Genus Rhododendron has its place in the plant kingdom as follows:
Species rhododendron are rhododendron varieties which grow naturally in the wild. Plant collectors have been collecting rhododendron species for over a century from all over the world. They may be propagated from seed or cuttings. Since they all include the word Rhododendron in their name, it is frequently abbreviated as R. The genus Rhododendron includes both rhododendrons and azaleas. Both are members of the genus Rhododendron, and have very similar blossoms. The basic difference is that azalea flowers have five pollen-bearing stamens while rhododendrons have 10 or more. There is some variation in a species at different locations in the wild. The most outstanding natural selections are named. For example, R. carolinianum Album is a white form of R. carolinianum.
Rhododendron are frequently divided into two categories: lepidote and elipidote. The lepidote rhododendrons have minute scales on the leaves and are generally the small-leaved species. Conversely, the elepidote rhododendrons don't have leaf scales and are generally the large-leaved species. Azaleas are among the elepidotes even though they usually have small leaves.
There are about 750 species in the genus rhododendron. The Genus Rhododendron is divided as follows
51 evergreen azaleas
|Hymenanthes||elepidotes rhododendrons & deciduous azaleas||
199 elepidotes & 20 deciduous azaleas
|Rhododendron||lepidotes rhododendrons & vireyas||
154 lepidotes & 319 vireyas
|Choniastrum||lepidotes from SE Asia||
2 lepidote species
|Therorhodion||lepidotes from NE Asia and Alaska||
5 lepidote species
R. alabamense, the Alabama Azalea, is a deciduous azalea found in Alabama and adjacent states. It has snowy white flowers with a prominent yellow blotch. Blooming in midseason, the flowers have a distinct lemon-spice fragrance and measure 0.8 to 1.5 inches across. Originally thought to be a white form of R. periclymenoides (R. nudiflorum) , this plant was first described by Dr. C. Mohr in 1883. It grows naturally in north central Alabama, and western to central Georgia and South Carolina. R. alabamense is low to medium in height, and spreads by underground stems or stolons. It propagates with relative ease from soft wood cuttings and makes a delightful landscape plant. [Return to Top]
R. albiflorum, the Cascade Azalea, is an upright, white-flowering, deciduous rhododendron that is found in western North America from British Columbia and Alberta to Oregon and Colorado near the timberline. It does not do well in cultivation except in Scotland. [Return to Top]
R. arborescens, the Sweet Azalea, is a deciduous azalea found from Pennsylvania south to Georgia and Alabama. It is a good late flowering, scented species that can be used in cultivation. It has white to blush pink flowers with red stamens, and a very strong fragrance similar to heliotrope. It blooms in late spring to early summer and individual flowers measure 1.5 to 2 inches across. First described by John Bartram in 1814, this species has a wide distribution in the eastern United States, but can usually be found growing near streams or moist areas. It is sometimes known as the "Smooth Azalea" because the stems are very smooth and do not have hairs similar to the other azaleas. An excellent landscape plant, R. arborescens can perfume a wide area when in bloom. Relatively easy to propagate, there are a number of excellent forms in the trade. [Return to Top]
R. atlanticum, the Atlantic Azalea or Coastal Azalea, is a low deciduous azalea found on coastal plains from Pennsylvania and Delaware south to Georgia. It is a good late flowering, scented species that can be used in cultivation. It is a common understory plant along the south eastern coastal plains of the United States. The white flowers are 1 to 1.5 inches across, but are often blushed with pink on the outside and some have a yellow blotch. Collected by John Clayton in 1743, this plant was appreciated more in England than in its native land. The plant habit is relatively low but stoloniferous. Spreading by underground stems, R. atlanticum can develop into very large colonies of an acre or more in sandy soils. R. atlanticum is easy to propagate, and makes a nice landscape plant in heavier soils which will restrict the spreading habit. [Return to Top]
