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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Identification of Native Azaleas
F. C. Galle, Director of Horticulture,
Callaway Gardens, Pine Mountain, Ga.

Reprinted from "Native and Some Introduced Azaleas for Southern Gardens", Callaway Gardens.

FIG.28. R. serrulatum, the late-flowered      Hammock-sweet azalea, is found in wooded      swamps and is clove scented.      Photo courtesy of the U. S. National      Arboretum		FIG. 34. Late blooming R. prunifolium                   varies from orange to deep red                   and should be planted in shady                   areas.                   Photo by Fred Galle
FIG.27. The delicate flowers of R. vaseyi                 lack the distinct tube typical of                 most azaleas.                 Photo by Fred Galle		FIG. 29. Pure white, sometimes with a distinct                  yellow blotch, the stoloniferous                  R. alabamense is a rare plant of                  dry, open woodland.                  Photo by Fred Galle

        Native azaleas, while our most attractive plants, are difficult and extremely frustrating to classify or identify. There is great variability within some species and strong morphological characteristics are frequently absent. The difficulty is increased when one considers the innumerable natural hybrid forms often encountered.
        Hybridization may be expected to occur whenever synchronous-flowering species grow close together. With a good knowledge of the characteristics of the various species, these can generally be recognized. To list all known hybrids would be confusion to most and consequently of little value. However, a few examples will serve to emphasize their occurrence.
        In central Georgia, both Oconee, R. speciosum, and Piedmont azalea, R. canescens, grow in the same general area. Hybrids between these two species are frequently observed with deep pink to salmon color forms. Usually neither swamp azalea, R. viscosum, nor Alabama azalea, R. alabamense, are found flowering with the above species. However, all four species have been observed flowering at the same time in north central Georgia. Also, in three years out of twelve, the following three species, Piedmont, Oconee, and swamp azalea, have been observed in flower at the same time. Numerous hybrids are available from hand pollinated crosses from plants that may never occur within the same area or flower at the same time. Thus, it requires only a small imagination to envision the colorful complex of hybrids that sometimes occur. It is likewise easy to understand that descriptive key including such hybrids would be too complex to use except by a few professionals.
        The identification keys presented here are based on the more distinctive and evident characteristics of a species. The key includes the recognized azalea species of the Eastern United States, omitting the Western azalea, R. occidentale.
        The key is constructed with a choice of two or more alternatives, only one of which should fit the plant in question. After deciding which description fits, proceed to the next set of choices. Continue this procedure until the species is determined.
        Make sure the azalea specimen is representative. A small hand lens is necessary to determine pubescence and glandular setae. Fragrance may be variable and is best observed in the early morning or late afternoon. While the key is based on the flowering plants, you will find references to winter floral bud characteristics. Thus, the study and identification of azaleas requires year round observation. Clear-cut identification is often very difficult due to the variability and intergradation of species.
        It is sometimes said that plant keys are of value only to the designer of the key. While an amateur may have difficulty at first, continued effort should give improvement. Hopefully, the more advanced gardener will find the key more readily useful. Suggested improvements on the key will be appreciated.

Key to Species of Native Azaleas of the Eastern United States
SUB-GENUS ANTHODENDRON

A. Corolla rotate or rotate campanulate, more or less two lipped; stamens 7-10, rarely 5-6.

Section Rhodora B. Stamens 10, corolla rose-purple, occasionally white; two lipped with two lower lobes divided to the base. Flowers appearing before the leaves. Leaves tomentulose beneath, glaucous. Low bush with ascending branches. This is the most northern species, found in moist woods and swamps extending from Labrador to Newfoundland south through New England, central New York to northeastern Pennsylvania and northern New Jersey. R. canadense (L.) Torrey Rhodora

BB. Stamens 5 - 7, corolla with short tube, slightly two lipped, pink or white, spotted. Flowers appearing before the leaves. Leaves glabrous, green beneath. Bush upright with irregular, spreading habit. A plant from the high mountains of western North Carolina, found in ravines and sometimes swamps. R. vaseyi   Gray Pinkshell azalea

AA. Corolla funnel-form, sub-regular; tube variable, being longer or shorter than the lobes. Stamens 5.

