JARS v37n3 - The Species of Rhododendron Native to North America

The Species of Rhododendron Native to North America
Martha K. Roane
Department of Plant Pathology and Physiology
Virginia Polytechnic Institute and State University, Blacksburg, Virginia
Josephine DeN. Henry
Henry Foundation for Botanical Research, Gladwyne, Pennsylvania

Editors Note; This article was originally printed in Virginia Journal of Science 32(2):50-68, 1981 and has been slightly modified for publication in this Journal.

Abstract - The evergreen and deciduous species of Rhododendron native to North America are widespread throughout the area but do not occur in the Great Plains. Interior Plains, and Mexico. The highest species concentration is in eastern North America and centers in Virginia and North Carolina. Descriptions and keys of the species are provided.

The Rhododendrons of North America are representatives of an old group of plants with floral parts typical of primitive angiosperms. Members of the genus existed in North America fifty million years ago in essentially the same form as those found today in the wild in Asia (Leach, 1961). According to Cronquist (1968), the family Ericaceae and order Ericales, to which the genus Rhododendron L. belongs, are found in the subclass Dilleniidae, one of four subclasses probably derived directly from the Theales and indirectly through the Theales from the Dilleniales, the order in the subclass most closely related to the Magnoliales of the Magnoliidae. The name Rhododendron is derived from the Greek words rhodon (rose) and dendron (tree) and was used by Pliny to describe the oleander ( Nerium oleander L.). It took on its present meaning when Linnaeus (1754) established the genus Rhododendron . As it is known today, the genus consists of about 1000 species that vary from tiny mats two inches high to giant trees (Leach, 1961). Ninety percent of the world's Rhododendrons are concentrated in southeastern Asia from the northwestern Himalayas, Tibet, western and central China, southward to Malesia and the Philippine Islands.
In North America, the twenty-seven native species occur in the arcticalpine areas of Canada and Alaska, along the Sierra Nevada and Cascade Ranges as far south as southern California and along the Rocky Mountains into Colorado. They are not found in the Great Plains, Interior Plains or in Mexico but appear again in the Ouachita belt in Missouri, Oklahoma and Arkansas, the Interior Plateau from Tennessee north into New York, the Coastal Plain from southeastern Texas through the Mississippi Embayment and north into New England and Piedmont. The highest concentration of species is in eastern North America and centers in Virginia and North Carolina. The narrowest endemic, R. vaseyi A. Gray, is restricted to the higher mountains of western North Carolina at altitudes between 3000 and 5500 feet. The native Rhododendrons are mainly inhabitants of the temperate deciduous forests and most are found on the wooded acidic slopes of hills and mountains, although R. roseum (Loisel.) Rehd. and R. oblongifolium (Small) Millais do grow on limestone soil. The separation of the North American and Asiatic species has been attributed to continental drift and/or land bridges that have recently subsided. In North America the separation of the western and eastern species probably is due to the formation of mountain ranges during the late Tertiary and the glacial period (Lee, 1965).
The data in this paper have been compiled from field observations and collections, herbarium specimens, and literature (Duncan & Pullen, 1962; Funk & Fuller, 1978; Hulten, 1968; Hitchcock et al., 1969; Munz, 1963; Peck, 1961; Read & Wrzesin-ski, 1978; Rehder, 1927; Roane, 1975; Rydberg, 1922; Shiskin & Bobrov, 1967; Small, 1914, 1933; Solymosy, 1976; Viereck & Little, 1972; Wilson & Rehder, 1921; Wood, 1961).

Rhododendron Linnaeus
Evergreen or deciduous shrubs with alternate, entire or ciliateserrulate, glabrous to tomentose, lepidote or strigose leaves. Buds with several to many imbricate scales. Flowers 5-merous, usually in terminal corymbs. Calyx 5-parted, often very small, persistent. Corolla white to yellow or orange, pink, purple or deep red, rotate to campanulate, or funnel-form, usually irregular, with a 5-lobed limb, the media.n lobe adaxial; deciduous. Stamens 5-10, usually exserted from the corolla. Ovary superior, 5-locular. Fruit, a septicidal 5-loculed capsule, usually ovoid to oblong, 0.5-1 cm or more long.

Key to Flowering Plants
1. Leaves evergreen, glabrous, lepidote or tomentose, not ciliate....2
1. Leaves deciduous, pubescent, often strigose and ciliate, never lepidote....8
2 (1). Plants lepidote....3
2 (1). Plants not lepidote....6
3 (2). Flowers 2.5-5 cm across, corolla campanulate-funnelform, erect shrubs....4
3 (2). Flowers 1-2 cm across, corolla rotate-campanulate; procumbent, depressed or suberect-depressed shrub
1. R. lapponicum
4 (3). Flowers opening after the leafy shoots of the season expanded
4. R. minus
4 (3). Flowers opening before or with the expanding leafy shoots of the season ....5
5 (4). Tubes magenta, lobes rose with magenta spots on largest lobe; filaments white, anthers magenta
2. R. chapmanii
5 (4). Tubes and lobes pale rose-purple; rarely white, usually not spotted; filaments pale pink; anthers pale yellow
3. R. carolinianum
6 (2). Calyx lobes much shorter than ovary, underside of leaves glabrous....7
6 (2). Calyx lobes as long as ovary, underside of leaves tomentose or pubescent
5. R. maximum
7 (6). Pedicels glabrous, corolla rosy-purple with red-brown spots
6. R. macrophyllum
7 (6). Pedicels pubescent, corolla lilac-purple with olive-green spots
7. R. catawbiense
8 (1). Stamens 10, in one case, 5-7; corolla rotate to rotate-campanulate, sometime two-lipped....9
8 (1). Stamens 5 corolla funnel-form, sub-regular....12
9 (8). Inflorescences axillary or lateral; corolla rotate-campanulate, not two-lipped, not divided to the base
8. R. albiflorum
9 (8). Inflorescences terminal; corolla irregular, two-lipped or divided to the base on the lower side....10
10 (9). Procumbent shrubs, flowers solitary or two together
9. R. camtschaticum
10 (9). Erect shrubs, inflorescences with three or more flowers....11
11 (10). Stamens 10; corolla two-lipped with lower lobes divided to the base, rose-purple, not spotted
10. R. canadense
11 (10). Stamens 5-7; corolla with short tube, slightly two-lipped, pink or white, with orange or red-orange spots
11. R. vaseyi
12 (8). Flowers yellow or orange to scarlet or red, rarely white or pink; corolla slightly subirregular with the uppermost lobe the largest; often with a conspicuous yellow or orange blotch on the largest lobe....13
12 (8). Flowers white to deep pink, carmine or crimson; corolla regular; usually with no conspicuous blotch....18
13 (12). Corolla abruptly dilated, tube longer than lobes....14
13 (12). Corolla gradually dilated, tube as long as lobes....15
14 (13). Flowers opening before leaves expand; tube glandular, purple or red; lobes yellow
12. R. austrinum
14 (13). Flowers opening with the leaves, tube glandless, tube and lobes unicolored, red to scarlet
15. R. flammeum
15 (13). Corolla white or pink; style and filaments white or pink to red
14. R. occidentale
15 (13). Corolla yellow, orange, scarlet or red; style and filaments not white....16
16 (15). Filaments twice as long as tube, tube strongly ridged, calyx lobes ciliate
13. R. bakeri
16 (15). Filaments three times as long as tube, tube not strongly ridged, calyx lobes glandular-ciliate....17
17 (16). Corolla 5 cm or more across, style and filaments yellow to orange, flowers opening with the leaves
16. R. calendulaceum
17 (16). Corolla 3.5-4 cm across, style and filaments brick red, flowers opening when leaves are about fully developed
17. R. cumberlandense
18 (12). Corolla without prominent rows of glands along large veins of lobes....19
18 (12). Corolla with prominent rows of glands along large veins of lobes....22
19 (18). Winter buds glabrous....20
19 (18). Winter buds pubescent....21
20 (19). Leaves glabrous; corolla tube pink, about as long as the lobes; filaments almost three times as long as tube; flowers not fragrant
19. R. nudiflorum
20 (19). Leaves pubescent beneath; corolla tube white, longer than lobes, filaments twice as long as tube; flowers fragrant
20. R. alabamense
21 (19). Filaments twice as long as corolla tube; lobes about as long as tube; entire corolla usually bright pink, rarely white; with clove-pink fragrance
21 R. roseum
21 (19). Filaments almost three times as long as corolla tube; lobes half as long as tube; tube usually deeper pink than lobes; flowers fragrant but not like clove-pink
22. R. canescens
22 (18). Low, stoloniferous shrubs with simple or little-branched stems usually less than 0.5 m tall; leaf blades and petioles with short, stipitate glands
18. R. atlanticum
22 (18). Much branched shrubs usually more than 0.5 m tall; leaf blades and petioles glandless....23
23 (22). Branchlets pubescent; style pubescent....24
23 (22). Branchlets glabrous; style glabrous....26
24 (23). Leaves 4-10 cm long; calyx lobes oblong to oblong-lanceolate, unequal, 1-3 mm long
23. R. oblongifolium
24 (23). Leaves 2-6 cm long; calyx lobes semi-orbicular to ovate, short, equal, 1 mm long....25
25 (24). Branchlets yellow or gray-brown, hirsute or moderately strigose; corolla slightly pubescent inside above the middle
24. R. viscosum
25 (24). Branchlets bright red-brown, copiously strigose, finely villous; corolla tube glabrous inside
25. R. serrulatum
26 (23). Corolla white or pink; tube glandular-pilose outside; flowers fragrant
26. R. arborescens
26 (23). Corolla crimson; tube glabrous outside; flowers not fragrant
27. R. prunifolium

