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The question does not have a simple answer. The horticultural answer is not the same as the botanical answer. Consider these facts.
- Plants do not always fall neatly into discrete groups of plants. Botanists created the concepts of genus and species to help mankind understand plant life, but plants do not read botany texts, nor always fit neatly inside sharp boundaries.
- Before Carl Linnaeus, the father of taxonomy, various plants we know today as azaleas, mountain laurel, and rhododendrons were all called, among other names, by the foot-long word, Chamaerhododendron (low-growing rose tree). Carl Linnaeus named the Azalea genus in 1735 in his first book, Systema Naturae (System of Nature). In Species Plantarum (Plant Species), first published in 1753, he placed the first six species in genus Azalea, including A. indica (evergreen) A. lutea (deciduous), and A. viscosa (deciduous). Note that in the Azalea genus, he included both deciduous species and evergreen species. Linnaeus’s A. indica, now Rhododendron indicum, is one of the ancestors of the Satsuki hybrid azaleas. A. lutea as documented by Linnaeus is now considered three separate species, R. periclymenoides (Pinxterbloom), R. canescens (Piedmont Azalea), and R. calendulaceum (Flame Azalea). A. viscosa is now named R. viscosum (Swamp Azalea).
- Linnaeus did not create the Rhododendron genus until 1753, in Species Plantarum, eighteen years after he defined the Azalea genus.
- A major reason for Linnaeus’s separating azaleas from rhododendrons is that the azaleas he knew had 5 stamens and the rhododendrons had 10.
- Around 1800, some botanists decided all except one of the known species in the Azalea genus should be merged with the Rhododendron genus. The lone remaining Azalea species, Azalea procumbens (Alpine Azalea) was later renamed to another genus, Loiseleuria, essentially eliminating the Azalea genus. Today it is known as Kalmia procumbens. Slowly, azaleas became accepted by plant experts as part of the rhododendrons.
- Taxonomists have made a decision that for the purposes of determining valid names for genera or species 1753, with the publishing of Species Plantarum, is the starting point. Both Azalea and Rhododendron appeared in this reference, putting them on equal footing. But following the spirit of the main principle behind botany’s naming rules (use the oldest valid name), moving Azaleas into Rhododendron was illogical, because Azalea was the much older genus. Rhododendron species should have been merged into the Azalea genus, not the other way around.
Over the next 150 years, some botanists continued to keep Azalea as a separate genus, especially for the deciduous plants, but their opinion did not sway most botanists. To go back and reestablish Azalea as a separate genus would require hundreds of botanical name changes, something the botany community was and is reluctant to do.
Plant naming rules created long after Linnaeus’s time prevent the name Azalea from being used in any official capacity (such as a subgenus of Rhododendron).
Yet, the name azalea will not die. Extensively used since its coinage in 1735, the name has never dropped from favor. Azalea has a sonorous sound, and the plants are some of the most popular in the plant kingdom. The word conveys clear meaning. But since azalea is no longer a valid botanical name, its use is determined primarily by the public at large, and the name is now entrenched in our vocabulary for both deciduous and evergreen azaleas. Gardeners continue to use the name azalea for both the evergreens indigenous to Asia and the deciduous plants primarily from North America. Scores of books have been written with azalea in the title. Thousands of articles on azaleas have been published. Azaleas have become such an important group within the rhododendron family that many books carry redundant titles like Rhododendrons and Azaleas to emphasize the fact that these popular plants are included in their texts along with what is more typically thought of as a rhododendron. Thousands of azalea hybrids have been produced and named. Towns and cities hold annual azalea festivals. Azalea lovers form organizations like the Azalea Society of America and the Azalea Chapter of the American Rhododendron Society.
The Botanical View
As far as most botanists are concerned, all azaleas are rhododendrons, but not all rhododendrons are azaleas. Some botanists divide the large genus Rhododendron (over 850 species) into three major groups: lepidotes (scaly leaves), elepidotes (non-scaly), and azaleas (e.g., Almut Seithe, H. H. Davidian). Other botanists divide them into four major groups: lepidotes, elepidotes, deciduous azaleas and evergreen azaleas (e.g., Kathleen Kron). In fact, DNA studies show that deciduous azaleas may be more closely related to the elepidote rhododendron subgroup than to evergreen azaleas. Further, Kron found deciduous azaleas R. molle (Chinese Azalea) and R. arborescens (Sweet Azalea) to be related closer to lepidote rhododendron R. edgeworthii and other lepidotes than to the evergreen azaleas. With rare exceptions (e.g., ‘Grierdal’), lepidotes and elepidotes are not known to hybridize with one another. Some deciduous azaleas and elepidote rhododendrons crosses have been made. However, crosses of evergreen azaleas with either deciduous azaleas, with elepidote or with lepidote rhododendrons are rare. Most often such hybrids are sterile and the offspring are weak. Hybridization data support these major distinctions within the large genus Rhododendron.