Section Pentanthera C. Early flowering before or with the leaves (see CC and CCC)      D. Flowers pale to deep pink, rarely white; fragrant.     E. Winter bud scales glabrous. Leaves glabrous - glabrescent. Corolla - tube setae eglandular. Plants variably stoloniferous. The pinxterbloom azalea is the northern counterpart of R. canescens and occupies a wide territory from the Carolinas and the Atlantic seaboard to Tennessee, Ohio, and Massachusetts. R. nudiflorum (L.) Torrey Pinxterbloom azalea

EE. Winter bud scales pubescent. Plants generally not stoloniferous.            F. Corolla tube, glandular; about as long as the lobes; lobes pointed. Stamens about twice as long as the tube. Leaves with soft pubescence beneath, sometimes with glandular setae. The glandular leaf phase of the roseshell azalea seems to be confined to the mountains of Virginia, but, in more variable form, this species follows the highlands as far west as Missouri via northern Indiana and Ohio to New England and southern Quebec.   R. roseum (Loisel) Rehder   Roseshell azalea

FF. Corolla tube glandular, distinctly longer than the lobes. Stamen about thrice as long as the tube. Leaves are usually felty pubescent beneath. Stoloniferous clones frequently occur. The southern Piedmont (pinxter) azalea is widespread from South Carolina and Florida to eastern Texas and Tennessee.       R. canescens (Michx.) Sweet Piedmont azalea

DD. Flower white and fragrant (note DDD)

G. Winter buds glabrous. Plants usually stoloniferous.

H. Corolla tube, broad, with prominent rows of conspicuous long stipitate glands at apex. Flowers, scented, white or pink flushed, without yellow blotch. Branchlets and leaf undersides usually glabrous, often with glandular setae. Bushes low and very stoloniferous. The coastal azaleas are found in moist woods and pinelands from southeastern Pennsylvania, Delaware to South Carolina.       R. atlanticum (Ashe) Rehder       Coastal azalea

HH. Corolla tube, narrow, glandular without prominent rows stipitate glands at apex. Flowers lemon scented and usually with yellow blotch. Branchlets with glandular setae. Crushed foliage has distinct odor unlike other species. Bushes low, stoloniferous. In 'best' form, the Alabama azalea has a limited distribution, centering in north central Alabama. (Hybrid forms frequently found, bushes tall, less stoloniferous and flowers usually with pink blush.)         R. alabamense Rehder   Alabama azalea

GG. Winter buds pubescent. Plants somewhat stoloniferous.       I. Leaves variably pubescent. Corolla tube thin and sparingly pubescent; flowers rather small. Plant of moist woods from northern Arkansas to eastern Oklahoma and southeast Texas.   R. oblongifolium (Small) Millais   Texas azalea

II. Leaves glabrous; except for abundant eglandular setae. Corolla tubes soft (villous) pubescent, glandular and thin. Winter buds densely gray-pubescent. Flowers many in a cluster and small. Assigned to R. viscosum by Rehder, this little plant of the drier woods of Georgia and Alabama is earlier flowering than R. viscosum and seems more akin to R. oblongifolium.         R. viscosum (L.) Torr. var. aemulans-Rehder

DDD. Flowers yellow to red. Plants not stoloniferous.          J. Winter buds pubescent, flowers yellow and fragrant. Leaves soft pubescent beneath, and leaves and branches often with glandular setae. Corolla tube glandular, usually reddish. The Florida azalea occupies a restricted southern range from northwestern Florida to southeastern Mississippi.     R. austrinum (Small) Rehder     Florida azalea     JJ. Winter buds glabrous, flowers yellow to red, not fragrant.

K. Corolla tube usually pubescent with glandular setae, about as long or shorter than lobes. Flower width variable, but averaging about 48mm. across wing petals; color clear yellow to orange and red. Branchlets and leaves variably pubescent. This is the large flower phase of the tetraploid flame azalea, which blooms in early May and occurs at lower elevations of the mountains from north Georgia to Virginia and perhaps to Pennsylvania and Ohio. R. calendulaceum (Michx.) Torrey Flame azalea

KK. Corolla tube finely pubescent and eglandular; slender tube longer than the lobes. Flower width variable, but average about 40 mm. across wing petals; color apricot yellow to orange-red and red. Leaves glabrescent to glabrous. The Oconee azalea follows a relatively narrow band across central Georgia and down the Savannah River.     R. speciosum (Willd.) Sweet (R. flammeum (Michx.) (Sarg.) Oconee azalea

CC. Mid-season flowering, with or after leaves, but before winter buds are formed. (see CCC.)

L. Flowers white to pink blush and fragrant.

M. Branchlets glabrous and smooth. Winter buds glabrous. Corolla tube sparingly glandular outside, usually pubescent inside. Style purplish red, usually glabrous. Leaves glabrous, occasionally eglandular setae. Bush non-stoloniferous. The sweet azalea is a plant of upland stream sides from New York and Pennsylvania to central Georgia and Alabama.         R. arborescens (Pursh.) Torrey Sweet azaleas

MM. Branchlets and leaves pubescent with bristle-like setae. Winter buds glabrous to pubescent, depending upon variety. Corolla tube thin and glandular.  Style not colored and usually pubescent. Plant is variably stoloniferous; also variable as to habit, leaf size, flower size and time of bloom, which sometimes occurs well into July. This moisture-loving species is widespread from Maine to South Carolina, Tennessee and Georgia.         R. viscosum Torrey Swamp azalea

