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.
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:
- lack scales on the undersurface of leaves or other plant parts,
- most are deciduous (most, but not all, rhododendrons are evergreen, exceptions
include R. dauricum and R. mucronuclatum),
- have 5 lobes per flower,
- most have 1 stamen per lobe, that is 5 stamens (most rhododendrons have 2
stamens per lobe, thus 10 or more stamens per flower),
- tend to have long straight hairs parallel to the leaf surface (appressed), usually along the midrib on the underside of the leaf (tend to be thinner, softer and more pointed than rhododendron leaves), and
- their flowers are usually funnel-shaped as opposed to bell-shaped.
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:
- Ghent Hybrids: resulting from crosses between several American azalea species and R. luteum. They have an upright habit, 6-10 ft (1.8-3 m) tall, 6-8 ft (1.8-2.4 m) wide, brilliant fall color, bloom mid to late season (i.e. May). Flowers are 4-6 cm wide, funnel shaped with a long tube, many are fragrant, colors range from off-white to pale-yellow to purplish red, and may be single or double. They prefer a cool climate and have good hardiness (-15o to -25oF). More or less trouble-free. Some examples:
- 'Baltic Amber' - yellow flowers, the leaves have silvery markings, hardy to USDA Zone 5.
- 'Corneille' - soft double pink flowers, USDA Zone 5?.
- 'Daviesi' - white flower with yellow centers, USDA Zone 5.
- 'Magic' - yellow flowers change to orange as they mature, USDA Zone 4.
- 'Nancy Waterer' - large, golden-yellow flowers, USDA Zone 5.
- 'Narcissiflora' - double light yellow flowers with pointed petals, USDA Zone 5.
- Mollis Hybrids: derived mainlty from R. japonicum (syn. R. mollis var. japonicum), some may not by hybrids. They are upright, to 8 ft (2.4 m), 6 ft (1.8 m) wide, bloom in mid to late season. Flowers are 5-6.5 cm wide (generally larger than Ghents), in clusters of 7-13, all singles, shorter tubes and more striking colors (yellow, orange, red, salmon-pink, apricot-orange) than Ghents. Because some Mollis azaleas are difficult to propagate from cuttings, some are seedling strains and hence not genetically identical, however they are sometimes sold under cultivar names. Currently this group does not appear to be well represented in North American nursery listings. A few examples of Mollis azaleas:
- 'Anthony Koster' - bright yellow flowers with a vivid orange blotch.
- 'Chirstopher Wren' - bright yellow flowers flushed with red, buds orange red.
- Knapp Hill-Exbury Hybrids: initially (1870) these hybrids were the result of crosses of Ghent hybrids with the Chinese azalea and other species by the Waterers of Knapp Hill Nursery, England. Later some of the seedlings were acquired by other hyrbridizers, e.g., Lionel de Rothschild of Exbury, Slock Nursery, Windsor Nursery, and still later Edgar Stead of Ilam New Zealand and others, and these were used in their own azalea development programs. The hybrids of this group are generally medium to large, 4-10 ft (1.2-3 m) high by 4-6 ft (1.2-1.8 m) wide, but with some dwarfs, and a spreading habit. Young foliage often has a reddish-orange tint and many have excellent fall color. Flowers are wide, 5-8 cm, tubular, in large clusters (trusses), from rounded to spreading, 18-30 flowers, usually fragrant, some doubles. Colors range from near white, orange, pink to vivid red. They bloom in mid to late season. Sometimes azaleas in this group are lumped and designated as Knapp-Exbury Hybrids, whereas others may classify individual selections as a Knapp Hill, Exbury, or Ilam azalea. A few selections:
- 'Cecile' (Exbury) - red flowers with an orange-yellow blotch.
- 'Frills' (Knapp Hill) - orange, semidouble, frilly flowers fade to a yellow-orange in the center.
- 'King Red' (Exbury) - vivid red, slightly ruffled, large ball cluster, compact plant.
- 'Yellow Giant' (Ilam) - flowers brillant yellow, darker flare, about 8 cm across.
- Occidentale Hybrids: developed from crosses of our native R. occidentale [Western Azalea] with Mollis Hybrids. Plants grow up to 8 ft tall and tend to bloom late, late May and early June. Flowers are similar to those of Mollis Hybrids but more "delicately colored" and usually highly fragrant. Flowers are 6-10 cm wide and colors range from white flushed rose and blotched yellow to red with orange blotch. A few examples:
- 'Candystsripe' - bright pink, fragrant flowers with an orange blotch.
- 'Centennial' ('Washtington State Centennial') - large, fragrant frilly flowers in light orange-yellow, marked with large yellow flare, flowers fade to white.
- 'Jock Brydon' - pinkish buds open to white flowers with a orange blotch on the upper petal.
- Northern Lights Hybrids: developed by the University of Minnesota Landscape Arboretum, first becoming commercially available in 1978. They are hybridized from Mollis hybrids, R. prinophyllum and other hybrids. They are very hardy azaleas, all are hardy to USDA Zone 4 and some to Zone 3b. They grow to 6-8 ft (1.8-2.4 m) tall with a similar spread and bloom heavily with fragrant sterile flowers in spherical clusters; bloom season is late (May-June).
- Eastern American Species and Hybrids: interest in azaleas native to eastern North America began in the 1950s and large collections have been developed at several locations, including Biltmore Estates, Callaway Gardens, Morris Arboretum and the USDA National Arboretum. Wild specimens and seedlings are being named as well as hybrids of these clones (Gallee, 1985).
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.