Lepidote rhododendrons have small scales on the underside of their leaves and sometimes also have hairs on their leaves and branches. These lepidotes mostly have small leaves and are usually dwarf or relatively low-growing plants. Elepidotes do not have those scales, usually have hairs, and tend to be large-leaved, big plants, the type of plant many associate with the classic rhododendron. Even though botanists know azaleas do not have scales and therefore are technically elepidotes, they still tend to consider them separately from the main group of elepidote rhododendrons, putting them mostly in subgroups of their own.
Rhododendrons, A Diverse Group
Rhododendron is not only a very large genus but contains a highly diverse group of plants. Almost all rhododendrons are found in the northern hemisphere. Some plants are only inches (a few cm) tall, some can grow to ninety feet (27 m), some have trunks over three feet (.9 m) in diameter. The genus is mostly evergreen, but some species are deciduous, and some are dimorphic, having two sets of leaves, one set of which is deciduous. A very few are semi-deciduous; that is, they become deciduous in the colder end of their hardiness range and remain more or less evergreen at their warmer end. Leaves vary in length from a fraction of an inch (1 cm) to almost three feet (.9 m) long. They can be thin or they can be thick like southern magnolia (Magnolia grandiflora) and lusterleaf holly (Ilex latifolia). Upper surfaces of leaves can be smooth or rough. The undersides can be smooth, scaly, or hairy. Rhododendrons are found from sea level to as high as 18,000 feet (5,486 m). Some are alpine plants living in arctic conditions or high altitudes under many feet of snow in winter and then flower and produce seed in cool short summers; other are tender plants living in subtropical rain forests. Only one (R. afghanicum) can be considered a high desert plant. A few are epiphytic, growing like orchids attached only to tree limbs, downed logs, or even rocks, just hanging on, waiting for the monsoon rains to nourish them.
Some rhododendrons have buds from which both leaves and flowers emerge, and others have separate flower buds and leaf buds. Most have flower buds only on the terminal ends of branches, while some have flower buds emanating from the axils of branch and leaf. Flower blossoms vary in size from less than half an inch (1.27 cm) to six inches (15 cm) across. Some have fragrance; most do not. Some rhododendrons bloom as early as January or February; some as late as August or even September, rarely into November. Rhododendrons come in a wide variety of flower colors in the white to pink to purple range and in the yellow to orange to red range, but true blue is a hybridizer goal more than a reality. Some plants have single flowers, and some have trusses of blossoms. Most have 5 petals on the flower corolla but some species have 7 or more. The number of stamens varies from 5 (most azaleas), to 10 (the majority), on up to 28 (rare exceptions). In fact, rhododendrons and their subclassification groups are so variable, it is difficult to make an absolute statement about them; someone is certain to point out an exception.
In botany, the term azalea no longer has any official standing; in botanists’ formal language Azalea is now a synonym attached to Kalmia (K. procumbens) and cannot be used in grouping and classifying other plants. It continues only as a vernacular name, a common name. To millions of gardeners and plant lovers, azaleas are delightful, floriferous, desirable evergreen shrubs. To a smaller number, it includes the deciduous rhododendrons primarily from North America, including the plant William Bartram called, “certainly the most gay and brilliant flowering shrub yet known.”
To a gardener, azaleas are distinct from other rhododendrons. Choosing an azalea and choosing a rhododendron are not synonymous. Books that commingle azalea species among other rhododendron species without mentioning the distinction or combine deciduous hybrids and evergreen hybrids in the same lists with no clue given of this major trait are not helpful to those trying to choose a plant. Such practice is common, however, because all are in the Rhododendron genus.
