Decoding our Native Azaleas

Decoding our Native Azaleas

Rhododendron viscosum. Photo courtesy of Dr. Charles Horn

Compleat Naturalist

By Hal and Laura Mahan

Botanist Dr. Charles Horn, who researches native azaleas of the Southeast, says, “Now I have more questions than I did a decade ago!” This Newberry College, South Carolina, biology professor has been engaged in a project which has an objective of refining the classification of our 20 species of native azaleas, which all fall under the scientific genus name of Rhododendron. The shrubs that we commonly call rhododendrons have thick, evergreen leaves and ten stamens. The rhododendrons that we commonly call azaleas have deciduous leaves and five to seven stamens. Confused? So are the botanists!

It is somewhat amazing to think that one of the most popular groups of horticultural plants is poorly understood, as far as its classification. Azaleas were first described in the 1750s by Carolus Linnaeus, the Swedish botanist whose system of classification is still in use today. Then the French botanist André Michaux, who famously traveled through our mountains in the late 1700s, named several other species including the beautiful flame azalea (Rhododendron calendulaceum). More recently (early 1900s), Alfred Rehder described all the detailed variations in the southeastern species, possibly being overly aggressive with distinctions.

Most rhododendron and azalea shrubs that are commonly planted in our landscapes are cultivated varieties of wild species that might have originated in either Asia or North America. Here in the Southeast, there are approximately 24 native species of shrubs that fall in the genus rhododendron, although some are further divided into subspecies.

Horn’s research started out over a decade ago with a phone call from the South Carolina Department of Natural Resources in March 2000, asking about an azalea that had been recently described as new to science, the May-white azalea (Rhododendron eastmanii). Further field work led to a total to date of 60 populations of this fragrant shrub that blooms in May.

Meanwhile, in Alabama, a new species known as red hills azalea was discovered by other botanists in 2006 (Rhododendron colemanii). Horn’s project quickly expanded. His project now seeks to understand how many species of native azaleas occur in the southeastern US, and how to clearly distinguish each species. Horn’s field work will be augmented by studies in the lab to determine each species’ DNA sequence (their genes). Then he will discover whether the plants’ outward appearances correspond with differences seen in the DNA. The molecular work is being conducted by Dr. Emily Gillespie of Marshall University. Currently their project is concentrating on the deciduous species (those that lose their leaves in the fall), but eventually the evergreen species will be studied as well.

If you are interested in learning more about azaleas, The North Carolina Arboretum is home to the National Native Azalea Repository. It is a woodland garden with azaleas representing nearly every species native to the US, along with many natural hybrids and selections. In late spring and early summer the garden is a diverse and eye-catching composition of color, form and fragrance set among native ferns, wildflowers, shrubs and trees. To reach it, follow the Arboretum trail maps to Bent Creek Road.

The specimen in our photograph is Rhododendron viscosum, an interesting plant that you might see if you hike the Mount Pisgah trail in late June. Its common name is swamp azalea. Mount Pisgah is not a swamp, you say? And it’s found abundantly in the coastal plain? Hmmm, could our mountain specimens be a different species? Stay tuned as scientists gather data, answer questions and come up with even more questions!

Laura and Hal Mahan are owners of The Compleat Naturalist, located at 2 Brook Street in the Historic Biltmore Village. To learn more, visit or call 828.274.5430.

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