glandular scales; petioles with glandular-peltate scales without broad rim and few ferruginous, long-crisped hairs; abaxial leaf surfaces sparsely to densely papillate, with few ferruginous, long-crisped hairs intermixed with conspicuous glandular- peltate scales.   1 Rhododendron columbianum
+ Twigs and petioles with ± dense covering of ferruginous, multicellular, long-crisped hairs; abaxial leaf surfaces (and especially midvein) with usually dense, ferruginous, multicellular, crisped hairs, these often intermixed with white, short, straight to curved hairs, glandular-peltate scales thus ± inconspicuous   (8)
       
8 (7) Pedicels with at least a few ferruginous, elongated, eglandular hairs, covered with ferruginous, long-crisped, unicellular and/or peltate scales, sometimes also with long-stalked, multicellular, glandular hairs; stamens 10; adaxial leaf surfaces with veins deeply impressed, abaxial with ferruginous, crisped hairs (usually more abundant on midrib, which is usually visible), sometimes forming dense, ± uniform mat; leaf blades ± linear (often much longer than wide); fruits borne on sharply recurved pedicels.   2 Rhododendron tomentosum
+ Pedicels with unicellular and/or glandular-peltate scales and sometimes ferruginous, long-crisped, multicellular hairs; stamens usually 5-10; adaxial leaf surfaces with veins usually not impressed, abaxial with ferruginous, crisped hairs forming usually dense, ± even covering, (usually concealing midvein); leaf blades ovate-lanceolate, sometimes narrowly elliptic to linear; fruits borne on broadly recurved pedicels.   3 Rhododendron groenlandicum
       
9 (1) Inflorescences lateral, 1-2-flowered; flowers pendulous, ± radially symmetric. ......   9 Rhododendron albiflorum
+ Inflorescences terminal, 3-24-flowered; flowers erect to horizontal, bilaterally symmetric   (10)
       
10 (9) Corollas glabrous (or sparsely stipitate-glandular-hairy) on outside, tube to 1/4 as long as lobes or absent; stamens 5-7 or 10; seeds with testa tightly appressed   (11)
+ Corollas often unicellular-hairy on outside, tube usually equaling or much longer than lobes; stamens 5; seeds with testa usually ± loose   (12)
       
11 (10) Corolla tubes absent due to deep division between 2 lower lobes and between lateral and lower lobes; upper corolla lobes 3-8 mm; stamens 10; filaments unicellular-hairy proximally; capsules moderately multicellular, stipitate-glandular- and eglandular-hairy; seed tails flattened; leaf margins usually revolute, sometimes plane, apex acute to rounded.   10 Rhododendron canadense
+ Corolla tubes 3-8 mm; upper corolla lobes 10-18 mm; stamens (5-)7; filaments glabrous; capsules very sparsely to moderately multicellular, stipitate-glandular-hairy; seed tails ± stellate-globular; leaf margins plane, apex acuminate.   11 Rhododendron vaseyi
       
12 (10) Flowers opening after leaves (i.e., essentially all leaves unfolded, and vegetative bud scales absent)   (13)
+ Flowers opening before or with (or sometimes right after in R. occidentale) leaves (i.e., at least some leaves still folded or vegetative bud scales still present)   (17)
       
13 (12) Twigs usually glabrous; abaxial surface of leaves glabrous   (14)
+ Twigs multicellular-hairy (glandular or eglandular) and/or unicellular-hairy; abaxial leaf surface glabrous or sparsely or densely eglandular-hairy   (15)
       
14 (13) Corollas red to orange-red or orange; capsules eglandular-hairy and sparsely unicellular-hairy; seed testa expanded, dorsiventrally flattened.   21 Rhododendron prunifolium
+ Corollas white or sometimes light pink (filaments and style dark pink to red); capsules multicellular, stipitate-glandular-hairy, sometimes also sparsely unicellular-hairy; seed testa not dorsiventrally flattened.   22 Rhododendron arborescens
       
15 (13) Corollas red, upper lobe with blotch or darker-colored spot, tube abruptly expanding into lobes; floral bud-scale margins glandular proximally (usually ciliate distally), abaxial surface glabrous.   23 Rhododendron cumberlandense
+ Corollas white (or sometimes pink), upper lobe with or without yellow or orange blotch, tube gradually expanding into lobes; floral bud-scale margins usually ciliate, glandular along proximal 2/3 or with glands and unicellular hairs mixed proximally, abaxial surface glabrous or densely unicellular-hairy   (16)
       