Key to Plants with Leaves and Capsules
1. Leaves evergreen, glabrous, lepidote, or tomentose, not ciliate....2
1. Leaves deciduous, glabrous, or pubescent, often strigose and ciliate, never lepidote....8
2 (1). Leaves lepidote....3
2 (1). Leaves glabrous....6
3 (2). Shrubs erect, capsules to 12 mm long....4
3 (2). Shrubs procumbent, depressed or sub-erect-depressed; capsules 4-5 mm long
1. R. lapponicum
4 (3). Leaves up to 13 cm long, blades acute or acuminate....5
4 (3). Leaves 2-5 cm long, blades obtuse or retuse
2. R. chapmanii
5 (4). Low compact shrubs to 2 m; leaves elliptic to narrow-elliptic, somewhat broadened upward, 5-8 cm long; capsules oblong-ovoid, 8-12 mm long
3. R. carolinianum
5 (4). Straggling shrubs to 3 m; leaves narrow-elliptic to lanceolate, not broadened upward, 4-10 cm long; capsules oblong, less than 8 mm long
4. R. minus
6 (2). Pedicels glandular or pubescent....7
6 (2). Pedicels glabrous
6. R. macrophyllum
7 (6). Capsules and pedicels glandular
5. R. maximum
7 (6). Capsules and pedicels pubescent
7. R catawbiense
8 (1). Twigs glabrous; leaves glabrous or sparingly strigose on lower midrib....9
8 (1). Twigs pubescent and/or strigose; leaves pubescent, more or less strigose, sometimes stipitate-glandular....10
9 (8). Capsules densely glandular
26. R. arborescens
9 (8). Capsules strigose, minutely pubescent
27. R. prunifolium
10 (8). Capsules axillary
8. R. albiflorum
10 (8). Capsules terminal....11
11 (10). Plants to 35 cm tall, capsule to 1 cm long; pedicels to 3 cm long
9. R. camtschaticum
11 (10). Plants to 1 m or more tall; capsule length to 1.5 cm or more; pedicel length 2 cm or less....12
12 (11). Capsules glandular....13
12 (11). Capsules without glands....23
13 (12). Leaves usually glandular....14
13 (12). Leaves usually eglandular....17
14 (12). Petioles usually glabrous, eglandular
11. R. vaseyi
14 (12). Petioles usually pubescent, stipitate-glandular....15
15 (14). Buds glabrous; shrubs stoloniferous, to 0.5 m tall
15. R. atlanticum
15 (14). Buds pubescent; shrubs not stoloniferous, more than 0.5 m tall....16
16 (15). Buds pink, twigs yellow-red or pink, shrubs to 1 m tall
10. R. canadense
16 (15). Buds gray, twigs red-brown, shrub to 3 m tall
12. R. austrinum
17 (13). Buds usually glabrous....18
17 (13). Buds usually trichomatous....21
18 (17). Twigs strigose, not pubescent; shrubs stoloniferous....19
18 (17). Twigs strigose and pubescent; shrubs not stoloniferous....20
19 (18). Leaves trichomatous; petioles villous and strigillose
20. R. alabamense
19 (18). Leaves glabrous; petioles strigillose, not villous
24. R. viscosum
20 (18). Petioles pubescent; capsule strigose, glandular; buds yellow with no brown margin
13. R. bakeri
20 (18). Petioles strigose; capsules villous, glandular-setose; buds yellow-brown with sharply marked dark brown marginal band
25. R. serrulatum
21 (17). Petioles pubescent; pedicels pubescent, eglandular
23. R. oblongifolium
21 (17). Petioles strigose, pubescent; pedicels pubescent, glandular....22
22 (21). Bud scales acuminate, aristate-mucronate, finely pubescent; leaves thinly pubescent on both surfaces
14. R. occidentale
22 (21). Bud scales mucronulate, densely gray pubescent; leaves sparingly pubescent above and densely gray villous beneath
21. R. roseum
23 (12). Buds gray, densely pubescent
22. R. canescens
23 (12). Buds not gray, glabrous....24
24 (23). Leaves pubescent, strigose; pedicels eglandular, strigose....25
24 (23). Leaves pubescent, not strigose; pedicels setulose, stipitate-glandular
16. R. calendulaceum
25 (24). Capsules strigose, finely pubescent; leaves strigillose
19. R. nudiflorum
25 (24). Capsules strigose, not pubescent; leaves strigose, pubescent....26
26 (25). Pedicels strigose, pubescent; twigs yellow-brown, sparingly strigose or glabrous
17. R. cumberlandense
26 (25). Pedicels strigose, not pubescent; twigs orange-brown densely strigose, finely pubescent
15. R. flammeum

1. Rhododendron lapponicum (L.) Wahl. Fl. Suec. p. 249. 1824.
Azalea lapponica L. Sp. PI. 1:151. 1753.

Lapland Rose Bay. Alpine Rhododendron.

Matted to erect much branched shrub 1-4 decimeters tall. Twigs stout, brown, densely lepidote. Bud scales rosy purple, densely lepidote, mucronate, white ciliolate. Leaves oblong, elliptic or oval, to 1.5 cm long, obtuse, leathery, slightly revolute, densely lepidote, slightly paler beneath than above. Petioles lepidote, 0.5-1 mm long. Flowers in clusters of 1 to several, pink to deep purple, sometimes white, fragrant. Pedicels lepidote, 6-12 mm long. Calyx lobes minute, ovate to oblong-ovate, lepidote, long-ciliate. Corolla rotate-campanulate, 1.5-2 cm dia., 7-10 mm long, slightly hairy at base within, with rather deeply cut oblong lobes. Stamens 5-8(-10), much exerted; filaments pink to purple, gently curved, hairy only at the base, 1-1.5 cm long; anthers short, cylindric, ochraceous, 1 mm long. Style pink to purple, gently curved, slightly exceeding stamens. Capsules ovoid or ovoid-oblong, lepidote, rusty, to 7 mm long.
Occasional to rare shrubs of tundra and open spruce forests at tree line. Circumboreal in North America from Greenland, Labrador, Baffin Island, Ellesmere Island, Canadian Arctic, the mountains of south central Alaska west to the Arctic coast and north slope of Brooks Range. Absent from the northern coastal plain of Alaska. Also disjunct locations in the Gasp, Maine, Wisconsin and the mountains of New York, New England; June to August.

2. Rhododendron chapmanii A. Gray. Proc. Acad. Phila. II. 4:61. 1877
Azalea chapmanii (A. Gray) Kuntze. Rev. Gen. PI. 2:387. 1891.
Rhododendron minus Michx. var. chapmanii (A. Gray) Duncan & Pullen. Brittonia 14:297. 1962.
Evergreen shrubs usually 1 m tall, rarely to 2 m, with erect rigid branches. Twigs gray-yellow, rusty-lepidote. Buds gray-orange with rusty-lepidote, white-ciliolate, mucronate, obovate scales. Leaves coriaceous, revolute, oblong to oval, 2-5 cm long, obtuse or retuse at apex, abruptly narrowed at base, somewhat lustrous above, lepidote beneath. Petioles short, to 3 mm long, lepidote. Flowers several, in an umbel-like cluster, appearing before leafy shoots of the season, tubes magenta, lobes rose with magenta spots on largest lobe, not fragrant. Pedicels densely lepidote, 0.5-1 cm long. Calyx lobes broadly deltoid, less than 1 mm long, densely lepidote with long delicately white-ciliolate margin. Corolla 2.5-3 cm long, 3 cm dia., funnelform-campanulate; tube to 1 cm long and 5 mm wide; lobes oblong-ovate, to 1 cm long. Stamens 10; filaments white, more than 2.5 cm long; anthers magenta, less than 2.5 mm long. Style magenta, shorter than stamens. Capsules brown, densely lepidote, cylindric, to 1 cm long.
Low pinelands, Gadsden, Gulf Franklin, Liberty, Leon and Clay Counties, Florida, late March.

3. Rhododendron carolinianum Rehder. Rhodora 14:99. 1912.
Rhododendron punctatum Small, Fl. S.E. U.S. p. 884, 1903. (non Andrews)
Rhododendron carolinianum var. margarettae Ashe. Rhodora 23:177. 1921.
Rhododendron carolinianum var. album (Rehder) Rehder. Jour. Arnold Arb. 4:250. 1923.
Rhododendron carolinianum var. foliatum Rehder. Jour. Arnold Arb. 7:33. 1926.
Rhododendron minus Michx. var. minus Duncan & Pullen. Brittonia 14:297. 1962.
Low compact shrubs with thick branches. Twigs green- or purple-lepidote. Buds ovoid, acute, with broadly ovate, mucronate, densely white-ciliolate, lepidote scales. Leaves coriaceous, oval or elliptic to elliptic-oblong, broadly cuneate at the base, acute or shortly acuminate at the apex, 5-8 cm long and 2.5-4 cm broad, above at first lepidote but soon becoming glabrous, somewhat lustrous, deep yellow-green, paler beneath and densely, or sometimes more sparsely, lepidote, slightly revolute at the margin; midrib above slightly, beneath strongly elevated. Petioles stout, relatively long, lepidote, slightly pubescent, 0.5-1.5 cm long. Flowers 4-9 in dense umbel-like terminal racemes, pale rose-purple, rarely white, usually not spotted, opening when young shoots are scarcely developed, not fragrant. Pedicels 1-1.5 cm long, lepidote. Calyx lobes semi-orbicular or broadly ovate, 2-3 mm long, lepidote on margin and outside. Corolla rotate-campanulate, 2.5 cm long and 3.5-4 cm wide, glabrous or sparingly lepidote outside; lobes broadly ovate, about as long or slightly longer than the short gradually widened tube. Stamens 10, slightly shorter than the corolla; filaments pale pink, villous at base, 1-2.5 cm long; anthers pale yellow, 2-3 mm long. Style glabrous, purple, slightly shorter than stamens. Capsules narrow-oblong, 8-1 2 mm long, 2-4 mm thick, brown, lepidote.
In full bloom in early May when buds of R. minus are still tightly closed. Higher mountains of North Carolina, South Carolina, and Tennessee.