Beginning in 1915, the American Joint Committee on Horticultural Nomenclature was formed by horticulturists, pharmacists, nurserymen, growers, and plant societies. It purpose was “so far as practical” to promote “the consistent use of a single standardized ‘scientific’ name, and a single standardized ‘common’ name for every tree, shrub, and plant in American commerce.” The Statement of the Problem issued in 1917 included the following example. “Azalea is now classed under Rhododendron by some botanists, yet for trade reasons it seems inexpedient to catalogue the Azaleas as Rhododendrons.” In their 1923 edition of Standardized Plant Names, they state, “Azalea is retained as a genus name in accordance with the universal horticultural distinction between Azaleas and Rhododendrons, although leading botanical authorities regard the distinction as of subgeneric rank and apply the name Rhododendron to both groups ...” In the 1942 edition, their stance is modified. Under Azalea they write, “Because of their outstanding horticultural interest, and because they are popularly (as well as by some botanists) generically distinguished from Rhododendrons, Azaleas are listed separately here, as well as under Rhododendron.” Under Rhododendron, is the notice, “Special attention is called to the fact that botanically all Azaleas are properly listed below under Rhododendron. In horticultural practice, however, it is deemed permissible to use the generic Latin term Azalea for the (mostly) deciduous forms of Rhododendron. Some horticulturists, however, prefer to use the technically correct botanical name Rhododendron for the entire group, confining the name Azalea to common and hort[icultural] names where appropriate. For convenience of reference, a botanical list is also included under Azalea ...”
Horticulturists and gardeners have no problem distinguishing between evergreen azaleas, deciduous azaleas, and the other rhododendrons. Almost any gardener can tell an azalea from other rhododendrons at a glance. What are the differences?
- Most, but not all, azaleas have 5 stamens; most rhododendrons have 10 stamens.
- Azaleas are mostly medium-sized shrubs, with unbranched (single stem or strand) hairs usually found on the leaves and branches.
- Azalea leaves are smaller and thinner than most rhodys, and generally the plants have smaller flowers. All azaleas are elepidotes and do not have scales on the undersides of their leaves.
- Azalea flowers are mostly funnel-shaped, while other rhododendrons are usually bell-shaped.
- Azaleas sometimes have flowers of different colors on the same plant, even within the same flower bud. Other rhododendrons are much more consistent in flower color.
- Most azaleas have 5 deeply-lobed flower petals. Occasionally, a plant will display blossoms with 6 or 7 petals.
- Stamen and sepal abnormalities resulting in hose-in-hose or double flowers are not unusual in azaleas; such characteristics are rare in the other rhododendrons.
- Azalea flowers tend to be at the ends of the branches. Some rhododendrons form flowers along the branch.
- Culture of azaleas is somewhat easier than rhododendrons. Azaleas are more tolerant of habitat than other rhododendrons. In general, they can tolerate more sun, more drought, more water, more fertilizer. Early-blooming azaleas seem to be less tolerant of early spring frosts than other early-blooming rhododendron. In the United States, azaleas can be successfully used as garden plants in more areas of the country than other rhododendrons.
- The azaleas we call evergreen azaleas are not actually true evergreens. They have dimorphic (two sets of) leaves: (1) deciduous leaves, which come out in the spring and drop in the fall or winter; (2) smaller, thicker leaves growing in summer at the ends of the new branch growth and usually remaining through winter, dropping after new spring leaves have formed. The leaves of some evergreens turn an interesting bronze or crimson color when cold weather sets in.
- Evergreen azaleas are bushy shrubs, usually compact and mounded, as wide as tall, usually smaller than other rhodys, but can be from low ground covers to plants over 10 feet (3 m).
- Both flowers and leaves emerge from the same terminal bud on evergreen azaleas.
- Almost all evergreen azaleas have single flowers, not a cluster of flowers, but the quantity of flowers is usually large. Most deciduous azaleas and other rhododendrons have clusters of flowers in a truss.
- Some evergreen azaleas, primarily R. oldhamii and recently developed hybrids, regularly bloom in both the spring and fall because they bloom from a second set of flower buds on new growth as well as from last year’s growth. While deciduous azaleas and other rhododendron occasionally re-bloom out of their normal bloom season, such re-blooming is an anomaly.
- The corolla of an evergreen azalea flower can be tubular, funnel-formed, or bell-shaped, with a large variety of petal shapes.
- The number of stamens on evergreen azaleas varies from 5 to 10. Some horticultural important species and hybrids have varying numbers up to 10. The stamens on evergreen azaleas (and other rhododendrons) do not extend much beyond the corolla.
- Flowers on evergreen azaleas can have unusual color combinations not just with different coloring on separate flowers but with spots, flecks, streaks, stripes, sectors, and edging in different colors. Other rhododendrons may have a strong blotch with colored spots (“bee tracks”) and some flushes of lighter or darker colors but are not found with flecks, streaks, or picotee edging.