LL. Flowers yellow to red; non-fragrant.

N. Bushes non-stoloniferous, upright habit of growth. Corolla tube, usually pubescent with glandular setae, about as long or shorter than lobes. Winter buds glabrous. Flower width variable, but averaging about 46 mm. across wing petals. The earlier description of R. calendulaceum applies in most respects, but the flowers are somewhat smaller and the blooming season later, from late May to mid-June. This is the high altitude, late phase flame azalea of the southern Appalachians (Wayah Bald; Soco Gap, etc.) to Virginia.         R. calendulaceum (Michx.) Torrey Flame azalea

NN. Bushes variably stoloniferous, sometimes vigorously so. Plants low and twiggy. Similar to R. calendulaceum with respect to pubescence of vegetative and floral parts, flower color, etc., but corolla tubes are thinner and flowers somewhat smaller, averaging about 42 mm. across wing petals. The diploid Cumberland azalea flowers in June and early July on the mountains of eastern Kentucky to Tennessee, western North Carolina, north Georgia, and Alabama.       R. bakeri (Lemon & McKay) Hume Cumberland (Camp's Red) azalea

CCC. Late flowering, appearing after the winter buds are at least partly formed.

O. Flowers white, fragrant.

P. Branches densely strigose. Winter buds with more than 15 aristate mucronate scales, usually pale with conspicuous dark margin. Leaves often pubescent beneath, serrulate, ciliate margin. Corolla tube slenderly cylindric nearly to summit; about twice the length of lobes; tubes copiously glandular - pilose and sparingly villous outside. Style glabrous or minutely pubescent only at base and usually not colored. Bushes tall, infrequently similar to those of R. viscosum, including the variability of winter bud pubescence, but its leaves may be slightly larger than average for R. viscosum and individuals may flower in late October. A close ally of R. viscosum, the hammock-sweet azalea occupies a disjunct area of distribution of the southern coastal plains from east central Georgia, central Florida to Louisiana.         R. serrulatum (Small) Millais         Hammock-sweet azalea

PP. Branches glabrous and smooth. Winter buds glabrous and light brown. Corolla tube sparingly glandular outside, usually pubescent inside. Style purplish red, usually glabrous. Leaves glabrous, occasionally with eglandular setae. Late flowering phase of sweet azalea; found in Georgia.         R. arborescens (Pursh.) Torrey         Sweet azaleas

OO. Flowers yellow to red, non-fragrant; winter buds glabrous.

Q. Branches with stiff bristles. Bushes variably stoloniferous, sometimes vigorously so, plants low and twiggy. See previous description. Scattered individuals of southern Appalachians have been observed in flower in August and September.         R. bakeri (Lemon & McKay) Hume         Cumberland (Camp's Red) azalea

QQ. Branches glabrous and smooth. Bushes tall, round topped, non-stoloniferous. Winter buds glabrous.  Leaves generally glabrous with eglandular setae on veins. Flowers generally apricot yellow to red with predominance of red-orange and red. The distinctive plumleaf azalea is restricted to the ravines of a small part of southwestern Georgia and adjacent Alabama, generally centering around Ft. Gaines, Georgia. It shows closest affinity to R. bakeri.   R. prunifolium (Small) Millais   Plumleaf (Prunifolia) azalea

Glossary

1. Apex: The tip or distal end.

2. Aristate: Bristle-like or elongated apex. 

3. Campanulate: Bell-shaped.

4. Ciliate: Marginally fringed with hairs.

5. Diploid: Two similar complements of chromosomes (n=13) (2n=26). 

6. Eglandular: Without glands.

7. Glabrescent: Becoming nearly glabrous with age. 8. Glabrous: Not hairy or pubescent.

8. Glandular: Bearing glands or gland-like appendages or protuberance. 

9. mm.: Millimeter, 25mm. equals approximately an inch.

10. Mucronate: Tipped with a short abrupt point. 

11. Pilose: With soft, long, straight hairs.

12. Pubescence: Covered with short, soft hairs. 

13. Rotate: Wheel-shaped.

14. Setae: A bristle.

15. Stipitate glands: Glands with stalk-like bases ('pinhead glands').

16. Stoloniferous: A plant producing runners or any basal branch that is inclined to root & give rise to a new plant.

17. Strigose: With sharp appressed, straight, stiff hairs, often swollen at the base. 

18. Style: Elongated part of pistil between the ovary and stigma.

19. Tetraploid: Complement of chromosomes (n=26) (2n=52).

20. Tomentulose: Finely tomentose or dense, fine wooly pubescence. 

21. Villous: Bearing long and soft hairs.

*Key from the booklet "Native and Some Introduced Azaleas for Southern Gardens".