4. Rhododendron minus Michx. Jour. Hist. Nat. 1:412. 1792.
Rhododendron punctatum Andr. Bot. Reposit. 36. 1798.
Rhododendron parviflorum Hort. ex DC. Prodr. 7:723. 1839.
Azalea minor (Michx.) Kuntze. Rev. Gen. PI. 2:386. 1891.
Branching, straggling evergreen shrubs 2-3.5 m tall. Twigs sparingly lepidote, villous. Buds densely lepidote with mucronate, white-ciliolate scales. Leaves deep green and glabrous above, paler green, densely lepidote and often glabrous beneath, mostly at the ends of branches, narrow elliptic to lanceolate, 4-10 cm long, acute or somewhat acuminate at both ends. Petioles stout, more or less pubescent, lepidote, 3-12 mm long. Flowers opening after leaves have unfolded, in clusters of 7-10, clear rose spotted with green, sometimes white, rarely with yellow blotch on upper lobe, not fragrant. Pedicels densely lepidote, to 1.5 cm long. Calyx lobes lepidote, deltoid, delicately long-ciliate, broader than long, to 2 mm long. Corolla funnelform-campanulate, 2-3 cm long, 2-3 cm dia., lobes usually flat but rarely crisped. Stamens 10, unequal; longer filaments exceed 2.5 cm; anthers ochraceous, 2.5 mm long. Style purple, 2-2.5 cm long. Capsules oblong or oblong-ovoid, lepidote, 8-12 mm long.
Woodlands of inner Coastal Plain to lower Blue Ridge of North Carolina to Georgia and Alabama. Late May, June.

R. minus
R. minus

5. Rhododendron maximum L. Sp. PI. 1:392. 1753.
Rhododendron purpureum G. Don. Gen. Hist. 3:843. 1834.
Rhododendron purshii G. Don. Gen. Hist. 3:843. 1834.
Rhododendron ashleyi Coker. Jour. Elisha Mitchell Sci. Soc. 51:189. 1935.
Hymenanthes maxima (L.) Copeland. Amer. Midland Nat. 30:614. 1943.
Rhododendron maximum var. leachii Harkness in Leach, Rhododendrons of the World, p. 192. 1961.

Rose Bay. Great Laurel. White Laurel.

Evergreen shrubs or small trees to 10 m tall. Twigs gray-brown, scurfy. Buds with red-brown, tomentose, ciliate scales. Leaves thick, coriaceous, bright green above, pale green beneath, oblong-obovate, 10-20 cm long, abruptly narrowed or tapering at the base, glabrous above, almost imperceptibly scurfy beneath. Petioles stout, scurfy, 2-4.5 cm long. Flowers opening after the new leaves expand, numerous, in an umbel-like cluster, white to rose, with yellow or orange spots, somewhat green in throat, not fragrant. Pedicels stipitate-glandular, 3-4 cm long. Calyx lobes ovate to oblong, mostly longer than wide, 2-4 mm long. Corolla rotate to campanulate, 2-3 cm long, 3.5-4 cm dia., lobes not crisped. Stamens 10, unequal, the longer filaments to 2 cm long, anthers 2 mm long. Style shorter than stamens. Capsules stipitate-glandular, narrowly oblong or cylindric-ob-long or slightly narrowed upward, 1 - 1.5 cm long.
In woodlands, on northern slopes and shady mountainsides, in sheltered stream valleys. Nova Scotia to southern Ontario and Ohio, south, especially in the mountains, to Georgia and Alabama. June, July.

R. maximum
R. maximum

6. Rhododendron macrophyllum G. Don. Gen. Hist. PI. 3:843. 1834.
Rhododendron californicum Hook. Curtis' Bot. Mag. 81: pi 4836. 1855.
Hymenanthes californica (Hooker) Copeland. Amer. Midland Nat. 30:614. 1943.
Rhododendron macrophyllum f. album Rehder. Jour. Arnold Arb. 28:254. 1947.
Hymenanthes macrophyllum (G. Don) Copeland. Leaflet West. Bot. 5:140. 1948.
Hymenanthes macrophyllum f. album (Rehder) Copeland. Leaflet West. Bot. 5:140. 1948.

Western Rose Bay. California Rose Bay.

Evergreen shrubs 1-5 m tall with coarse glabrous branches. Twigs puberulent when young, then glabrate and coarse. Buds glabrous with long-awned scales. Leaves coriaceous, dark green above, paler and somewhat papillose beneath, oblong to elliptic, 6-20 cm long. Petioles stout, glabrous, 1-2 cm long. Flowers many, in terminal clusters, white to pink or rose-purple with red-brown or green spots on upper lobes, not fragrant. Pedicels glabrous, 1.5-5 cm long. Calyx very shallowly lobed, glabrous, lobes 1 mm long and considerably broader, margin entire. Corolla tubular-campanulate, 2.5-4 cm long, glabrous, deeply 5-lobed, lobes ovate, spreading, crisped-undulate. Stamens 10, unequal, the longer ones well exserted; filaments white to pink or red, sparsely short-trichomed on the lower half, 1 -3.5 cm long; anthers pale yellow or pale pink, oblong, 2-3 mm long. Style pale pink or rose-purple, glabrous, as long as stamens. Ovary and young capsule strigose, stipitate-glandular. Mature capsules ovoid to oblong, woody, 1.5-2 cm long.
Dryish to damp, more or less shaded woods below 4000 feet from Vancouver Island and southern British Columbia along the coast of Washington and Oregon into northern California. April-July.

7. Rhododendron catawbiense Michx. Fl. Bor. Am. 1:258. 1803.
Hymenanthes catawbiensis (Michx.) Copeland. Amer. Midland Nat. 30:614. 1943.

Mountain Rose Bay. Purple Laurel. Rose Bay. Catawba Rhododendron.

Evergreen shrubs or small trees to 6 m tall. Twigs gray to red-brown, scurfy. Bud scales red-brown, slightly scurfy, glandular-ciliate. Leaves thick, coriaceous, dark green above, pale green beneath, oval or elliptic, 5-15 cm long, broadest near the middle, rounded, obtuse or subacute at both ends, sometimes subcordate at the base, abruptly pointed at apex, glabrous on both sides, aciliate, midribs sparingly hirsutulous. Petioles sparingly hirsutulous, stout, 1.5-2.5 cm long. Flowers many, in an umbel-like cluster, somewhat larger than those of R. maximum, rose-purple to lilac, rarely white, with olive-green spots on upper lobe, not fragrant. Pedicels 2.5-3.0 cm long, hirsutulous, eglandular. Calyx lobes broadly triangular or semicircular, broader than long, less than 1 mm long. Corolla rotate to campanulate, glabrous, to 6 cm dia.. Stamens 10, to 3 mm long; filaments purple, anthers white, 2-3 mm long. Style about as long as stamens, purple. Capsules cylindric, 2-2.5 cm long, densely hirsute, eglandular.
Mountains of Virginia and West Virginia to Georgia and Alabama, rarely eastward to near the Coastal Plain. May, June.

8. Rhododendron albiflorum Hook. Fl. Bor. Am. 2:43. 1834.
Azalea albiflora Kuntze. Rev. Gen. 2:387. 1891.
Azaleastrum albiflorum (Hook.) Rydb. Mem. N.Y. Bot. Gard. 1:297. 1900.

Deciduous shrubs with erect branches, 1-2 m tall. Twigs finely puberulent, copiously red strigose-hirsute, glandular-strigose. Bud scales thin, caducous, strigose-hirsute. Leaves alternate, clustered or approximate on young shoots, thin, deep green above, lighter beneath, elliptic-ob-lanceolate, entire to undulate, 4-9 cm long, slightly ciliate, appressed-pilose and copiously strigose-hirsute, midribs appressed-villous. Petioles pubescent, strigose, 5-10 mm long. Flowers white or ochroleucous with yellow to green spots, in axillary clusters of 1-4 along the stems, not fragrant. Pedicels stipitate-glandular, coarsely red-hirsute, 1-1.5 cm long. Calyx divided to base; lobes oblong, ovate or obovate, finely pubescent, red-hirsute, stipitate-glandular, ciliate, to 1 cm long. Corolla rotate or rotate-campanulate, 1-1.5 cm long, with very broad lobes about as long as the tube. Stamens 10, exserted, 1-1.5 cm long, with lower half of white filaments very pubescent, anthers obovoid or globoid, yellow, 1 -2 mm long. Style white, pubescent at base, as long as filaments. Capsules ovoid, 6-10 mm long, heavy-walled, pubescent.
Often in wet places, usually along streams, chiefly montane. British Columbia south to Oregon and east to western Montana and Colorado. June through August.