16 (15) Corollas densely glandular-hairy on outer surface (i.e., tube densely scattered, conspicuous, stipitate-glandular-hairy, hairs extending in line up corolla lobes), upper lobe without blotch; flowers strongly scented at night; styles white or greenish to pinkish.   24 Rhododendron viscosum
+ Corollas weakly glandular-hairy on outer surface, upper lobe with yellow to orange blotch; flowers strongly scented during mid day; styles white.   25 Rhododendron eastmanii
       
17 (12) Upper corolla lobes with contrasting blotch often appearing as darker-colored area   (18)
+ Upper corolla lobes without contrasting or darker-colored blotch   (22)
       
18 (17) Corolla lobes spreading nearly as broadly as tube is long, tube usually abruptly expanding into lobes, corolla yellow to orange, red-orange, or red   (19)
+ Corolla lobes shorter than or equaling tube, tube gradually expanding into lobes, corolla white, sometimes pink tinged, or yellow to orange   (20)
       
19 (18) Floral bud scales with glandular margins, abaxial surface glabrous; corollas scattered, multicellular, stipitate-glandular-hairy on outside.   12 Rhododendron calendulaceum
+ Floral bud scales with ciliate margins, abaxial surface glabrous or densely unicellular-hairy; corolla tubes multicellular eglandular-hairy abaxially, sometimes very weakly glandular.   13 Rhododendron flammeum
       
20 (18) Corollas yellow to orange (tube often orange-red or red), upper lobe with indistinct, darker yellow, orange, or red blotch; floral bud-scale margins glandular.   14 Rhododendron austrinum
+ Corollas white, sometimes pink tinged, upper lobe with contrasting yellow blotch; floral bud-scale margins unicellular, sometimes with unicellular hairs mixed with glands, or glandular   (21)
       
21 (20) Capsules moderately to densely unicellular-hairy and multicellular eglandular-hairy; floral bud scales glabrous or glabrate abaxially, margins unicellular-ciliate.   15 Rhododendron alabamense
+ Capsules sparsely unicellular-hairy (unicellular hairs rarely absent) and multicellular eglandular- and stipitate-glandular-hairy, or without eglandular hairs; floral bud scales sparsely to densely unicellular-hairy abaxially, margins unicellular-ciliate or with glands and cilia mixed, or with only glands.   16 Rhododendron occidentale
       
22 (17) Corolla lobes multicellular stipitate-glandular-hairy (hairs forming lines that continue along outer surface), corolla white, usually flushed pink or rose; plants to 0.5-1 m, strongly rhizomatous   17 Rhododendron atlanticum
+ Corolla lobes scattered, multicellular stipitate-glandular- or eglandular-hairy on outer surface (hairs not forming distinct lines), corolla pink to white; plants to 3-6 m, not strongly rhizomatous   (23)
       
23 (22) Corollas pink, scattered, multicellular eglandular-hairy on outer surface; floral bud scales glabrous abaxially, margins moderately unicellular-ciliate; leaf blades glabrous or nearly so.   18 Rhododendron periclymenoides (in part)
+ Corollas pink or white with pink tube, multicellular stipitate-glandular-hairy; floral bud scales glabrous or sparsely to densely unicellular-hairy abaxially; leaf blades sparsely to densely unicellular-hairy   (24)
       
24 (23) Floral bud scales glabrous abaxially; pedicels (and sepal margins) usually eglandular, often not unicellular-hairy or only sparsely unicellular-hairy; leaf blades glabrous or only sparsely unicellular-hairy..   18 Rhododendron periclymenoides (in part)
+ Floral bud scales sparsely to densely unicellular-hairy abaxially; pedicels (and sepal margins) eglandular or glandular, (sparsely) moderately to densely unicellular-hairy; leaf blades sparsely to densely unicellular-hairy abaxially   (25)
       
25 (24) Pedicels usually eglandular, 4-17 mm; leaf blade margins inconspicuously ciliate (cilia appressed to margins); capsules densely unicellular-hairy.   19 Rhododendron canescens
+ Pedicels usually glandular, 5-26 mm; leaf blade margins conspicuously ciliate (cilia ascending away from margins); capsules sparsely unicellular-hairy   20 Rhododendron prinophyllum

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