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Home , Rhododendron, Rhododendron arborescens, Rhododendron atlanticum, Rhododendron austrinum, Rhododendron luteum, Rhododendron occidentale

All of the remaining 12 of IDENTIFYING THE NATIVE subsection Pentanthera oc- cur in the southeastern U.S. The easi- Dr. Kathleen A. Kron est first line of investigation for iden- Winston-Salem, NC tification purposes is color. Three main groups can be identified using this criterion: (1) white, (2) pink, The azaleas are among the showiest of our native . Their and (3) orange to . Within these primary beauty is due to their , which range in color from white to color groups the timing of flowering pink, and orange to red. Early in the history of European collecting in and expansion, the type of pubes- this country, native azaleas were used to develop numerous (Ghent, cence (hairs) and characteristics of the Knap Hill, Exbury, and Mollis azaleas). These azaleas were the result flower scales are useful characters of initial crosses of (from and ) and the North for identifying species. American azaleas, or (from the Caucasus region) and the North American azaleas. Although these cultivars offer marvelous displays of Among the white-flowered group are color in the spring, the native azaleas have their own delicate beauty. Rhododendron alabamense, R. arborescens, R. atlanticum, and R. viscosum. Like other members of the Rhododendron, the native azaleas have the Although the flowers of these species petals fused into a funnel or cup-shaped corolla. Most of the native azaleas are consistently white, they may oc- belong to a subgroup of named R. Pentanthera [Rhodo- casionally be pale pink or tinged with dendron vaseyi, a rare found in the southern Appalachians, is in the pink. Of these, R. alabamense is unique closely related R. section Rhodora (1)]. The members of R. section Pentanthera because it possesses a yellow blotch share several unique characters that indicate a unique common ancestry. These on the upper corolla lobe and a very characters include a narrow corolla tube and five that are much longer delicate sweet fragrance. This azalea than the petals. Several members of the section have a darker-colored blotch is almost entirely confined to on the upper petal, others lack this blotch. This is in contrast to the and its periphery. It flowers early in azaleas (Rhododendron Tsutsusi) that have spotted petals, usually ten the spring, before the are ex- stamens that are not much longer than the petals, and wide corolla tubes. panded. is re- stricted to the Atlantic Coastal Plain. There are 15 species of R. section Pentanthera. Rhododendron molle (Mollis It is usually a strongly rhizomatous azalea, R. subsection Sinensia) is the most distinctive of the Pentanthera , often less than a meter in azaleas. It has a broader corolla tube than the other members of section, with height, growing in acid sands. The spots on the interior surface. Its color ranges from yellow, in China and Japan, flowers of R. atlanticum bloom before to orange-red, in Japan only. This species has been extensively used in the the leaves expand and have a very development of many cultivars of azaleas. The remaining 14 species of section strong musky-sweet odor. The flow- Pentanthera (R. subsection Pentanthera) are very closely related and therefore ers are densely covered with gland- the differences among these species are often subtle. All of these species tipped hairs that are extremely sticky. possess a narrow corolla tube and stamens that extend beyond the end of the and R. petals. These azaleas occur as understory shrubs, often under a canopy of viscosum are two closely related oaks. They are less frequent on north-facing slopes because, although they are understory , they require a fair amount of sunshine. If the canopy is too dense, azaleas will be absent. They can vary in height from less than a meter to ten meters tall. Although in the past most of the azaleas were reported as non- rhizomatous, they are actually often rhizomatous or stoloniferous. Two species occur outside of the southeastern . (Western azalea) occurs along the west coast of southern Oregon to southern . This species is extremely variable in flower color, but is generally white with a yellow blotch on the upper corolla lobe. It is the only member of R. section Pentanthera that occurs along the west coast. Rhododen- dron luteum (Pontic azalea) is native to the Black Sea and Caucasus region. Its flowers are bright yellow with a deeper yellow blotch. This species is reputed to have caused Alexander the Great's troops to fall ill after eating made from its flowers. This is likely due to the andromedotoxin that is prevalent in the R. viscosum, the clammy azalea of many members of the genus Rhododendron.