R. albiflorum
R. albiflorum

9. Rhododendron camtschaticum Pall. Fl. Ross, l:48. 1784.
Rhododendron camtschaticus Lindl. Paxt. Fl. Gard. 1:pl22. 1853.
Chamaecistus kamtschaticus Rgl. Dendrol. 111:196. 1873.
Therorhodion camtschaticum (Pall.) Small. N. Amer. Fl. 29(1):45. 1914.
Therorhodion glandulosum Standley. N. Amer. Fl. 29(1):45. 1914.
Rhododendron kamtschaticum var. pumilum E. Busch. Fl. Sib. and Dal'm Vost. ll:37. 1915.
Rhododendron kamtschaticum var. pallasianum Kom. Fl. Kamtsch. ll:360. 1929.
Rhododendron kamtschaticum ssp. typicum Hult. Flora Kamtsch. IV:14. 1930.
Rhododendron kamtschaticum ssp. glandulosum (Standley) Hult. Flora Kamtsch. IV:14. 1930.
Rhododendron kamtschaticum ssp. intercedens Hult. Flora Kamtsch. IV:14. 1930.

Kamchatka Rhododendron.

Deciduous shrubs to 3 decimeters tall, with depressed stems, much branched, with leaves approximate in rosettes at ends of branches, spreading underground by means of suckers. Twigs gray-brown to red-brown, coarse, sparingly glandular-pilose, with long scattered trichomes. Buds yellow-brown, glabrous, scales thin, persistent, mucronate, strigose, ciliate. Leaves cuneate to obovate or oval, 1-4 cm long, obtuse or rounded, mucronulate at apex, bristly-ciliate, glandular-ciliate, sparingly pilose and strigose on veins beneath, sessile. Flowers opening with the leaves, rose-purple, with deeper rose-purple spots, very rarely white, 1-3, terminating leafy shoots, not fragrant. Pedicels 1 cm or more long, glandular-pilose with 2 foliaceous bracteoles. Calyx lobes oblong-oval, 1-2 cm long, glandular-pubescent, bristly-ciliate. Corolla rotate, 2.5-5 cm dia., glabrous to pubescent, deeply lobed; tube to 5 mm long; lobes erose, often ciliolate, two-lipped, lower lip deeply cleft, 2-2.5 cm long. Stamens 10, unequal; filaments purple, villous at base, shorter than corolla, lower 5 twice as long as upper 5; anthers oval or obovoid, purple. Style villous at base, exceeding stamens, purple. Capsule ovoid, thin-walled, villous, sub-coriaceous, 5-10 mm long.
Dry rocky tundra of Aleut an Islands and alpine zone of forested regions of Alaska. June through August.

10. Rhododendron canadense (L.) Torr. Geol. Survey New York Assembly, No. 50. 1839. Cat. PI. p. 151.
Rhodora canadensis L. Sp. PI. Ed. 2. 1:561. 1762.
Hochenwartia canadensis Crantz. Inst. ll:469. 1766.
Rhododendron rhodora G.F. Gmelin. Syst. Nat. II pt. 1:694. 1791.
Rhodora congests Moench. Meth.:68. 1794.
Rhododendron pulchellum Salisb. Prodr. :287. 1796.
Azalea canadensis (L.) Kuntze. Rev. Gen. ll:386. 1891.


Much branched deciduous shrubs to 1 m tall, with slender upright or ascending branches. Twigs puberulous, bright yellow-red or pink, often slightly bloomy. Buds finely pubescent with ovate, acuminate to acute, pink, ciliolate scales. Leaves elliptic to oblong, obtuse or acute, cuneate at base, 2-4.5 (-6) cm long; with ciliate and revolute margin; dull blue-green, somewhat strigillose, with finely villous midribs above; thinly gray-tomentulose, with scattered fulvous trichomes and short stipitate glands, sparingly strigose midribs beneath. Petioles puberulous, sparingly strigose, 2-5 mm long. Flowers opening before leaves in 3-7-flowered umbel-like racemes, rose-purple, not fragrant. Pedicels puberulous, glaucous, sparingly glandular-pilose, 3-7 mm long. Calyx lobes very short, unequal, puberulous, setosely-ciliate. Corolla glabrous, 1.5-2 cm long, two-lipped, the lower lip divided nearly to the base into two nearly distinct narrow-oblong lobes, the upper lip with three short ovate lobes. Stamens 10, unequal, about as long as corolla; filaments pubescent on lower third, purple; anthers broadly ellipsoidal, purple, 2 mm long. Style slightly longer than stamens, 1.5-2 cm long, glabrous or minutely pilose at base, purple. Capsules ovoid-oblong, curved near the base, slightly grooved, setose, finely puberulous, sparingly short stipitate-glandular, light red-brown with a pink bloom, 1 -1.5 cm long.
River banks, moist woods and swamps from Labrador and Newfoundland to southwestern Quebec and south through New England, central New York to northeastern Pennsylvania and northern New Jersey. April-May; June in the north.

11. Rhododendron vaseyi A. Gray. Proc. Amer. Acad. XV:48. 1879.
Azalea vaseyi (Gray) Rehder. Mller's Deutsch. Grtn. - Zeit. XIV:332. 1899.
Biltia vaseyi (Gray) Small. FL. S.E. U.S. p. 884. 1903.

Pinkshell Azalea.

Deciduous, upright, irregularly branched shrubs with spreading branches, to 5 m tall, not stoloniferous. Twigs light red-brown, puberulous, sparingly pilose, glabrescent. Buds with orbicular-ovate, mucronulate scales, outer acuminate, glabrous, inner ones minutely puberulous, white-ciliate, sometimes glandular-ciliate. Leaves elliptic, elliptic-oblong, acuminate, cuneate, 5-12 cm long, 2-5 cm broad; margins ciliate, usually slightly undulate; glabrous above and sparingly short stipitate-glandular near midribs beneath; midribs sparingly finely-villous above and sparingly pilose beneath. Petioles glabrous or sparingly pilose, 3-7 mm long. Flowers opening before leaves, in 5-8 flowered racemes, white or light rose with orange or red-orange dots, not fragrant. Pedicels short stipitate-glandular, 0.5-1.5 cm long. Calyx oblique with shallow, rounded lobes erose-glandular on margins. Corolla rotate-campanulate, 2.5-3 cm long, two-lipped, glabrous; tube very short, to 5 mm long; lobes oblong, rounded at apex, upper lip less deeply divided, with middle lobe exterior in bud. Stamens usually 7, sometimes 5 or 6, unequal, the longer exceeding the corolla; filaments glabrous, white or pink; anthers ellipsoid, white, pink, or purple, 1-2 mm long. Style glabrous or with a few stipitate glands near the base, white, longer than stamens, 2.5-3 cm long. Capsules narrow-oblong, with thin narrow keel on back of valves, stipitate-glandular, 1-1.5cm long.
Mountain slopes and summits. North Carolina. May.

R. vaseyi
R. vaseyi

12. Rhododendron austrinum (Small) Rehder in Bailey, Stand. Cycl. Hort.VI:3574. 1917.
Rhododendron nudiflorum var. luteum Curtiss in Rehder. A. Monograph of Azalea, p. 146. 1921.
Azalea austrina Small. FL. S.E. U.S. Ed. 2. p. 1356. 1913.

Florida Azalea.

Deciduous shrubs to 3 m tall, with irregular branches, not observed to be stoloniferous. Twigs red-brown, with soft puberulence, sparingly strigose, copiously stipitate-glandular. Buds with ovate, acuminulate scales, densely gray-pubescent. Leaves elliptic to obovate or oblong-ovate, 3-9 cm long, acute or obtuse and mucronulate at apex, cuneate at base, setosely ciliate or glandular-ciliate, finely pubescent on both sides but pubescence denser on lower. Petioles pubescent and stipitate-glandular, sparingly strigose, 3-8 mm long. Flowers opening before or with leaves, yellow to orange with no yellow to orange blotch, the tube more or less purple, slightly fragrant, in 8-15 flowered umbel-like racemes. Pedicels pubescent, glandular-setose, 0.5-1 cm long. Calyx lobes broadly ovate to oblong, unequal, 1-2.5 mm long, glandular-ciliate, pubescent. Corolla funnelform; tube 1.5-2 cm long, cylindric, abruptly dilated at apex, finely pubescent, stipitate-glandular; lobes 1-1.5 cm long, short-acuminate with recurved points, finely pubescent outside. Stamens three times as long as tube, filaments pubescent below the middle, anthers ochraceous, 2-3 mm long. Style slightly exceeding stamens, 5-6 cm long, short-pilose near base. Capsules oblong-cylindric, 1.5-2.5 cm long, with thin, fine pubescence, long-strigose, partly gland-tipped trichomes.
River banks and sometimes flood plains. Originally reported from Gadsden and Liberty Counties in Florida but also from Georgia, Alabama and Mississippi. Late April.

13. Rhododendron bakeri (Lemmon & McKay) Hume. Azaleas, Kinds and Culture, p. 28. 1949.
Azalea bakeri Lemmon & McKay. Bartonia 19:16-17. 1938.