R. austrinum, the Florida Azalea, is a deciduous azalea found from northwest Florida to Georgia, Alabama and southeast Mississippi. This plant blooms in early spring as the leaves are beginning to expand. The fragrant blossoms come in shades of orange through gold and yellow, and measure approximately 1 to 1.5 inches across. This species has very long stamens and the tube of the flower is often flushed with red but there is no blotch. Discovered by Dr. A. W. Chapman before 1865, R. austrinum is similar in many respects to R. canescens including the sticky glanular hairs on the flower tube, but differs in the color variations which are orange to yellow rather than pink to white. R. austrinum makes an excellent landscape plant as well as a valuable hybridizing resource, especially in southern gardens where heat tolerance is important. [Return to Top]
R. calendulaceum, the flame azalea, is one of the most brightly colored of native deciduous North American shrubs, bearing large clusters of 2-inch flowers in early summer. Plants usually grow 4 to 9 feet tall in Zones 5-8, hardy to -10° F, and occasionally reach 15 feet. It bears clusters of 2-inch clove-scented bright scarlet, orange or yellow flowers in late spring or early summer flowers that are long lasting, even in full sun. The leaves are 3 inches long and drop in the fall. [Return to Top]
R. camtschaticum formerly called Therorhodion camtschaticum is an deciduous elepidote rhododendron found from Northern Japan to Alaska. This low growing rhododendron has the unusual trait that the one to three flowers appear at the end of young leafy shoots of the current year, and not from special buds. It does well in cultivation in alpine regions of Northern Germany and Eastern Scotland. It is difficult to grow in warmer climates. It was first described in 1784 by Pall. [Return to Top]
R. canadense, the Rhodora Azalea, is a low deciduous azalea found from Eastern Quebec to Newfoundland and Nova Scotia, and south the northern parts of Pennsylvania and New Jersey. It has showy lavender flowers. It has a white form that comes true from seed. It does well in moist acidic soils. Rhododendron canadense is a very unusual native azalea species and was originally considered an entirely separate genus, Rhodora . The top three petals of the flower are fused together almost to the end to form a single lobe, whereas the bottom two are completely separate lips. The purplish pink blossoms are approximately 1.5 inches across and have 10 stamens, twice the number of most east coast natives. First described by Linnaeus in 1762, R. canadense is a low stoloniferous shrub that is the most northern of the east coast native azaleas, the species is very cold hardy but a difficult plant where summers are hot and dry. [Return to Top]
R. canescens, the Florida Pinxter or Piedmont Azalea, is often confused with R. periclymenoides. Both are medium deciduous azaleas that are found in the Carolinas but can be separated by the flower tubes, which in R. periclymenoidesare fuzzy. R. canescens also has tiny hairs, but they are sticky and glandular. Another noticeable difference is that when a flower of R. periclymenoides dies, a ridge on the corolla tube tends to catch on the end of the pistil so that a flower cluster past its prime consists of several dangling blossoms. Although widespread in the eastern half of the U.S., these two wild azaleas differ in distribution. In South Carolina, for example, R. periclymenoides is a Piedmont plant, with almost no specimens reported from the Sandhills or Coastal plain, while R. canescens is predominantly a Low Country plant absent from the Piedmont, except in counties that border the Savannah River. In general, if it grows wild north of South Carolina, it's likely R. periclymenoides; south of the Palmetto State and it's probably R. canescens. Both species prefer moist, humus-laden, acidic soil but seem to do equally well in shade or sun. Old specimens can reach heights of 12-15 feet and have multiple stems or trunks up to 5" in diameter. [Return to Top]
R. colemanii, the Red Hills azalea, from the upper Coastal Plain of Alabama and Western Georgia, was named in 2008. It was initially collected and propagated by S. D. Coleman, Sr. It is one of the tallest, most fragrant and most richly colored of all the native azaleas. Different plants may show flower colors ranging from pure white to deep pink, and even yellow or nearly orange. It's also late-blooming, typically flowering in early May. In the field, the azalea can be distinguished from R. alabamense and other coastal azaleas by its late spring (early to mid May) flowering time, its wide range of flower color (white, pink or yellow), longer flower buds, its often warty seed capsules with glandular hairs, and its taller stature (3-7 m). Its natural distribution extends from southwestern Alabama to the Chattahoochee Valley in Georgia. [Return to Top]
