Rhododendron? Azalea? Why and Where Have All
the Azaleas Gone?
Henry R. (Hank) Helm
Bainbridge Island, Washington
What is an azalea? Are azaleas rhododendrons? Are rhododendrons azaleas? Are there both deciduous azaleas and evergreen azaleas? How do you tell if a plant is an azalea or a rhododendron? Are there blue azaleas? Are there yellow evergreen azaleas? A brief, and admittedly incomplete, review of the history of rhododendron classification and subsequent placement of plants within the genus Rhododendron by taxonomists will perhaps answer these questions as well as clear up some of the confusion and misunderstandings that seem to exist about the name "azalea."
The Swedish Botanist Linnaeus devised a system of plant classification in the 18th century. At that time he knew of eleven plants that are now (all but one) classified in the genus Rhododendron. He determined that these plants should be in two genera. One genus was Rhododendron where he placed the species Rhododendron ferrugineum, R. hirsutum, R. dauricum and R. maximum, which all had ten stamens. The genus Azalea was where he placed Azalea indica, A. pontica, A viscosa, A. lutea, A. lapponica and A. procumbens as they all had five stamens. Of these six latter plants placed in genus Azalea, the first is an evergreen, the next three are deciduous, the fifth is a scaly rhododendron that just happens to have only five stamens and the last is a shrub that is now not included in genus Rhododendron at all but is in a different genus and is classified as Loiseleuria procumbens.
Very little work was done on rhododendron classification from 1753 when Linnaeus's Species Plantarum was published until 1834 when George Don published his General History. Don's work united rhododendrons and azaleas into one genus (Rhododendron) and he did not use the term "azalea" (which he reserved for Loiseleuria). Maximovicz's work published in 1870 reinstated the genus Azalea. Between 1870 and 1921, several taxonomists made revisions to rhododendron classification and attempted to deal with the increasing numbers of rhododendrons being introduced from Asia by plant hunters. Most of these classifications/revisions accepted hierarchical ranks of subgenus or section. Among these publications was the Monograph of Azalea published in 1921 by A. Render and E. H. Wilson.
In 1930, The Species of Rhododendron was published which utilized the work of Sir Issac Balfour, Regis Keeper of the Royal Botanical Garden at Kew, Edinburgh, Scotland (14). A second edition was published in 1947. This Balfourian classification, as it became known, dealt with the massive quantities of material that had been sent to Edinburgh. Seed collected and sent to Scotland was distributed to enthusiasts, nurseries, and estates who grew large numbers of rhododendrons. Balfour created an artificial classification system utilizing "series" to group plants as an aid for horticulturists and which was based on cultivated plant material almost exclusively. The system created and used in this classification of the known species in cultivation eventually utilized forty-five series (all of which were given equal rank) to divide up the genus Rhododendron. One of these series was Azalea. This series contained both evergreen and deciduous plants as well as plants that did not fit into other series. This system of classification was used for nearly fifty years, during which time the Royal Horticultural Society published the classification in the Rhododendron Handbook. This was "the reference" for rhododendron enthusiasts. The Balfourian system of classification became firmly entrenched in the horticultural world. "Azalea" became a term widely used and accepted. An interesting side note is the fact that the vireyas, which comprise nearly one-third of the genus Rhododendron, was ignored by the Balfour classification in spite of the fact that many were grown and hybridized in the early part of the century.
The assemblage of plants in the Azalea Series included the well known and recognized Rhododendron occidentale, R. molle, R. kiusianum, R. schlippenbachiiand many of the eastern United States native rhododendron species (just to name a very few). The plants in this grouping really do not belong together in a classification as is easily seen when we compare, say, R. occidentale and R. kiusianum. One is a large deciduous shrub with large fragrant white to pink flowers usually with a yellow flare and the other is mostly a dwarf evergreen plant with purple to pink or occasionally white small flowers.
Hermann Sleumer published a re-classification of the genus in 1949 in which he placed the plants of the Balforian classification of the Azalea Series in four subgenera. Sleumer's classification was not widely known or accepted until work was begun in Edinburgh, Scotland, on a complete revision of the genus utilizing his work. The first portion of this work was the revision of subsection Lapponica published in 1975 by W. R. Philipson and M. N. Philipson (12) followed by there vision of subgenus Rhododendron and Pogonanthum by J. Cullen in 1980 (4) and subgenus Hymenanthes by D. F. Chamberlain in 1982 (2). The Philipsons published a revision of subgenera Azaleastrum, Mumeazalea, Candidastrum and Therorhodion in 1986 (13). K.A. Kron published a revision of section Pentanthera in 1987 (9), Chamberlain and S. J. Rae published a revision of subgenus Tsutsutsi in 1990 (3), and W. S. Judd and Kron published a revision of Pentanthera (sections Sciadorhodion, Rhodora and Viscidula) in 1995 (8). These collective works have become known as the Sleumer/Edinburgh Revision.
The Royal Horticulture Society published The Rhododendron Handbook 1980 where some of the work being done at Edinburgh was introduced (10). An outline of the classification was presented alongside the Balfourian system for comparison.
Fred Galle's extensive work resulted in a detailed treatment and publication in 1982 of Azaleas which became a valuable resource widely used (6). Reference is made in his book to the re-classification work being done at that time as well as his proposal for accepting the reclassification proposed by the Philipsons. "Azalea" was still used by Galle as an all encompassing term for essentially the same group of plants from the Balfourian system's Azalea Series.
With the reclassifications known as the Sleumer/Edinburgh Revision, the entire system of nomenclature of rhododendrons has changed. The hierarchy system as laid out by Sleumer was adopted, with modifications, which matches the system that is used for other plant genera. This system has now been widely accepted by many in the rhododendron world. Within this reclassification, the name "azalea" disappeared! It has even been suggested that the name should be reserved for Loiseleuria because of nomenclature rules!