Deciduous shrubs or small trees regularly to 3 m tall, with irregularly whorled branches, not stoloniferous. Twigs light green, puberulent, sparingly strigose. Buds yellow, conical; scales glabrous, ciliate, mucronate or awned; awn shorter than body of scale. Leaves narrowly to broadly elliptic, acute, mucronulate, 2-8 cm long, ciliate; blades glabrous or pubescent above, glabrous, shiny or pubescent beneath; midribs strigose above and pubescent and strigose beneath; veins strigillose beneath. Petioles pubescent, 2-5 mm long. Flowers in clusters of 3-9, expanding after leaves, yellow or red-orange with no distinct yellow to orange blotch, not fragrant. Pedicels villous, stipitate-glandular, 2-6 mm long. Calyx lobes unequal, elliptic-acuminate to deltoid-ovate, setose, ciliate. Corolla funnelform, 5-6 cm dia.; tube strongly ridged, pubescent, stipitate-glandular, gradually dilated, 1.5-2.5 cm long; lobes ovate to elliptic, acute, slightly pubescent and stipitate-glandular, 1.5-2.5 cm long. Stamens 5, exserted; filaments more than twice the length of the tube with the exserted portion glabrous, carmine and the remainder pubescent, lighter; anthers ochraceous, 2 mm long. Style carmine, with pubescent base, 7.5 cm long. Capsules cylindric, strigose with appressed gland-tipped trichomes, 1.5 cm long.
Blue Ridge Mountains of Georgia and North Carolina above 3000 feet. June-August.

14. Rhododendron occidentale (Torr. & Gray) A. Gray. Bot. Calif, l:458. 1876.
Azalea calendulacea Hooker & Arnott. Bot. Capt. Beechey Voy. 362. 1841. (not Michaux)
Azalea nudiflora var. ciliata Kellogg. Proc. Calif. Acad. Sci. l:60. 1855.
Azalea californica Torrey & Gray. Durand. Jour. Acad. Philadelphia. Ser. 3, 111:94. 1855. (not R. californicum Hooker)
Azalea occidentalis Torrey & Gray. Rev. Explor. Surv. Miss. Pacif. Ocean IV:116. 1856.
Rhododendron sonomense Green. Pittonia 11:172. 1891.

Western Azalea. Pacific Azalea.

Deciduous shrubs to 3 (-8) m tall with loose branching, shredding bark. Twigs stiff, divaricate, glabrous to pubescent, sometimes stipitate-glandular, brown. Buds finely pubescent or nearly glabrous with ovate to ovate-lanceolate, acuminate, aristate-mucronate or mucronulate scales. Leaves elliptic, oblong-lanceolate, 3-9 cm long, acute or obtuse, mucronulate, cuneate or attenuate at base, ciliate, thinly pubescent on both surfaces, rarely nearly glabrous; midribs pubescent above, sparingly strigose beneath. Petioles rather stout, short, pubescent, strigose, 1 -5 mm long. Flowers opening with or after leaves, 5-25 in umbel-like racemes, white to pink with yellow blotch, usually fragrant. Pedicels pubescent, more or less glandular, sometimes pilose, eglandular, rarely nearly glabrous, to 1.5 cm long. Calyx lobes broadly ovate to oblong-ovate, obtuse, 1.5-5 mm long, densely ciliate with usually setose gland-tipped or glandless hairs. Corolla funnelform, somewhat irregular; tube narrowly funnelform, gradually dilated upward, villous, glandular-pilose, 2-3 cm long; lobes about as long as tube, broad-ovate, acute. Stamens 5, exserted, more than twice tube length; filaments pilose below middle, white to pink or red; anthers white or brown, 2.5-3 mm long. Style as long or longer than stamens, white to pink or red, pubescent above base. Capsules ovate-oblong, setosely pilose, stipitate-glandular, 1-2 cm long.
Moist thickets and stream banks among the mountains to 6000 feet elev. Oregon, California. June, July; as early as April or as late as August.

R. occidentale
R. occidentale
photo by Frank Mossman

15. Rhododendron flammeum (Michx.) Sargent. Rhododendron Soc. Notes 1(3):120. 1917.
Azalea nudiflora coccinea Aiton. Hort. Kew. 1:202. 1789.
Azalea coccinea Curtis. Bot. Mag. 5, t. 180. 1792.
Azalea fulva Michaux. Jour. Hist. Nat. 1:410. 1792. (nom. nud.)
Azalea calendulacea a flammea Michaux. Fl. Bor. Am. 1:151. 1803.
Azalea speciosa Willdenow. Berl. Baumz. Ed. 2:49. 1811.
Azalea periclymenoides var. coccinea Pursh. Fl. Am. Sept. 1:152. 1814.
Azalea nudiflora Loiseleur-Deslongchamps. Herb. Amat. IV:213, t. 1820. (not Linnaeus)
Azalea coccinea major Loddiges. Bot. Cab. VII; t. 624. 1822.
Azalea speciosa a major Sweet. Hort. Brit. p. 265. 1826.
Rhododendron speciosum a major Sweet. Hort. Brit. Ed. 2. p. 343. 1830.
Rhododendron nudiflorum coccineum Sweet. Hort. Brit. Ed. 2. p. 343. 1830.
Azalea speciosa a coccinea De Candolle. Prodr. Vll:717. 1839.
Azalea calendulacea Darby. Bot. S. States p. 422. 1855. (in part)
Rhododendron calendulaceum Chapman. Fl. S. U.S. p. 265. 1860. (in part)
Rhododendron calendulaceum f. speciosum Voss. Vilmorin's Blumengart. l:588. 1894.

Oconee Azalea.

Deciduous shrubs to 2 m tall with slender, irregularly whorled branches, stoloniferous. Twigs finely pubescent, densely strigillose, trichomes rust-brown; branches orange-brown becoming gray to dark brown-gray, somewhat decorticating. Buds glabrous with scales ovate, mucronate, sometimes aristate, to 3 mm long, ciliolate. Leaves obovate, elliptic or oblong, acute or obtuse, mucronulate, broad-cuneate at base, 3-6 cm long, 1-2 (-3) cm broad; strigillose, sometimes glabrescent above, finely pubescent, densely pubescent on veins, strigose on midribs beneath; setosely ciliate. Petioles pubescent, strigillose, 3-5 mm long. Flowers opening with the leaves in 6-15-flowered umbel-like racemes, scarlet or bright red with large orange blotch on upper lobe, not fragrant. Pedicels strigillose, eglandular, 6-1 2 mm long. Calyx lobes round-ovate to oblong, long-ciliate, pubescent or nearly glabrous, 0.5-3 mm long. Corolla funnelform, 3-5 cm dia.; tube cylindric, rather slender, 2-2.5 cm long, longer than lobes, abruptly dilated at apex, villous, pilose, occasionally sparely stipitate-glandular; lobes ovate, abruptly acuminate, 1.5-2 cm long. Stamens 5, much exserted, more than twice as long as tube; filaments pubescent below middle, rose-pink; anthers ochraceous to yellow, to 2 mm long. Style slightly longer than stamens, finely pubescent on lower third, usually purple above. Capsules ovoid to narrow-oblong, narrowed toward apex, strigose, 2-3 cm long
Dry open woods and sand hills. Piedmont of South Carolina, Georgia. Late April, early May.

16. Rhododendron calendulaceum (Michx.) Torr. Fl. U.S. p. 425. 1824.
Azalea lutea L. Sp. PI. 1:150. 1753. (in part)
Azalea nudiflora L. Sp. PI. Ed. 2. 1:214. 1762. (in part)
Azalea flammea Bartram. Travels 1:327. 1791. (nom. nud.)
Azalea calendulacea Michx. Fl. Bor. Am. 1:151. 1803. (excluding var. a)
Azalea aurantiaca Dietrich. Darst. Verz. Zierpfl. 4,t.1. 1803.
Azalea pontica A. calendulacea Persoon. Syn. 1:212. 1805.
Rhododendron luteum (L) Schneider. III. Handb. Laubholzk. ll:500, fig. 329 q-r, 320a. 1911. (not Sweet)

Flame Azalea. Yellow Azalea.

Deciduous shrubs 1.5-3 (-5) m tall, with erect and spreading branches, not observed to be stoloniferous. Twigs gray-brown, strigose with fine dense pubescence. Buds glabrous, brown with broadly ovate, apiculate, densely white-ciliolate scales. Leaves oblong to obovate or lanceolate, ciliate, 4-8 cm long, acute, mucronulate, broadly cuneate at base, finely pubescent above, densely pubescent beneath when young, particularly on and along the midribs. Petioles pubescent, 2-5 mm long. Flowers expanding with or shortly after the leaves, in clusters of 5-7, yellow or orange to scarlet, with orange blotch on upper lobe, not fragrant. Pedicels setulose, stipitate-glandular, 0.5-1 cm long. Calyx lobes oblong to ovate, obtuse, setulose, glandular-ciliate, 2-3 mm long. Corolla funnelform, over 6 cm dia.; tube gradually dilated above the middle, 1.5-2 cm long, glandular-pilose and pubescent; lobes ovate, undulate, abruptly contracted into short points, slightly pubescent outside, 2 cm long, 1.5 cm broad. Stamens 5, nearly 3 times as long as tube; filaments yellow to orange, pubescent toward the base; anthers yellow to orange, 3 mm long. Style yellow to orange, pilose toward base, 6-7 cm long, as long or longer than stamens. Capsules ovoid-oblong, 1.5-2 cm long, setose, pubescent.
Appalachian mountain region from Pennsylvania to northern Georgia in open woods and along streams. Also in Fairfield County, Ohio. May-June.