R. cumberlandense, the Cumberland Azalea, is a low deciduous azalea found in Kentucky, Virginia, Tennessee, North Carolina, Georgia and Alabama. It has long been grown under the name, R. bakeri. It is an excellent, low-growing, late blooming orange to red-flowered azalea suitable for small gardens. The flowers are not large, about 1.5 to 1.75 inches across, and typically range from yellowish-orange to deep red. This species is sometimes difficult to distinguish from the larger flowered R. calendulaceum , but the blossoms generally appear several weeks later after the leaves have fully expanded and the undersides of the leaves are usually waxy white or bluish in color. The species distinction for R. cumberlandense was first described by Lemon and McKay in 1937. This native azalea makes an excellent landscape plant in its own right, but it also hybridizes easily with many of the other species, producing beautiful hybrids in a broad range of colors. R. indicum, or the Satsuki azalea, is one of the oldest evergreen Japanese azaleas, bred since ancient times, and it is the type most commonly used for ornamental pruning. It is a slow-growing evergreen shrub that eventually becomes 3 to 6 feet tall and is hardy to Zone 5, to -15° F. Its 1 1/2-inch-long oval leaves grow on finely twigged branches. In the species, the flowers are red tinged with purple, about 2 to 3 inches wide, but it has been bred in many other colors. [Return to Top]
R. eastmanii, the May white azalea, is a deciduous azalea found in South Carolina in 13 counties of the piedmont and coastal plain to date, officially named, described and proposed as a new species in Novon in September of 1999. It has snowy white flowers with a prominent yellow blotch. Blooming in mid May, the flowers have a distinct, strong, fresh fragrance and measure .8 to 1.5 inches across. Originally thought to be R. alabamense, this plant was first described, noticed and thought to be different from known S.C. species by Charles Eastman in the early 1980s, one hundred years after Dr. C. Mohr discovered R. alabamense. R. eastmanii is low to medium in height, and has a clump habit but does not spread by underground stems. It propagates with relative ease from woody cuttings and makes a delightful landscape plant. [Return to Top]
R. flammeum, the Oconee Azalea (synonymous with R. speciosum ), is a deciduous azalea found in the lower Piedmont from central Georgia to South Carolina. It is seldom cultivated in cooler climates but does well in the southeast. Its blossoms are approximately 1.2 to 1.8 inches across and come is shades of yellowish orange, through orange to deep red. This species can be distinguished from the earlier blooming R. austrinum in that the flowers usually have a blotch, they are not fragrant, nor do they have sticky glandular hairs on the corolla tube. Exact date of discovery for R. flammeum is not known, but plants of this species were first described by Aiton at Kew Gardens in 1789 and were probably sent there by William Bartram prior to that date. This species is a heat tolerant shrub and holds much breeding potential where hot summer stress is a problem. [Return to Top]
R. kiusianum, the Kyushu azalea, is a low-growing Japanese species, only 18 inches high. Its leaves are deciduous when the plant is young but evergreen in maturity, remaining on the plant all winter, though often changing color. In its original form the Kyushu azalea is covered in midspring with 8- to 10-inch clusters of lilac pink flowers, but there are many named hybrids derived from this species. It blooms while still young, bearing flowers 1 to 1 1/2 inches across. It is hardy in Zone 7, +5° F. This semi-evergreen spreading shrub grows 3 feet tall, but up to 5 feet wide. The shiny 1-inch leaves sometimes turn red in winter. [Return to Top]
R. prinophyllum, formerly R. roseum, is a medium deciduous azalea found from southwestern Quebec, through New England, to Appalachian Mountains in Oklahoma and Arkansas at the higher elevations. The flowers are typically rose pink measuring 1.2 to 1.8 inches across and are very fragrant. Assumed to be a form of R. periclymenoides (R. nudiflorum) since its first mention in 1787, it was first described as a distinct species in 1914 by Small but had been under cultivation as A. rosea in Europe before 1812. R. prinophyllum can be distinguished from periclymenoides by a number of characteristics including the fact that it is usually deeper pink in color and with a strong cinnamon to clove fragrance. R. prinophyllum is a good landscape plant for northeastern gardens, but may be more difficult in the south because of summer heat. [Return to Top]