The Rhododendron Handbook 1998 published by the Royal Horticulture Society in 1997 incorporates the work done at Edinburgh and for the first time incorporates vireyas and what were classified as Ledum (15). The Balfourian classification is no longer included. Peter and Kenneth Cox's The Encyclopedia of Rhododendron Species published in 1997 adopts the Sleumer/Edinburgh Revision with a few modifications (1).
There are several groups of plants in the Sleumer/Edinburgh Revision that are still referred to as azaleas by many. These groups have a wide diversity and include the current subgenera classifications of Pentanthera (these are mostly deciduous), Tsutsutsi (these are mostly evergreen), Azaleastrum (deciduous or semi-deciduous that do not fit in other places, including one elepidote), Candidastrum (a single species - R. albiflorum), Mumeazalea (a single species -R. semibarbatum) and Therorhodion (a single species - R. camtschaticum).
There are approximately 975 (plus or minus depending on the authority) species within the genus Rhododendron. Of these, approximately 600 have scales on parts of the plant (usually on the under surface of the leaves). These scales can be seen with a magnifying glass in most cases. None of these species are within the groups that have been known as azaleas. They are all within the scaly group of plants now known as subgenus Rhododendron. Some of these plants also have hairs along with scales. The group of plants that are generally called azaleas almost all have hairs on parts of the plant. If scales are not present, the plants are a branch of the genus called subgenus Hymenanthes or are plants that fall within other groups where the catch-all name of azalea is often used. Some of the species within Hymenanthes also have hairs; however, their hairs are quite different when looked at with a microscope. The hairs of those species within Hymenanthes all branch. So-called azaleas have hairs that never branch.
We can now get back to the questions raised in the first paragraph of this short article that remain to be answered. There are no blue flowers on plants that are in the subgenera that are called azaleas. There are blue flowered rhododendrons that look somewhat similar to those in subgenus Pentanthera at least superficially; however, they are in subgenus Rhododendron and have scales. There are no yellow flowered plants in subgenus Tsutsutsi which is that group of plants that have been known as the evergreen azaleas.
In summary, azalea is a term that has been applied to plants that are now in several different subgenera and is a carry-over from past classifications. It does not accurately represent any current taxonomic thinking (as accepted by most people) about one group of plants. As taxonomists have had the opportunity to study more material and utilize modern technology, a great many changes have occurred. The Linnaeus and Balfour classifications have been left behind.
In spite of the latest classification of the genus, the term "azalea" will undoubtedly be used to refer to plants in the above five different subgenera for some time to come. It has been widely used for nearly seventy years. Some taxonomists do not agree with the Sleumer/Edinburgh classification, most notably H. H. Davidian (5). It is doubtful that the Azalea Society of America will change its name and I make no such suggestion here!
Do you think we will ever go out in our gardens and describe any of our plants as Pentanthera, Tsutsutsi, Mumeazalea, Azaleastrum and Candidastrum to our garden visitors?
1. Cox, P. A. & Cox, K. N. E. 1997. The Encyclopedia of Rhododendron Species. Perth, Scotland: Glendoick Publishing, p. 386.
2. Chamberlain, D. F. 1982. A Revision of Rhododendron II Subgenus Hymenanthes. Notes from the Royal Botanic Garden Edinburgh. Vol. 39: 209-486.
3. Chamberlain, D. F. & Rae, S.J. 1990. A Revision of Rhododendron IV Subgenus Tsutsutsi. Notes from the Royal Botanic Garden Edinburgh. Vol. 47: 89-200.
4. Cullen, J. 1980. A Revision of Rhododendron I Subgenus Rhododendron. Notes from the Royal Botanic Garden Edinburgh, Vol.39: 10207.
5. Davidian, H. H. 1995. Rhododendron Species Volume 4 Azaleas, Portland, OR: Timber Press, Oregon, p. 184.
6. Galle, F. C. 1985. Azaleas. Portland, OR: Timber Press, p. 600.
7. Greer, H. E. Greer's Guidebook to Available Rhododendrons, Third Edition. Eugene, OR: Offshoot Publications, p. 228.
8. Judd, W. S. & Kron, K. A. 1995. A Revision of Rhododendron VI Subgenus Pentanthera (Sections Sciadorhodion, Rhodora & Viscidula). Edinburgh Journal of Botany. 52: 1-54.
9. Kron, K. A. 1993. A Revision of Rhododendron V Section Pentanthera. Edinburgh Journal of Botany. Vol. 50: 249-364.
10. Leslie, A., ed. 1980. The Rhododendron Handbook 1980. London: The Royal Horticultural Society.
11. Luteyn, J. L., ed. 1978. Contributions Toward a Classification of Rhododendron. New York: New York Botanical Garden, p. 338.
12. Philipson, W. R. & Philipson, M. N. 1975. Revision of Subsection Lapponica. Notes from the Royal Botanic Garden Edinburgh. Vol. 34, No. 1.
13. Philipson, W. R. & Philipson, M. N. 1986. A Revision of Rhododendron III Subgenera Azaleastrum, Mumeazalea, Candidastrum and Therorhodion. Notes from the Royal Botanic Garden Edinburgh. Vol. 44: 1-29.
14. Stevenson, J. B., ed. 1930, 2nd edition 1947. The Species of Rhododendron. London: The Rhododendron Society, p. 861.
15. Wilson, Karen, ed. 1997. The Rhododendron Handbook 1998. London: The Royal Horticultural Society, p. 352.
Hank Helm's most recent contribution to the Journal was the article "In the Footsteps of the Great Plant Hunters" appearing in the Fall 1999 issue.