R. calendulaceum
R. calendulaceum

17. Rhododendron cumberlandense Braun. Rhodora 43:33. 1941.

Red Azalea. Cumberland Azalea.

Deciduous compactly-growing shrub to 3 m tall but usually lower growing, sometimes stoloniferous. Twigs yellow-brown, sparsely strigose or glabrous. Buds glabrous, yellow-brown with ciliate, mucronate inner scales, outer scales with awn as long as body of scale. Leaves narrowly obovate, 3-5 cm long, less than half as wide, glabrous, full grown at anthesis; midribs strigose with fine white pubescence above, sparingly strigose, finely pubescent beneath. Petioles strigose, pubescent, 3-5 mm long. Flowers opening after leaves are expanded, several in a short raceme, orange-red to red with large orange blotch, not fragrant. Pedicels strigose, finely pubescent, 3-7 mm long. Calyx lobes round-ovate, sparsely hirsute, glandular-ciliate, less than 1 mm long. Corolla 3.5-4 (-5) cm dia., funnel-form, pubescent, strigillose, with short almost sessile glands outside; tube 1-2 cm long, gradually dilated; upper lobe broader than laterals, sometimes twice as broad, almost orbicular but contracted to a short acuminate tip. Stamens 5, exserted; filaments brick red, nearly glabrous, 3 times as long as tube; anthers ochraceous, 1-2 mm long. Style brick red, glabrous, 6-7 cm long. Capsules ovoid, strigose, to 2 cm long.
Mesophytic oak woods. Cumberland and Kanawha Plateaus, Cumberland Mountains of Virginia, Kentucky, Tennessee. June.

18. Rhododendron atlanticum (Ashe) Rehder in Wilson & Rehder. A monograph of Azalea, p. 147. 1921.
Azalea atlantica Ashe. Bull. Charleston Mus. Xlll:26. 1917.

Dwarf Azalea. Coast Azalea.

Low, stoloniferous, deciduous shrubs rarely more than 0.5 m tall, with slender upright usually sparingly branched or simple stems. Twigs red-brown, glabrous to sparingly strigillose and sometimes setosely glandular. Buds light brown, glabrous or slightly silky pubescent; scales ovate, mucronulate, white-ciliolate. Leaves obovate to oblong-ovate, bright green or bluish-green, 3-6 cm long, setosely ciliate, glabrous above except sometimes short stipitate-glandular, glabrous or short stipitate-glandular beneath; midribs villous above, sparingly strigose, sometimes pubescent, short stipitate-glandular beneath. Petioles sparingly strigillose, short stipitate-glandular, 1-5 mm long. Flowers opening before or with the leaves, white usually flushed pink or purple, no blotch present, in clusters of4-10, very fragrant. Pedicels to 1 cm long, hirsute, stipitate-glandular. Calyx lobes broadly ovate, glandular-ciliate, 2-4 mm long. Corolla funnelform, 4 cm dia.; tube cylindric, gradually dilated at apex, 2-2.5 cm long with numerous short gland-tipped trichomes, otherwise glabrous or slightly villous; lobes ovate to ovate-oblong, acute, 1.3-2 cm long, prominently stipitate-glandular along the middle. Stamens 5, twice as long as the tube, filaments white to pale buff, villous below middle, anthers white to pale buff, 2-3 mm long. Style exceeding stamens, 4.5-5.5 cm long, pubescent below the middle, purple above. Capsules ovoid-oblong, setose, stipitate-glandular, 1.5-2 cm long.
Coastal Plain pine barrens, Delaware to South Carolina. Late April, early May.

R. atlanticum
R. atlanticum

19. Rhododendron nudiflorum (L.) Torr. Fl. U.S. p.424. 1824.
Azalea lutea L. Sp. PI. 1:150. 1753. (in part)
Azalea nudiflora L. Sp. PI. Ed. 2. 1:214. 1762. (in part)
Rhododendron venustum Salisbury. Prodr. p. 287. 1796. (in part)
Azalea periclymenoides Michaux. Fl. Bor. Am. 1:151. 1803.
Azalea periclymena Persoon. Syn. 1:212. 1805.
Azalea nudiflora periclymenoides (Michx.) Heynhold. Nomencl. Bot. Hort. 1:108. 1840.
Anthodendron nudiflorum (L.) Reichenbach in Moessler, Handb. Gewchsk. Ed. 2, l:309. 1827.
Rhododendron periclymenoides (Michx.) Shinners. Castanea 27:95. 1962.

Wild Azalea. Early Azalea. Pinxter-flower. Pinksterflower. Honeysuckle. Pinxterbloom Azalea.

Deciduous, upright shrubs, 1-2 (-3) m tall with irregularly whorled, intricate branching, occasionally stoloniferous. Twigs slightly pubescent, sparingly strigose. Buds glabrous or slightly pubescent, brown, with ovate, abruptly acuminate, finely ciliate scales. Leaves elliptic, oblong, obovate or oblong-obovate, acute or abruptly acuminulate, rarely obtuse, mucronate, finely ciliate, 3-8 cm long, 1.5-3 cm wide, glabrous except for finely pubescent midribs above, strigillose and sometimes pubescent midribs beneath. Petioles strigillose, sometimes slightly pubescent, 2-4 mm long. Flowers opening before or with expanding leaves in clusters of 6-12 or more, white to pink or pink-purple, without blotch, not fragrant. Pedicels strigose-pilose, sometimes finely pubescent, rarely sparingly stipitate-glandular, 4-6 mm long. Calyx lobes semi-orbicular or ovate, long-ciliate, 0.5-2 mm long. Corolla funnel-form, 3.5-5 cm dia.; tube gradually dilated above, finely pubescent, pilose or strigose-pilose, sometimes stipitate-glandular, 1.5-2 cm long; lobes ovate to oblong-ovate, abruptly acuminate, finely pubescent to glabrescent outside, 1.2-1.6 cm long. Stamens 5, nearly 3 times tube length; filaments pubescent below middle, white, 4-6 cm long; anthers yellow-brown or yellow-orange, 2-2.5 mm long. Style 5-6 cm long, exceeding stamens, finely pubescent on lower third, usually purple above. Capsules oblong to narrow-oblong, narrowed upward, finely pubescent, usually strigose, 1-2 cm long.
Moist or dry open woods, bogs, along mountain streams, chiefly in Appalachian Mountains to 3800 feet elev. but extending into Piedmont and Coastal Plain. Massachusetts to North Carolina, from the Atlantic coast west to central New York, Pennsylvania, southern Ohio, eastern Kentucky, also Calloway County in western Kentucky, eastern Tennessee. April, May.

20. Rhododendron alabamense Rehder in Wilson & Rehder. A monograph of Azalea, p. 141. 1921.
Azalea nudiflora var. alba Mohr. PI. Life Ala. p. 653. 1901. (not Pursh)

Low deciduous shrubs 1-1.5 m high, with irregularly whorled branches, stoloniferous. Twigs gray or yellow-brown, densely or sparingly strigose. Buds brown, glabrous, with ovate, mucronate, densely white-ciliolate scales. Leaves obovate, elliptic to elliptic-oblong or obovate-oblong, base cuneate, apex mucronate, 3-6 cm long, 1-3 cm wide, finely strigillose and sparingly puberulous above, glaucescent or pale green and densely short-villous beneath; midribs villous above and sparingly strigose beneath. Petioles short, rarely over 5 mm long, villous, strigillose. Flowers opening with expanding leaves, in umbel-like racemes of 5-15, white, white with yellow blotch, shades of yellow, white with shades of pink and rose, pink, intensely fragrant. Pedicels villous, stipitate-glandular, rarely eglandular, to 1.5 cm long. Calyx lobes unequal, round to ovate, 0.5-1.5 mm long, densely setose, eglandular. Corolla funnelform with cylindric tube, gradually dilated at apex, 2-3 cm long, stipitate-glandular or hirsute outside; lobes ovate, acute, 1.5-2 cm long, distinctly shorter than tube, not undulate. Stamens 5, twice as long as tube; filaments white, villous on lower half; anthers yellow, 2-3 mm long. Style exceeding the stamens by 1 cm, finely villous at base, white. Capsules cylindric-oblong, finely villous, stipitate-glandular or eglandular and strigose, 1.5 cm long.
Very steep slopes, rocky hillsides in deciduous or open mixed forests in light, well-drained soil. Alabama, Georgia, South Carolina. April, May.

21. Rhododendron roseum (Loisel). Rehder in Wilson & Rehder. A monograph of Azalea, p. 138. 1921.
Azalea rosea Loiseleur-Deslong-champs in Duhamel, Trait Arb. Arbust. Ed. 2., v. 224, t. 64. 1812.
Azalea canescens Pursh. Fl. Am. Sept. 1:152. 1814. (in part)
Azalea nudiflora ee rosea Sweet. Hort. Brit. p. 265. 1826. (nom. nud.).
Azalea nudiflora Darlington. Fl. Cestrica p. 26. 1826.
Rhododendron nudiflorum ee roseum Sweet. Hort. Brit. Ed. 2 p. 344. 1830.
Rhododendron nudiflorum (Darlington) Darlington. Fl. Cestrica Ed. 2 p. 262. 1837. (not Torrey)
Rhododendron canescens Porter. Bull. Torr. Bot. Club XVI:-220. 1889. (not Sweet)
Azalea prinophylla Small. N. Am. Fl. 29(1):42. 1914.
Rhododendron prinophyllum (Small) Millais. Rhododendrons, p. 229. 1917.