R prunifolium, the Plum Leaf Azalea, is a medium deciduous azalea found on the Georgia-Alabama border. The color ranges from orange-red to red, and occasionally orange or yellow. This species blooms very late, usually in late June or in July. First collected by R.M. Harper in 1913, R. prunifolium prefers more shade than most deciduous azaleas to prolong the flowers during hot summer months. [Return to Top]
R. schlippenbachii, the royal azalea, is a deciduous Korean species. It has soft green leaves that grow in whorls around the stem and turn yellow, orange and crimson in the fall. It is so lovely that man can hardly hope to improve upon its soft pink flowers. They are 3 inches across, and freckled on the upper petals and have a delicate fragrance. Schlippenbachii tolerates less acid soils in Zones 5-8 than the other species. It is hardy to -10° F and grows to 6 to 10 feet tall. [Return to Top]
R. stenopetalum 'Linearifolium' (spider azalea) is an unusual species. This species is normally 4-foot-tall shrub with narrow, ribbon-like evergreen leaves, 3 inches long and 1/2 inch wide, and rose-pink flowers with deeply divided petals. The variety 'Linearifolium' (formerly R. macrosepalum 'Linearifolium' or R. linearifolium) is somewhat smaller, 3 feet tall, and the leaves are deciduous and 3 inches long. But the flowers are even more curiously shaped than those of the species; they have 1 1/4-inch-long sepals that overshadow the petals. The flowers of this variety are fragrant and have been bred in several colors; in one, 'Polypetalum', flower petals and leaves are so similar in size that the plant seems to have pink and green leaves. [Return to Top]
R. vaseyi, the Pink Shell Azalea, is a medium to tall deciduous azalea found in North and South Carolina. It does very well in cultivation, especially in moist soils. It has showy dark pink flowers. It also has a white variation. It has showy fall foliage when the willow-like leaves turn yellow and red. Discovered by George Vasey in 1878, this native azalea has a relatively restricted natural habitat in four mountainous counties of North Carolina. Growing at elevations of 3000 to 5500 feet, the rare Pinkshell azalea can be seen in bloom along the Blue Ridge Parkway in early spring. [Return to Top]
R. viscosum, Swamp Honeysuckle, is a medium deciduous azalea found from Maine to Georgia and west to Texas. Rhododendron viscosum now includes two closely related forms that were previously considered separate species, R. serrulatum and R. oblongifolium . The fragrant flowers are generally white to pale pink, and bloom after the leaves have fully expanded. Flowers vary in size from 0.75 to 1.5 inches across depending upon the form, but have a long narrow tube covered with sticky glandular hairs. The species has a wide distribution from Maine to Florida, and westward to Texas. R. viscosum was the first North American azalea grown in England. Bishop Henry Compton raised the plant in 1680 from seed collected by John Bannister, an English Missionary. The species was an important parent in early hybridizing efforts with deciduous azaleas. It can be distinguished from the other late blooming white, R. arborescens in that the stamens are greenish white rather than red, and the stems are not smooth but contain hairs. [Return to Top]
R. catawbiense, the Catawba rhododendron, is hardy to Zone 5, or -15° F. This species grows 6 to 12 feet tall in gardens in 5 to 10 years and has 5- to 6-inch clusters of lilac-purple flowers in the spring. This native of North America is an excellent landscape plant. Its dark green leaves are 3 to 5 inches long. Forms in cultivation range from the white R. 'Catawbiense Album' to the violet R. 'Catawbiense Boursault'. The variety R. 'Catawbiense Album' is one of the best white rhododendrons for cold climates.
R. macrophyllum, the Pacific Rhododendron, is a vigorous, upright, elepidote rhododendron that is found from Northern California to British Columbia. Discovered in 1792 by Archibald Menzies, it has pink to rose colored flowers. It is seldom found in cultivation. It is related to R. ponticum and R. catawbiense [Return to Top]
R. maximum, the Rosebay rhododendron, is hardy to Zone 4, or -25° F. Because it becomes 10 to 20 feet tall after 15 to 25 years and because its rosy lavender flowers are partly obscured by new growth in late spring, this native evergreen American species is most useful as a background plant. It long slightly narrow leaves have a thin indumentum on the underside. It is a large hardy plant with small trusses. It has slender dark green leaves 5 to 10 inches long and thrives only in the shade.
