RHODY

These pages are the result of information I have picked up over 35 years at American Rhododendron Society meetings & conventions, in courses at Longwood Gardens, and in the numerous books which I have made available in the Rhododendron and Azalea Bookstore. The goal of this site is to share this information with those who share a love for the Genus Rhododendron. All proceeds from the Rhododendron and Azalea Bookstore are contributed to the ARS Research Fund and the Rhododendron Species Foundation.

Genus Rhododendron

RHODYRHODYRhododendrons are shrubs of the genus Rhododendron which is in the heath family (family Ericaceae) which includes heathers, Kalmia (mountain laurels), blueberries and cranberries. Since almost all of these plants require acidic soil, they are often referred to as the family of acid-loving plants. Rhododendrons are found in the wild, chiefly in mountainous areas of the arctic and north temperate zones. They typically have large, shiny, leathery evergreen leaves and clusters of large pink, white, or purplish flowers. North American species include the great laurel, or rose bay (R. maximum), West Virginia's state flower; and the Western rhododendron (R. macrophyllum), Washington's state flower. Azaleas are in the same genus. Most cultivated rhododendrons are hybrids that are propagated from cuttings or tissue culture. Rhododendrons are referred to as the King of Shrubs since they are regarded by many as the best flowering evergreen plants for the temperate landscape.

Rhododendrons are shrubs for all seasons. In winter some stand out with large evergreen leaves. In spring the flowers are showy; throughout the summer and fall the leaves add a pleasing, deep green color to the garden. Some deciduous azaleas add bright fall color before the leaves drop. The spectacular spring flowers of azaleas and rhododendrons make them among the most popular garden shrubs.

Also see details and pictures of the 27 species of Rhododendrons and Azaleas that are native to North America.

History of the Genus Rhododendron

Rhododendrons belong to the family Ericaceae. There are very few rhododendron fossils. The ericaceous plants (recognizable by their fossilized pollen grains that are arranged in groups of four) appeared first in the Maastrichtian Stage about 68 million years ago. The Maastrichtian is the last stage of the Cretaceous Period and immediately precedes the extinction of dinosaurs. The earliest rhododendron fossils are leaf imprints in rocks of early Tertiary age (about 50 million years old) from Alaska. Other fossils are known from the Tertiary and Pleistocene deposits of North America. These records indicate that rhododendrons have been in existence for at least 50 million years. They could have been in existence before that time, but not before 68 million years ago when the Ericaceae first appeared. This is of interest because it indicates that rhododendrons probably did not originate in the region of mountainous regions of SE Asia or in the high-island archipelago stretching between mainland Asia and Australia - the islands of Java, Sumatra, Borneo, New Guinea, and the Philippines (where they are now most abundant and most diverse) because these regions did not exist 50 million years ago.

E. Irving and R. Hebda of Victoria, Canada, have studied fossils and the genetics of rhododendrons and inferred the possible way they became distributed the way they are today. The essence of their thesis is that, following a period of mild climate when rhododendrons were widely and more-or-less continuously distributed across North America and Eurasia, their range became much reduced as a result of global climatic deterioration. This deterioration began about 25 million years ago, was clearly marked by about 15 million years ago, and became extreme with the onset of the current glacial period about 3 million years ago. Small populations of the subgenera Hymenanthes and Rhododendron (but not azaleas) situated on the edges of their main range were able to enter and retain a foothold in the newly developing region of extreme relief on the southeastern fringe of the Tibet-Himalayan region. There they proliferated, and it was from there that the vireyas spread into the mountains of the high-island archipelago stretching between mainland Asia and Australia. By taking advantage of special newly developing conditions, these small, originally populations have become now the most numerous and diverse. The place of origin of rhododendrons is not known, but it was not the region of extreme relief on the southeastern fringe of the Tibet-Himalayan region where they are nowadays most diverse and abundant.

The azaleas are not in the mountains of SE Asia although they live close by in terrain of about one half or one third the altitude. Presumably, because of their genetic make-up and historical circumstances, they were unable to take advantage of the dynamic environment of that region. In this respect, the paleogeographical evolution of azaleas appears therefore to have been very different from that of other rhododendrons.

