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Rhododendron                     Ericaceae

Rhododendron Species

There are over 800 species in the genus Rhododendron, some say 1000!  Azaleas are are included in the genus Rhododendron.   Rhododendrons are native to many parts of the world, both tropical and temperate.  The more hardy types are found in China, Japan, and eastern and western North America.  In the US, the culture of rhododendrons is best in eastern areas and the Pacific Northwest.  This geographically diverse and complex genus is divided into eight subgenera, as shown below (in alphabetical order):

These subgenera are in turn subdivided into sections and even into subsections.  This large taxonomic tree can be viewed and examined in the web site of the American Rhododendron Society.

      Two of the subgenera are especially large and important, Hymenanthes and Rhododendron (Huxely, 1992).  Plants in the subgenus Rhododendron have small scales ('lepidote scales') on their lower leaf sufaces, flower stalks and calyx as well as other surfaces.  These small scales, which can be seen with the aid of a hand lens, are more or less mushroom shaped but vary in size, structure, color, and density.  Plants in this subgenera are commonly known as the Lepidote Rhododendrons.  Another identifying characteristic of this group is that the young leaves in buds are flat or curved.  The growth habit of Lepidote Rhododendrons varies from almost prostrate spreaders to tall shrubs.  Leaf size ranges from very small to medium sized.  Flower color covers the specturm except for true red.  This group contains some of the most winter hardy evergreen forms (e.g., R. dauricum) as well as the tender Vireya types.
      Plants in the subgenus Hymenanthes completely lack scales and are known as Elepidote Rhododendrons; furthermore the young leaves in buds have their magings rolled under (revolute).  The growth habit of species in this group range from low, mound forming shrubs to trees.  They have evergreen leaves that are medium-sized to very large, and produce flowers in various shapes, sized and colors.

Hybrid Rhododendrons

      Because many Rhododendron species form spectacular flowers and hybridize easily, many persons, often gardener-hobbists, have and are trying to "improve" rhododendrons.  Because of this activity many thousands of selections exist, most of which are of hybrid origin.  These hybrid plants, often with complex parentage, cannot be classified as to a species or possibly even as subgenera.  They are designated only as to genus, i.e., Rhododendron.  For example, the well known rhododendron Pink Pearl, is designated only as Rhododendron ‘Pink Pearl’.  There are over 10,000 hybrid rhododendrons in the International Register, of which about 2,000 are generally available.  Harold Greer, in his book, "Greer's Guidebook to Available Rhododendrons: Species & Hybrids", briefly describes about 530 Rhododendron species and 780 hybrids (cultivars).


      Azaleas are now classified in the genus Rhododendron, but essentially all are assinged to two major groups (subgenus), the Pentanthera and Tsutsutsi.  The former includes Deciduous Azaleas and the latter the Evergreen (mostly) Azaleas, each subgenus is further divided into several taxonomic groups (sections, subsections), each of which includes one or more species.  Azaleas have been hybridized for hundreds of years and according to the Azalea Society of America, over 10,000 different cultivars, or cultivated varieties, have been registered or named, although far fewer are being propagated and sold.

            Is it an azalea or a rhododendron?   There is no simple, unambiguous means to distinguish azaleas from other rhododendrons, but generally, azaleas:

Deciduous Azaleas

       Deciduous azaleas are in the Pentanthera subgenus of Rhododendron.  Most used in landscaping are hybrids of two, but usually more, Rhododendron species.  Many of the species used in hybridization are native to eastern North American with additional species from Japan or Europe.  Successful hybridization of deciduous azaleas was initiated in the 1820's by a Belgian baker living in Ghent.  The plants from this series are known as the Ghent Azaleas.  Many other series (types) of hybrid deciduous azaleas were developed in the 1800's and new ones continue to be released.  Below are some of the more or less distinct groups of deciduous azaleas: Evergreen Azaleas

            Hybrids in this group are sometimes called Japanese Azaleas, which is appropriate since the oldest ones originated in the gardens of Japan several hundred years ago and the parental species are mainly natives of Japan.  All of the evergreen azaleas are native to Asia.  Evergreen azaleas have dimorphic leaves, known as spring and summer leaves.  The spring leaves unfold at the beginning of the growing season and are dropped in autumn.  Summer leaves emerge in early summer and are smaller, thicker, darker, and more leathery than spring leaves.  They remain on the plant during the dormant period and drop in the spring, however, summer leaves may persist for several years in warm climates.  Some "evergreen" azaleas are deciduous in colder climates.
            Evergreen azaleas began to be established in Europe in the early 1800's.  The first ones were not hardy and were used for greenhouse and indoor decoration, and are know as Indian or Belgian Indian Hybrids. The name "indian" was derived from a misidentification of Rhododendron indicum as being associated with this group.  A mixed group developed in the southern US beginning in the middle of the 1800's, in part from the Belgian Indian types, is known as Southern Indian Hybrids.  A large group of evergreen azaleas, known as Kurume Hybrids, has its origin near the Japanese city of Kurume.  They became widely known in the West through the work and writing of E.H. Wilson, who selected and imported 50 cultivars from Japan, arriving at Arnold Arboretum in Boston 1919.  A third major group is the Kaempheri Hybrids, derived in part from Rhododendron kaempferi and developed in Holland after World War I.  Another group, Satsuki Azaleas, are highly prized in Japan and have been used by hybridizers in the West.  The Inter-Group Evergreen Azalea Hybrids include relatively new groups of hybrid azaleas that have been derived from plants in several groups, such as Satsuki, Kaempheri, and Kurume.  The popular Glenn Dale hybrids are assigned to this group, they were developed, starting in 1935, by B.Y. Morrison in Glenn Dale, Maryland.

            Reportedly all parts of Rhododendrons are poisonous, including the nectar, causing mouth burning, followed later by many symptoms, including coma and convulsions.

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