R. metternichii, a Japanese leather-leaf rhododendron, grows 8 feet tall and has oval evergreen leathery leaves, 6 inches long, glossy on top, downy and rusty brown on the undersides. The leatherleaf rhododendron is less cold resistant and grows best in Zones 6-10, to -5° F. The flowers are rose pink. [Return to Top]
R. williamsianum is a Chinese evergreen species rhododendron that grows about 4 feet high. This is a tender plant, hardy to Zone 7, +5° F. In northern climates this plant can be container grown, and protected in the winter. This dwarf plant forms attractive mounds of bright green foliage and is haloed in midspring with pale pink bell-shaped flowers. [Return to Top]
R. groenlandicum, Labrador Tea, is an upright lepidote rhododendron that is found from the northern USA, through Canada and into Greenland. It blooms heavily with white trusses. It was introduced in 1763.
R. keiskei, the evergreen Japanese Keisk rhododendron, grows no higher than 2 1/2 feet tall and has yellow flowers 2 inches across. Keiskei is hardy in Zone 5, to -15° F. Its oval, pointed evergreen leaves, about 2 inches long, are olive green on top, rusty brown on the undersides; in autumn, they turn bronze red. [Return to Top]
R. lapponicum, the Lapland Rosebay, is a very low lepidote rhododendron found from Maine to Wisconsin and north from Quebec to Newfoundland. It has clusters of pink to lavender flowers. It is found on mountain tops and in sub arctic areas. It was first described by Wahlenberg in 1812.
R. minus (formerly called R. carolinianum), the evergreen Carolina rhododendron, is a native American plant that grows wild in the Blue Ridge Mountains of North Carolina. It is 3 to 6 feet high with a naturally rounded shape and dark green leaves 3 inches long. In midspring it is covered with 3-inch clusters of rose-pink flowers. The Carolina rhododendron is hardy to Zone 5, -15° F. This is one of the most satisfactory species for northern gardens, becoming a thickly branched, rounded shrub that usually grows 3 to 4 feet tall in 10 years. In midspring it is smothered with 3-inch clusters of small rosy pink flowers. The 1 1/2- to 3-inch dark green leaves are brown underneath. There is also a variety with pure white flowers and lighter green leaves, R. carolinianum 'Album'. [Return to Top]
R. neoglandulosum, Trapper's Tea or Glandular Labrador Tea, is an upright lepidote rhododendron that is found in the northwestern USA. It blooms heavily with white trusses. It was first introduced in 1894. It was long grown as Ledum glandulosum.
R. subarcticum, formerly Ledum subarcticum, is an upright lepidote rhododendron that is found in the arctic regions of North America, Europe and Asia. It blooms heavily with white trusses. It does well in alpine gardens. [Return to Top]
Rhododendrons grow best in partial shade, since full sunlight tends to bleach the flowers. They need an acid soil with a pH of 5.0 to 6.0, well mulched with organic material. Mix garden loam with equal parts of coarse sand and ground bark or oak leaves before planting. Soil around the rhododendrons' shallow roots must be kept cool and moist but well drained. All except leatherleaf rhododendrons transplant well in the spring, or in the fall if mild winter weather does not damage the shallow-rooted plants. Do not fertilize at the time of planting, as this might injure the roots, but water deeply. Supplemental feeding later is not normally needed, but if a plant appears pale or droopy, apply cottonseed meal or a fertilizer for acid-loving plants around its base in early spring. A year-round mulch of rotted oak leaves will also provide natural nutrients and will help keep the soil cool and moist.
Prune rhododendrons after the flowers have faded to induce new growth. Most evergreen rhododendrons may be propagated from stem cuttings of new growth taken in late summer and rooted in a mixture of perlite or vermiculite and peat moss. Deciduous rhododendrons are propagated by seed, grafting or cutting. For deciduous azaleas, cuttings may need bottom warmth of 75° and artificial light to promote root growth.
Do not cultivate around the shallow roots of rhododendrons and azaleas, but pinch off their faded flowers to improve bloom the following year. Also, prune out dead, diseased or damaged branches, and cut old branches back to the soil level to encourage new growth. [Return to Top]
Rhododendrons belong to the Ericaceae or heath family. Other members of the Ericaceae include the heaths (Erica), and heathers (Calluna), mountain laurel (Kalmia), lily-of-the-valley shrub (Pieris), cranberry (Vaccinium), Leucothe and Andromeda. Almost all of the heath family make good garden plants.