Also see:

History of Discovery and Cultivation of Rhododendrons and Azaleas

History of the First 50 Years of Hybridization of Rhododendrons and Azaleas

RHODYAzaleas, Where Fit In

Azaleas are shrubs of the genus Rhododendron and members of the heath family. There are 8 divisions of the genus Rhododendron. Azaleas comprise two of those divisions. Technically classified as rhododendrons, azaleas are generally more floriferous but have less impressive foliage. Typically non-azalea rhododendrons are evergreen and have flowers that are in trusses (a truss is a flower-like structure composed of many flowers). Typically an azalea is either evergreen with flowers singly rather than in a truss or are deciduous and may have flowers singly or in a ball-shaped truss.

When Linnaeus created the botanical grouping called genus Rhododendron in 1753, he created a separate genus for Azaleas containing 6 species. In 1796 Salisbury pointed out that Azaleas and Rhododendron could not be maintained as distinct genera. In 1834, George Don subdivided the genus Rhododendron into 8 sections. Azaleas comprise two of these sections, Subgenus Pentanthera typified by deciduous Rhododendron nudiflorum and Subgenus Tsutsusi typified by evergreen Rhododendron tsutsusi.

If flowers grow from terminal buds, new leaves and shoots grow from lateral buds and leaves are deciduous, then the rhododendron is an azalea in the Pentanthera subgenus.

If flowers and leaves grow from the same terminal buds, and the flowers have 5 to 10 stamens, then the rhododendron is an azalea in the Tsutsusi subgenus.

In 2004, these 8 subgenera were rearranged into 5 subgenera by Goetsch, et. al. Today, Azaleas are grouped into the Pentanthera Subgenus and Azaleastrum Subgenus.

Most grow in damp acid soils of hills and mountains, and are native to North America and Asia. Native American azaleas include the flame azalea (R. calendulaceum) and the fragrant white azalea (R. viscosum), also called swamp honeysuckle. Most of the brilliantly flowered garden varieties are from China and Japan. Some have deciduous leaves and are usually very hardy, while other are evergreen and frequently less hardy. The deciduous varieties are usually hard to propagate, but much progress had been made in this area. Evergreen varieties are usually easy to propagate. Many hybrid and species azaleas are in the commercial trade. They typically bloom early in the season and are popular for the color they add to the landscape.

One term that is used in describing many azaleas is hose-in-hose. This term is meant to describe what looks like a flower inside a flower. This actually is a flower with a large calyx. The sepals of the calyx are shaped like the petals of the corolla. It appears as two similar corollas, one inside the other and rotated so that all the petals are visible.

Another term that is more common with azaleas is double. A double flower looks like the interior is filled with petals. This is because the stamens grow into petal-like structures. The pistol may also be transformed into a petal-like structure or may be absent.

Another term is semi-double. In this case the stamens are partially transformed into petal-like structures. Occasionally extra petals are present and all stamens are present also.

Another version is hose-in-hose double. A perfect example of this is Gable's Rosebud azalea. The name is descriptive of the flowers appearance.

Also see details and photos of the 16 species of Azaleas that are native to North America.

Return to Top

Help Save The Azaleas at the U.S. National Arboretum

RHODY
RHODY
All Gardening Sites
Return to Top

Visit Henning's Home Page

This page was created in Adobe GoLive, Adobe DreamWeaver, and BBEdit by . Valid HTML 4.01 Transitional

Rhododendron and Azalea Web Pages

If you have a link to this site and I do not have a link to your site, please email so I can link to you.
Return to Top

I want to extend a big Thank You to the following sites for their links to this site and I recommend others visit the sites of these enthusiastic Rhododendron and Azalea gardeners:

Return to Top

The following links also have valuable information for Rhododendron and Azalea gardeners.

Return to Top

Also a big Thank You to the following sites which gave nice reviews of this site and they may be helpful in finding links to other sites:

Return to Top

References: Books & Websites Used In Preparing This Site
(Many of these books are available at The Rhododendron and Azalea Bookstore)

Return to Top

Help Save The Azaleas at the U.S. National Arboretum

RHODY
RHODY
All Gardening Sites
Return to Top

Visit Henning's Home Page

This page was created in Adobe GoLive, Adobe DreamWeaver, and BBEdit by . Valid HTML 4.01 Transitional