Election Pink. Early Azalea. Mountain Azalea. Roseshell Azalea.

Deciduous, upright shrubs 1-3 (-5) m tall with irregularly whorled branches, stoloniferous. Twigs gray-brown to light brown, finely pubescent, sparingly strigose, sometimes stipitate-glandular. Buds gray, pubescent, with generally ovate, obtuse, mucronulate, ciliolate scales 1 cm long. Leaves elliptic, obovate, obovate-oblong, acute or short-acuminate, cuneate at base, 3-7 cm long, 1-3 cm broad, ciliate, usually eglandular but sometimes stipitate-glandular, sparingly pubescent above, densely gray-villous beneath; midribs pubescent above, sparingly strigillose, sometimes stipitate-glandular beneath. Petioles soft-pubescent, sparingly strigose, 2-5 mm long. Flowers expanding with leaves, in clusters of 5-9, bright pink to magenta, rarely white, without blotch, with clove-pink fragrance. Pedicels finely villous, glandular-setose, 0.5-1.5 cm long. Calyx lobes semi-orbicular to ovate, unequal, finely pubescent, glandular-ciliate, scarcely exceeding 1 mm long. Corolla funnelform with oblique limb; tube cylindric, gradually dilated toward apex, with thin villous tomentum interspersed with numerous gland-tipped trichomes of unequal length, 1 5-2 cm long; lobes as long as or slightly shorter than tube, ovate, abruptly pointed. Stamens 5, slightly over twice tube length; filaments yellow-brown, pubescent below middle; anthers ochraceous, to 2 mm long. Style purple above, sparingly or densely pubescent below, exceeding stamens, 4-5 cm long. Capsules oblong, narrowed toward apex, sparingly puberulous, setosely glandular, 1.5-2 cm long.
Moist or dry woods or stream banks in Appalachian and Ozark Mountains. Southwestern New Hampshire, central Vermont, southwestern Quebec through New York, western and central Massachusetts, northwestern Connecticut, northern Rhode Island, Pennsylvania, northern New Jersey to eastern Ohio, southwestern Virginia; also western Tennessee, west central Arkansas, south and southeast Missouri, eastern Oklahoma. May, June.

22. Rhododendron canescens (Michx.) Sweet. Hort. Brit. Ed. 2 p. 343. 1830.
Azalea lutea L. Sp. PI. 1:150. 1753. (in part)
Azalea nudiflora L. Sp. PI. Ed. 2. 1:214. 1762(in part)
Azalea nudiflora e bicolor Aiton. Hort. Kew. 1:203. 1789.
Azalea canescens Michx. Fl. Bor. Am. f 150. 1803.
Azalea bicolor Pursh. Fl. Am. Sept. 1:153. 1814.
Rhododendron bicolor Sweet. Hort. Brit. Ed. 2. p. 344. 1830.
Rhododendron nudiflorum (L.) Chapman. Fl. S .U.S. p.265. 1865. (not Torrey)
Azalea nudiflora var. canescens Rehder in Bailey. Cycl Am. Hort. 1:12. 1900. (in part)

Wild Azalea. Piedmont Azalea. Florida Azalea. Wild Honeysuckle. Hoary Azalea.

Deciduous shrubs to 4 or 5 m tall with irregularly whorled upright branches, stoloniferous. Twigs yellow-brown, finely pubescent, sparingly strigose. Buds densely gray-pubescent, with broadly ovate, acuminulate scales. Leaves oblong-ovate to oblanceolate or oblong, rarely elliptic or obovate, acute, rarely obtuse, mucronulate, cuneate at base, 4-9 cm long, setosely ciliate; above, sparingly pubescent or glabrescent with finely villous midrib; below, densely pubescent or gray-tomentose with densely pubescent, sparingly strigose midribs. Petioles finely pubescent, sparingly strigose, 2-7 mm long. Flowers expanding before or with leaves in 6-15-flowered clusters, pink with tube usually deeper pink than the lobes which may be almost white, slightly fragrant. Pedicels villous-pubescent, hirsute, sometimes stipitate-glandular, 0.5-1 cm long. Calyx lobes unequal, semi-orbicular to ovate, scarcely exceeding 1 mm, ciliate or glandular-ciliate. Corolla funnelform, to 4 cm dia.; tube cylindric, abruptly dilated at apex, 1.5-2.5 cm long, sometimes nearly twice as long as lobes, densely and finely villous, stipitate-glandular; lobes ovate, acute or obtuse, 1.2-1.5 cm long, no yellow to orange blotch. Stamens 5, much exserted, nearly 3 times the tube length, with pink filaments pubescent below the middle, ochraceous anthers 1.5-2 mm long. Style pink, finely villous toward base, 4-6 cm long, as long or longer than stamens. Capsules cylindric-oblong, narrowed upward, slender, pubescent, sparingly setose, 1.5-2 cm long.
Moist sandy soil, often along streams. Southern Atlantic and Gulf Coastal Plain from Virginia to Florida and west to Oklahoma, southeastern Texas; Piedmont of Virginia, North Carolina, Georgia, Tennessee, Mississippi Embayment. April, May, sometimes as early as January.

23. Rhododendron oblongifolium (Small) Millais. Rhododendrons, p. 219. 1917.
Azalea oblongifolia Small. Fl. S.E. U.S. p. 883, 1336. 1903.

Texas Azalea.

Deciduous shrub 2 m tall or less, with irregularly whorled branches, sometimes stoloniferous. Twigs with short, sparse villous pubescence, sparingly strigose, sometimes nearly glabrous, orange-brown. Buds with ovate, acuminulate or obtuse, gray-pubescent scales. Leaves obovate, elliptic-obovate, oblong-oblanceolate, rarely oblong, 4-10 cm long, 1.5-4 cm broad, acute, mucronulate, cuneate, ciliate; dull green, glabrous, sometimes strigilose, minutely pubescent above, light green, rarely glaucescent, more or less pubescent or nearly glabrous beneath; midribs villous above, pilose and strigillose beneath. Petioles finely pubescent, 2-3 mm long. Flowers expanding after the leaves in 7-12-flowered clusters, white, no blotch, fragrant (lemon oil scent). Pedicels sparingly villous, glandular-hirsute, 5-7 mm long. Calyx lobes unequal, ovate, oblong, lanceolate, obtuse to acute, 1 -3 mm long, long ciliate, sparingly villous, corolla funnelform; tube cylindric, 2.5-3 cm long, abruptly dilated at apex, thinly villous, sparingly glandular-hirsute; lobes oblong-ovate, acute, 1.5-2 cm long; stipitate-glandular along middle. Stamens 5, twice as long as tube, slightly exceeding lobes; filaments white, villous below middle; anthers ochraceous, 2.5-3 mm long. Style exceeding stamens, 5-6 cm long, finely pubescent at lower third or sometimes glabrous, white. Capsules oblong, ovoid-oblong, or narrow-oblong, finely villous, stipitate-glandular, 1-2 cm long.
Sandy soils along high stream banks, open woods, occasionally in sandy, freshwater bogs, on slopes with exposed rocks. Arkansas, Louisiana, Oklahoma, Texas. Late April, May.

24. Rhododendron viscosum (L.) Torrey. Fl. U.S. p. 424. 1824.
Azalea viscosa Linnaeus. Sp. PI. 1:151. 1753.
Azalea viscosa palstris Marshall. Arbust. Amer. p. 16. 1785.
Rhododendron venustrum Salisbury. Prodr. p. 287. 1796. (in part)
Azalea nitida Lindley. Bot. Reg. V, t. 435. 1819. (not Pursh)
Anthodendron viscosum (L.) Reichenbach in Moessler, Handb. Gewchsk. Ed. 2, l:309. 1827.

Swamp Honeysuckle. Swamp Azalea.

Deciduous shrubs to 3 or nearly 5 m tall, sometimes low, with irregularly whorled branches, stoloniferous. Twigs yellow-brown or gray-brown, hirsute or strigose. Bud glabrous, sometimes pubescent, usually brown with 8-12 broadly ovate scales rounded at apex and usually mucronate, ciliolate; basal scales sometimes long-pointed. Leaves ovate, elliptic-obovate or oblong-lanceolate, acute or rounded, mucronulate, cuneate, 2-6 cm long, 0.5-2 cm broad, ciliate, glabrous; midrib slightly villous above, more or less strigose beneath. Petioles strigillose, 1-3 mm long. Flowers appearing after the leaves, white, rarely pink, no blotch, in clusters of 4-9, fragrant. Pedicels minutely pubescent, stipitate-glandular, 0.5-1 (-1.5) cm long. Calyx lobes semi orbicular to ovate, setosely glandular-ciliate, about 1 mm long. Corolla funnelform, 1-5 cm dia.; tube cylindric, somewhat dilated near the apex, finely villous, stipitate-glandular, about 1.5 cm long. Stamens 5, exserted, somewhat longer than lobes; filaments villous on lower two-thirds, white, 4-5.5 cm long; anthers yellow, 1-2 mm long. Style finely pubescent below middle, white, sometimes purple toward stigma, 4-6 cm long, exceeding stamens. Capsules oblong-ovoid, finely pubescent, glandular-setose, sometimes setose and eglandular, 1-2 cm long.
Common in swamps of Coastal Plain to higher mountains. Maine to Florida; west to Ohio, mountains of Pennsylvania to North Carolina, Tennessee; Mississippi, Arkansas, Louisiana, southeast Texas. April to June.