72 • THE AZALEAN/December 1996 species with white flowers that bloom The pink group of decidu- after the leaves have expanded. Rhodo- ous azaleas are all early flow- dendron arborescens is restricted to the ering species that bloom be- mountains, and the flowers have a fore the leaves are fully ex- bright red style with a cinnamon-like panded. These azaleas are R. fragrance. The entire plant of R. periclymenoides (Piedmont aza- arborescens is smooth and shining due lea), R. canescens (sweet aza- to the lack of soft white (unicellular) lea), and R. prinophyllum hairs that are usually common on (roseshell azalea). Their flow- plants of R. viscosum. ers range in color from pale pink (almost white) to a deep The most widespread species of cherry color and all lack a deciduous azalea is the clammy aza- blotch on the upper corolla lobe. lea, . Its geo- Rhododendron prinophyllum graphic distribution ranges from Maine can be distinguished from the south to peninsular , west to other two pink azaleas by its eastern Texas and from eastern Okla- longer flower stalks (pedicels) homa to southern Arkansas east to the covered with gland-tipped Atlantic coast. This is the most vari- hairs, a broader corolla R. calendulaceum, the flame azalea able of the southeastern azaleas and tube, gland-tipped hairs on has been divided into several segre- the margins of the sepals and gate species: R. coryi, R. oblongifolium, on the , and conspicuously well as gland-tipped hairs. This is in and R. serrulatum. In the northern part fringed (ciliate) leaf margins. This aza- contrast to the closely related R. peric- of its range the white corolla is often lea is more northern in its distribution lymenoides, which has very few unicel- tinged with pink. The flower bud than R. canescens, which is restricted lular hairs on the bud scales and scales of this species often have a dis- to the southern Coastal Plain. It is the corollas lacking gland-tipped hairs. tinctive dark brown line around the only pink azalea that occurs in Arkan- edges. This characteristic has been sas and Oklahoma, as well as in the There are five species that have used to separate R. serrulatum from R. northeast as far north as New Hamp- orange to red flowers—R. austrinum, R. viscosum in the southeast, where the shire. Because it is often covered with calendulaceum, R. flammeum, R. dark brown band is most common. soft unicelluar hairs it has been con- cumberlandense, and R. prunifolium. The However, the dark brown line can be fused with R. canescens. However, R. plumleaf azalea, R. prunifolium, is the found far north in some populations canescens has very narrow corolla tubes rarest of the deciduous azaleas and has of R. viscosum. Rhododendron coryi and and the lack the gland-tipped been proposed for federally endan- R. oblongifolium have been separated hairs common in R. prinophyllum. gered status. Restricted to just a few from R. viscosum due to their long nar- Rhododendron canescens is common in counties on the southern Alabama- row corolla tubes. Plants named R. swamps or bottomlands in the Gulf border, R. prunifolium flowers coryi are stoloniferous in habit, Coastal Plain. This azalea has a sweet, after the leaves have expanded. Its whereas plants named R. oblongifolium sometimes slightly musky odor and deep red-orange to apricot-colored have hairier leaves than many indi- flowers just before or as the leaves ex- flowers essentially lack any hairs, and viduals of R. viscosum. However, all pand. Rhododendron of these "species" have white flowers canescens is very variable with gland-tipped hairs that are in in pubescence (hairi- lines up the outside of the corolla tube. ness) and in corolla They lack a blotch on the upper co- color. In some Missis- rolla lobe and have greenish-whitish sippi populations R. colored filaments and styles. They all canescens has such flower after the leaves have fully ex- densely hairy (unicellu- panded and have a sweet-musky fra- lar hairs) leaves that grance. When all of the characters are they appear whitish. compared over the entire range of The flower bud scales of Rhododendron viscosum it is apparent R. canescens are also of- that R. coryi, R. oblongifolium. and R. ten white with dense serrulatum represent random local vari- hairs. The outer sur- ants of R. viscosum, and therefore face of the corolla is should not be recognized as distinct densely covered with R. prunifolium, the plumleaf azalea, species. unicellular hairs as at Calloway Gardens

December 1996/THE AZALEAN • 73 the bud scales and leaves are also calendulaceum (flame azalea) is a com- upper corolla lobe) the hybrid indi- smooth and shining. This species is mon component of the understory in viduals are easy to identify. Usually the most heat tolerant of the red-flow- the mountains of Georgia, North Caro- they are pink to white with a yellow ered azaleas. lina, and . This species is blotch on the upper corolla lobe. Other (Florida azalea) is restricted to the pan- tremendously variable in flower color aspects of these hybrids are also inter- handle of Florida and a few nearby and leaf pubescence. Flowers of this mediate between the parents. Plants counties in Alabama, Georgia, and species are generally larger than those of R. canescens are tall with a round . Although it can resemble of R. cumberlandense. This