- Yellow and true orange are colors not found on evergreen azaleas.
- Most evergreen azaleas have no fragrance.
- Evergreen azaleas were once thought too tender to be grown outside in the U.S. or Great Britain. They were considered hothouse plants. Since almost the beginning, deciduous azaleas were called the hardy azaleas, primarily grown out of doors.
- Cuttings from evergreen azaleas are much easier to root than those from deciduous azaleas or other rhododendrons.
- All evergreen azaleas come from Asia. There are 60 or so evergreen species (some sources state as many as 110), and many, many hybrids. Determining what is a species and what is a man-made hybrid is difficult. Evergreen azaleas have been cultivated in Japan and China for over 1000 years. None are native to America, though to see many gardens in the United States one would think the azalea is the U.S. national flower (no, it is the rose).
- Deciduous azaleas are, of course, deciduous, dropping their leaves each fall or winter and often providing good fall color. In Florida they can be almost evergreen. The vast majority of other rhododendrons are evergreen.
- Leaves of deciduous azaleas are for the most part longer and proportionally narrower that those on the evergreen azaleas.
- While mature deciduous azaleas can be short (e.g., R. atlanticum), they are typically taller and more upright than evergreen azaleas, occasionally reaching 15 feet (4.6 m) and more, and on rare occasions having trunks greater than 5 inches (12.7 cm) in diameter.
- Some deciduous azaleas are stoloniferous, with multiple stems growing out of the ground at a distance from the main plant. In favorable circumstances over a long period of time, single stoloniferous plants have been known to cover an acre or more. Evergreen azaleas and other rhododendrons are not stoloniferous.
- Deciduous azaleas have separate flower buds and leaf buds.
- On deciduous azaleas, multiple leaf buds form at the stem terminal below any flower buds, resulting in upright limbs often having one or two feet (.3 to .6 m) of bare stem between swirls of branches. Evergreen azaleas tend to branch more often, producing more compact plants.
- Most deciduous azaleas have funnel/tubular-shaped flowers, with longer tubes than on the corollas of evergreen azaleas.
- With two exceptions, all deciduous azaleas have 5 stamens, usually extending well beyond the corolla. R. vaseyi has 5 to 7 and R. canadense has 10. The corollas of these two deciduous azaleas are also considerably different with no or almost no corolla tube.
- Some deciduous azaleas bloom only in late summer and into fall (e.g., R. prunifolium, R. arborescens var. georgianum, R. viscosum var. serrulatum); except for re-blooming evergreen azaleas, other rhododendrons are not late bloomers (finishing in May or June, depending on climate).
- Unlike most evergreen azaleas, deciduous azaleas have trusses of flowers, with multiple flowers coming from a single flower bud. There can even be multiple flower buds on the ends of vigorous branches, creating a pom-pom effect. Almost 100 flowers at a terminal branch have been reported.
- When deciduous azalea flowers first begin to open, all the still-closed flowers in a bud stand up like fingers or claws. This is a particularly attractive stage, often with multicolored fingers.
- Deciduous azaleas can be multi-colored with flushes of colors blending on the petals and with a large solid blotch on the upper petal. Spots and flecks do not normally occur on deciduous azaleas, but coloring along the center of the petal and on one side or the other can yield a peppermint effect. Of the deciduous plants, only a few (e.g., R. vaseyi, R. albrechtii) have colored bee tracks (dots) in the upper petal blotch.
- Several deciduous azalea species are in the yellow and orange range, colors not usually found in evergreen azaleas or other rhododendrons. While lavender or light purple can occasionally be found on some deciduous azaleas (R. periclymenoides), purple is a color reserved for evergreen azaleas and other rhododendron.
- Many deciduous azaleas have distinctive fragrances (e.g., clove, lemon, vanilla, heliotrope, honeysuckle). Fragrances can be weak or strong, depending on species and other factors, like time of day and the sensitivity of one’s nose. Some species of other rhododendrons are fragrant.
- Deciduous azaleas come primarily from North America, where currently 17 species are defined. Georgia can claim the most species, and some states, like Indiana, have none. Asia has a few species and one species is found in eastern Europe along the Asia border.