R. viscosum
R. viscosum

25. Rhododendron serrulatum (Small) Millais. Rhododendrons, p. 241. 1917.
Azalea viscosa Hooker. Comp. Bot. Mag. 1:100. 1835. (Not Linnaeus)
Rhododendron viscosum Chapman. Fl. S. U.S. p. 265. 1865. (not Torrey)
Azalea serrulata Small. Fl. S.E. U.S. p. 883. 1903.
Rhododendron viscosum var. serrulatum (Small) Ahles. Jour. Elisha Mitchell Sci. Soc. 80:-173. 1964.

Hammocksweet Azalea.

Deciduous tall shrubs or small trees to 7 m tall with irregularly whorled branches and radiating white rhizomatoid root systems. Twigs bright red-brown, copiously strigose, finely villous. Buds glabrous or sometimes gray-pubescent with 15-20 ovate, mucronate or mucronate-aristate scales usually light yellow-brown with a sharply marked dark brown band along the white-ciliolate margin. Leaves elliptic, obovate or obovate-oblong, rarely oblanceolate, 4-8 cm long, 1.5-3.8 cm broad, acute, rarely obtuse, mucronulate, cuneate at base, serrulate-ciliate margins, glabrous above and beneath, midribs sparingly short-villous above and strigose beneath. Petioles strigose, 1-4 mm long. Flowers expanding after the leaves and after winter buds are partially formed, white, in clusters of 6-10, no blotch, very fragrant clove-pink scent. Pedicels minutely pubescent, stipitate-glandular, 1-2 cm long. Calyx lobes semi-orbicular to ovate, long glandular-ciliate, to 1 mm long. Corolla funnelform, 2-3 cm dia.; tube cylindric, slender, 2.5-3.5 cm long, 2 mm wide, slightly dilated at apex, copiously stipitate-glandular, sparingly villous; lobes ovate, lanceolate, acuminate, 1-1.5 cm long. Stamens 5, exserted about 1 times tube length; filaments yellow, lower two-thirds villous; anthers ochraceous, oblong, 2-2.5 mm long. Style considerably exceeding stamens, slightly short-pubescent near base or glabrous, purple above, 4.5-6 cm long. Capsules ovoid-oblong, minutely villous, densely glandular-setose, 1-1.5 cm long.
Wet woods, Coastal Plain of Virginia, North and South Carolina, Georgia, Florida westward to Louisiana, June to early August.

26. Rhododendron arborescens (Pursh) Torr. Fl. U.S. p. 425. 1824.
Azalea arborescens Pursh. Fl. Am. Sept. p. 152. 1814.
Azalea verticillata Carr. Cat. Trees & Shrubs Bartram's Bot. Gard. p. 11. 1814.
Azalea arborea Bartram ex Pursh. Fl. Am. Sept. p. 152. 1814. (as synonym)
Azalea fragrans Rafinesque. Ann. Nat. p. 12. 1820. (not Adams)

Smooth Azalea. Sweet Azalea.

Deciduous tall shrubs or small trees to 6 m tall, not observed to be stoloniferous. Twigs yellow to light gray-brown, glabrous. Buds glabrous; scales light brown, ovate, ciliate, with rounded base, mucronulate apex. Leaves oblanceolate, bright green, ciliate, glabrous except for midribs, sparingly short-villous above and sparingly strigose beneath. Petioles glabrous, 3-8 mm long. Flowers expanding after leaves have unfolded, white to pink, no blotch on lobes, in clusters of 3-8, very fragrant. Pedicels red stipitate-glandular, 0.5 cm long. Calyx lobes ovate to linear-oblong, glandular-ciliate, 3-6 mm long. Corolla funnel-form, 3-4.5 cm dia.; tube 2-3 cm long, sparingly red stipitate-glandular, slightly dilated at apex; lobes ovate-oblong, acuminate, 1.5-2 cm long. Stamens 5, exserted, about twice as long as tube; filaments purple above, pubescent below the middle; anthers 2-3 mm long, ochraceous. Style as long or longer than stamens, purple above, glabrous. Capsules oblong-ovoid, 0.5-2 cm long, densely glandular-hispid.
Banks of mountain streams, rarely on the borders of swamps. Appalachian Mountains, New York to Georgia, Kentucky, Alabama. Late June, early July.

27. Rhododendron prunifolium (Small) Millais. Rhododendrons, p. 230. 1917.
Azalea prunifolia Small. Fl. S.E. U.S., Ed. 2 p. 1356. 1913.

Plumleaf Azalea.

Deciduous shrubs to small trees over 5 m tall with irregularly whorled branches, often stoloniferous. Twigs glabrous, dark purple-red. Buds glabrous, orange to red brown, with broadly ovate, rounded, mucronulate, sometimes nearly aristate, ciliolate scales 1-1.25 cm long. Leaves elliptic, sometimes obovate to oblong, acute or short acuminate, mucronulate, cuneate, 3-10 (-13.5) cm long, 1.5-4 cm broad, ciliate, glabrous above, very sparingly strigillose beneath, midribs slightly villous above and sparingly strigose beneath. Petioles glabrous or rarely sparingly strigose, 3-6 mm long. Flowers opening after leaves have expanded, in 4-5-flowered clusters, crimson, no blotch on lobes, not fragrant. Pedicels hirsute, 5-7 mm long. Calyx lobes semi orbicular to ovate, glabrous, long-ciIiate, 1-1.5 mm long. Corolla funnelform; tube gradually widened above middle, 2-2.5 cm long, glabrous, sparingly hirsute or sparingly stipitate-glandular; lobes broadly ovate, abruptly acuminulate, 1.5 cm long, 1-1.5 cm broad, more or less finely villous, sometimes slightly glandular-pilose. Stamens 5, much exserted, nearly 3 times tube length, 5-6.5 cm long; filaments yellow, villous on and below the middle; anthers ochraceous to yellow, 2-3 mm long. Style to 8 cm long, much exceeding stamens, glabrous, purple above. Capsules ovoid-oblong, strigose, finely puberulous, 2 cm long.
Shady ravines on stream banks. Southwestern Georgia, eastern Alabama. June-September.

Putative Hybrids and Species Transferred to Other Genera

Rhododendron fastigifolium (Lemmon) Hume. Azaleas, Kinds and Culture, p. 28. 1948.
Azalea fastigifolia Lemmon. Bartonia 19:15. 1938.
Rhododendron furbishii (Lemmon & McKay) Hume. Azaleas, Kinds and Culture, p. 28. 1948.
Azalea furbishii Lemmon & McKay. Bartonia 21:5-6. 1940.

These two taxa are putative hybrids described from areas known to have hybrid swarms of Rhododendron . Leach (1959) reported that in the crosses he made with R. bakeri and R. arborescens in an effort to duplicate "R. furbishii" , resulting hybrids were almost identical with those described by Lemmon. He also found that plants grown from seeds resulting from self-pollination of wild "R. furbishii", proved to be a segregating population with characteristics of R. bakeri and R. arborescens . A similar situation obtains for R. fastigifolium although the crosses have not been reported. Lee (1965) says "Material described under fastigifolium , with smaller red flowers and orange-yellow blotch, probably falls within the speciosum-canescens complex. Of the thirteen plants of fastigifolium originally discovered and transplanted, only a few are apparently still in cultivation." These taxa should be regarded as hybrids with the names R. x furbishii and R. x fastigifolium .

Rhodothamnus leachianum (Henderson) Copeland. Amer. Midi. Nat. 30:565. 1943.
Rhododendron leachianum Henderson. Rhodora 33:205. 1931.
Kalmiopsis leachiana (Henderson) Rehder. Jour. Arnold Arb. 13:31-32. 1932.

Rehder considered this taxon to have enough characters different from Rhododendron that he placed it in a new genus, Kalmiopsis . However, Copeland found that Kalmiopsis was sufficiently like Rhodothamnus that it should no longer be maintained as a separate genus. Therefore, he merged it with Rhodothamnus , which has nomenclatural priority.

The use of facilities of and/or loans of specimens from The Academy of Natural Sciences, Philadelphia; Arizona State University; Auburn University; Florida State University; George Mason University; Longwood College; Lynchburg College; The Morris Arboretum; Mountain Lake Biological Station; The National Arboretum; Oklahoma State University; Old Dominion University; Oregon State University; Southern Methodist University; University of Georgia; University of Missouri-Columbia; University of North Carolina; University of Richmond; Vanderbilt University; Virginia Commonwealth University; Virginia Polytechnic Institute and State University; West Virginia University; and The College of William and Mary is acknowledged with thanks. Funding from the Flora Committee of the Virginia Academy of Science for collecting trips is greatly appreciated. Living plants representing most of the species that were collected and planted on the grounds of the Henry Foundation for Botanical Research by Mary G. Henry were studied. We also wish to thank Duncan Porter and Leonard Uttal for the suggestions for and comments on the manuscript.

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