When Linnaeus created the botanical grouping called genus Rhododendron in 1753, he created a separate genus for Azalea containing 6 species. In 1796 Salisbury pointed out that Azalea and Rhododendrons could not be maintained as distinct genera. In 1834, George Don subdivided the genus Rhododendron into 8 sections which were recognized until 2004. Azalea comprised two of these sections, Subgenus Pentanthera typified by Rhododendron nudiflorum and Subgenus Tsutsusi typified by Rhododendron tsutsusi.
The extensive collections of Rhododendron in the early 1900's from southern Asia made by Forrest, Rock, Kingdom-Ward, and others stimulated taxonomic work at Edinburgh Botanic Garden and Herbarium under the direction of Bayley Balfour. The horticulturally based classification consists of about 45 series. The artificial classification was intended as a temporary measure, but it became firmly entrenched following the publication of The Species of Rhododendron in 1930 by the Royal Horticultural Society. This system was designed to cope with the vast quantities of new plant material received from the Orient, North America, etc. Balfour intended to revise the temporary system but died before it could be accomplished.
The Series system does not attempt to classify species under their subgenus and section as proposed in the International Rules of Botanical Nomenclature. Further, it is based largely on cultivated plants rather than Rhododendron found in the wild. [Return to Top]
The Eight Subgenera prior to 2004
With many species discovered over several centuries the genus Rhododendron has been subdivided into groups and revised several times. Up until 2004 the genus was divided into eight subgenera under the taxonomic system of Chamberlain et al. (1996):
The first four of these subgenera contain all species the entire genus except for five. Of the last four Candidastrum, Mumeazalea and Therorhodion each include only one species (R. albiflorum, R. semibarbatum and R. camtshaticum respectively) while Azaleastrum includes two species (R. ovatum and R. stamineum).
Subgenera Rhododendron and Hymenanthes include the plants that gardeners recognize as the "true" rhododendrons. Some of these, the lepidote rhododendrons, have small scales on their leaves and make up the subgenus Rhododendron. Elepidote rhododendrons, those without leaf scales, form the subgenus Hymenanthes.
Subgenus Pentanthera covered the deciduous azaleas and Tsutsutsi covered the evergreen azaleas.
A list of all species and their classification is provided in the Genus Rhododendron Taxonomic Tree. [Return to Top]
The Five Subgenera proposed in 2004
Based upon genetic and chemical data, in 2004, Loretta Goetsch, Andrew Eckert and Benjamin Hall of the University of Washington proposed and in 2005 published a recommendation that the Genus Rhododendron be revised and subdivided into five subgenera:
Some of the subgenera are divided into sections, which are further divided into subsections. There are also groupings known as alliances and aggregates composed of very closely related species. [Return to Top]
For taxa outside of subgenus Rhododendron, this system eliminates three subgenera and two sections that are present in the 1996 taxonomic system of Chamberlain et al.
Section Pentanthera is included within subgenus Hymenanthes.
Sections Sciadorhodion and Viscidula and R. vaseyi (section Rhodora) from the discontinued subgenus Pentanthera are combined with sections Azaleastrum, Tsutsusi and Brachycalyx to form an expanded and revised subgenus Azaleastrum. Sister groups in this subgenus are the sections Tsutsusi (largely evergreen) and Sciadorhodion (entirely deciduous)
Since Choniastrum lacks lepidote scales on the leaves, Goetsch et al. propose that Choniastrum be considered a separate subgenus. [Return to Top]
A list of all species and their proposed classification is provided in the Proposed Genus Rhododendron Taxonomic Tree. This chart assumes subsections and alliances are carried over from the old Taxonomic Tree. This is probably not a safe assumption but is a pragmatic one since I don't have the faintest idea what is being considered.
If you understand the basic ideas of genus, subgenus, section and species you will have a better understanding of rhododendron naming conventions. A rhododendron species can be uniquely identified with the genus and species. Hence Rhododendron albiflorum defines a unique species. There may be another genus that uses the name albiflorum since it is Latin for "white flowering". Return to Top
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