On The Fence
In such a large genus as Rhododendron, one should not be surprised to see some plants that do not fit cleanly into one category or another. Some declared species may look like transition species between certain rhododendron and azalea groups or between deciduous and evergreen azaleas. As mentioned, leaves can be deciduous, semi-deciduous, dimorphic, or evergreen. There are a few species with some deciduous-azalea-like characteristics but notable differences (e.g., R. dauricum, R. mucronulatum, and R. albiflorum). The semi-deciduous R. dauricum and the deciduous R. mucronulatum are lepidotes and thus have scales. Each flower bud opens to a single flower, a trait not occurring in typical deciduous azaleas. While R. albiflorum of the North American Rocky Mountains is deciduous and has no scales, it has 10 stamens and flowers that look like little bells appearing along the stems (instead of at the terminal end). A common name for this plant is Cascade Azalea, though it has seldom been considered a deciduous azalea. It is also difficult to grow and quite rare in gardens.
A small group of deciduous azaleas has long been considered more closely related to the evergreen azaleas than to the majority of the deciduous azaleas. These azaleas (including R. mariesii, R. reticulatum, and R. weyrichii) usually have 10 stamens, instead of the usual 5, and flowers and leaves emerge from the same buds.
Reclassification and More Dilution
To make some sense of such a large genus as Rhododendron, it is not surprising that the whole has been divided into subgenera and even smaller divisions. The divisions have changed over the years, but for the most part deciduous azaleas and evergreen azaleas have been placed in two subgenera, Pentanthera and Tsutsusi. In a recent proposed move (Goetsch et al, 2004), Goetsch demoted subgenus Tsutsusi and moved these evergreen species under subgenus Azaleastrum, which with some irony is identified as the “False Azaleas” by Davidian. While there is phylogenetic logic to her reclassification, to the horticulturist at least, the Azaleastrum of Goetsch does not appear to be a coherent whole, but more like a group of interesting cats and dogs. In the 2004 revision, deciduous azaleas (with the exception of a few placed with the evergreen azaleas in subgenus Azaleastrum), were also demoted from subgenus status and moved under the existing subgenus that contains almost all the elepidote rhododendrons (Hymenanthes). Of all American azaleas, R. vaseyi and R. canadense are the most distinct in flower characteristics, and these somewhat similar species have usually been classified together. While Goetsch moved deciduous R. vaseyi to join the evergreen azaleas in subgenus Azaleastrum, deciduous R. canadense remained with the deciduous azaleas in subgenus Hymenanthes. For azaleas, the net effect of these proposed reclassifications is to separate even further evergreen azaleas from most of the deciduous azaleas and to demote both evergreen and deciduous azaleas from major category status (subgenera) within the genus Rhododendron.
What is the conclusion? What is an azalea? Don’t ask a taxonomic botanist, because the botanist will tell you that there is only one species that is (or was) an azalea, now renamed Kalmia procumbens. Instead, ask any gardener, and the gardener will smile, get up off of his soil-stained knees, and describe two most delightful groups of floriferous plants.
Davidian, H. H. 1982. The Rhododendron Species: Vol 1, Lepidotes. Portland OR: Timber Press
_____. 1989. The Rhododendron Species: Vol 2, Elepidotes. Portland OR: Timber Press.
_____. 1992. The Rhododendron Species: Vol 3, Elepidotes continued. Portland OR: Timber Press.
_____. 1995. The Rhododendron Species: Vol 4, Azaleas. Portland OR: Timber Press.
Goetsch, L. A., A. J. Eckert, and B. D. Hall. 2005. The molecular systematics of Rhododendron (Ericaceae): A Phylogeny based upon RPB2 gene sequences. Systematic Botany 30 (3): 616–626.
Kron, Kathleen. Rhododendrons. <http://users.wfu.edu/kronka/kakbek/rhododendrons.html> and associated links. Accessed March 27, 2013.
Kron, Kathleen, Walter S. Judd and Arne A. Anderberg. 2008. Validation of Kalmia buxifolia (Bergius) Gift & Kron and Kalimia procumbens (L.) Gift & Kron. Nordic Journal of Botany, 26: 47-48.
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_____. 1753. Species Plantarum. Stockholm: Laurentius Salvius. PDF from Biodiversity Heritage Library, <http://www.biodiversitylibrary.org/page/358106#page/1/mode/1up>
Seithe, Almut. 1980. Rhododendron hairs and taxonomy. In Contributions Toward a Classification of Rhododendron. Luteyn, James L. and Mary E. O’Brien, eds. Bronx, NY: New York Botanical Garden. 89-115.
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_____. 2000. How azaleas came to our gardens: a snapshot. The Azalean 22 (3): 56-62; 22 (4): 75.