may be due top, while plants of R. flammeum are R. canescens because of the general to the fact that R. calendulaceum is a shorter with a distinctive flat-topped shape of the corolla (the flower color tetraploid, whereas all other members shape. Hybrid individuals are inter- varies from yellow to orange with a of R. section Pentanthera are diploid. mediate in height between R. canescens dark pink tube), R. austrinum has a The flame azalea (R. calendulaceum) and R. flammeum and have neither the deep yellow to orange blotch on the flowers before or as the leaves expand flat-top of R. flammeum, nor the upper corolla lobe. Like R. canescens it and has gland-tipped hairs that are rounded shape of R. canescens. has densely pubescent bud scales, but prominent on the pedicels and sepal (unlike R. canescens) the bud scales margins. It is much more common Although hybrids can be locally have glands along the margins. In ad- than the Cumberland azalea (R. common, as in the case of Stone Moun- dition gland-tipped hairs are common cumberlandense) and flowers early in the tain, Georgia, the phenomenon is not on the petioles, lower edges (margins) spring. Some may be wondering as pervasive as often suggested in of the leaves, and flower stalks of R. "What about Rhododendron bakeri?" the literature. Much of the emphasis austrinum. This species is generally This name has been and is currently on problems of hybridization has been restricted to the bluffs above the used by many for the diploid flame due to collectors' emphases on areas Apalachicola River and thus is ecologi- azalea. Unfortunately, the name R. where hybrids are known to occur, e.g. cally isolated from R. canescens, which bakeri was based on a specimen that is Gregory Bald in the Great Smoky prefers wet or poorly drained soils. a hybrid of R. flammeum and R. Mountains National Park. However, The Oconee azalea (R. flammeum) is canescens! Therefore, it can only be much of the variability in flower another red-flowered species with a re- applied to hybrids of R. flammeum and color and pubescence is due to stricted range. This species is found R. canescens, and there is already an natural variation in the species of in upland woods on dry slopes and older name for such a hybrid (although native azaleas. ridges, or on bluffs of rivers or streams. naming natural hybrid individuals is It is found in the Flint River gorge area not recommended). The correct name Literature Cited of Georgia and along the bluffs of the for the diploid flame azalea, then, is Savannah River. Rhododendron Rhododendron cumberlandense (2). (1) Judd, W. S. and K. A. Kron. 1995. flammeum is almost entirely eglandular A revision of Rhododendron VI. [lacks glands, ed.], and flowers after The example of R. bakeri (= R. Subgenus Pentanthera (sections or as the leaves expand. Plants of R. flammeum X R. canescens) points to the Sciadorhodion. Rhodora, and flammeum are generally flat-topped and importance of natural hybrids among Viscidula). Edinburgh Journal of the leaves tend to be smooth and lack- the native azaleas. Most species of 52, pp. 1-54. ing very many hairs. Perhaps the most azalea can be crossed and produce fer- difficult species to distinguish within tile offspring. The natural barriers to (2) Kron, K. A. 1993. A revision of the red-orange group are R. calendula- hybridization among the native aza- Rhododendron section Pentanthera. ceum (flame azalea) and R. leas are primarily related to habitat Edinburgh Journal of Botany 50, cumberlandense (Cumberland azalea, preference and the time of flowering. pp. 249-364. diploid flame azalea). Both species are For example, R. flammeum is found in variable in flower color although R. well-drained upland sites, often fac- Photographs by the author cumberlandense is usually darker red ing rivers or streams. Usually this is and flowers after the leaves have ex- not the preferred habitat for R. Dr. Kathleen A. Kron, Dept. of , panded. This species also has smaller canescens, which is almost always found Wake Forest University Winston-Salem, flowers and leaves that are usually gla- in poorly drained soils. However, at NC 27109-7325 brous and shining on the upper sur- the northern edge of the range of R. face and glaucous (covered with a canescens it occurs in habitats similar Dr. Kron.'s Ph.D. dissertation was a whitish-bluish wax) on the lower sur- to that of R. flammeum. Under these revision of the Rhododendron sect. face. In addition, plants of R. circumstances hybridization can occur. Pentanthera, She also published a revi- cumberlandense can be distinguished A particularly well-documented case sion of the rest of the deciduous azaleas is that of R. canescens and R. flammeum from R. calendulaceum by the lack of with W. S. Judd. She's currently working gland-tipped hairs on the sepal mar- and their numerous hybrid individu- gins and pedicels. Rhododendron als on Stone Mountain, Georgia. Be- on the evolutionary relationships among cumberlandense is primarily found at cause R. canescens (pink-white flowers, the major groups of Rhododendrons using higher elevations in the Cumberland no blotch) is distinctive from R. DNA sequence data in the Department of Plateau and Mountains. Rhododendron flammeum (red-orange, blotch on Biology at Wake Forest. ❑

74 • THE AZALEAN/December 1996