An In-Depth Look At Evergreen Azaleas
August E. Kehr, Ph.D.
Hendersonville, North Carolina
This article developed from a talk prepared for the American Rhododendron Society Southeastern Regional Meeting held at Atlanta, Georgia, May 5, 1988. This article is not intended to be an entertaining one with beautiful color pictures. It is instead a serious "think" type paper. It may even be controversial to some. It probably has errors and deficiencies. I hope, however, that it may create the basis for serious thought about evergreen azaleas. But above all, I wish it may prompt some actions that will benefit future generations of azalea enthusiasts. It is this latter objective which is especially close to my most sincere aspirations and hopes.
If we were to take an in-depth look at evergreen azaleas, we would sooner or later be drawn to their beginnings, and this is what I hope to do. Thus we will look at the history of the development of evergreen azaleas for three purposes. (1). First, we shall look at their genetic base and assess what species were used in their development. (2). Secondly, I will discuss the limitations of this germplasm baseline and what should be done to broaden it. (3). Finally we will consider some actions which should be taken for future improvement.
In accomplishing these three objectives I will make only three simple, easy-to-understand points, and end up with four basic recommendations (or challenges if you wish to call them that) for future azalea improvement.
The Evergreen Azalea Species
I should like to start with a look at evergreen azalea species as they are presently classified in the Obtusum Subseries. It has been my observation that, by and large, most members of our Society do not have the appreciation and understanding of the species that make up the Subseries Obtusum to the degree that they have for all other rhododendron species in general. Thus we know, appreciate, and collect the species of deciduous azaleas, scaly rhododendrons, non-scaly rhododendrons, and Malesian rhododendrons. We even collect some species in the other azalea subseries such as R. canadense , R. schlippenbachii , R. nipponicum , and perhaps even R. tashiroi . But am I wrong when I say most of us can hardly name more than four or five of the total of 54 species of the Subseries Obtusum, let alone collect them.
There may be reasons for this lack of appreciation of evergreen azalea species. Perhaps foremost is their out-of-date nomenclature. I can sum up the present state of our understanding of this group of plants in one word: "confusing".
The confusion started at least a century and a half ago when the species R. indicum became badly confused with R. simsii . This confusion persists even today. For example, we speak of the Belgian Indicas under the mistaken belief they were derived from R. indicum . They were not. They were derived primarily from R. simsii .
Even the species R. obtusum which gives its name to the Subseries is now considered by many people as nothing more than a hybrid. Despite that, the fogginess in classifying evergreen azaleas has gradually been lifting. A big advance was made by the classic publication, Monograph of Azalea by Wilson and Rehder in 1921, and by Fred Galle's book Azaleas in 1985. I hope no one will be offended if I say the fog surrounding evergreen azalea classification has been stirred up but not yet lifted.
A great deal of the confusion that exists among the members of our Society can be placed at the door steps of early-day horticulturists and breeders for the outlandish practice of using Latin names for plants of nothing more than hybrid origin. These Latin names are often mistaken for bona fide species names.
Let me give a few examples: 'Narcissiflora', 'Rosaeflora', 'Balsaminaeflora', 'Vittata fortunei', 'Vittata punctata', 'Amoena', 'Laciniatum', 'Crispiflora' - All of these are of hybrid or garden origin. They are not species despite their Latin names. These improper names confuse us, and it is unfortunate that they were ever applied to cultivars. We can be thankful such improper use of Latin names for cultivars is no longer acceptable.
Another reason evergreen azalea species are not more popular with the rank and file of our membership is that they are not readily available from our nurserymen. The best I could find were 8 species listed in one of our larger nurseries in the U.S. I am glad to report that Transplant Nursery in Georgia not only lists 8 Obtusum species, but lists them in a special section of their catalog as evergreen azalea species. The Rhododendron Species Foundation lists 16 species in a 1984 inventory.
My first point then is that evergreen species of azaleas are not well known, not well appreciated, nor well classified. What should be done to correct this situation? I have two recommendations.
First, a serious effort should be made to update and revise the classification of evergreen azalea species. I know of no serious effort at present to undertake such a complete revision. My first recommendation therefore is that our Society sponsor and support a revision of the Subseries Obtusum on the scale of the 1921 Monograph of Azalea by Wilson and Rehder. It is my opinion that only by genuine active sponsorship of our Society will such a work be undertaken in our lifetime. Can't we set aside a fund to encourage this badly needed research? I sincerely hope so.
Secondly, I would highly recommend that either some chapter of the ARS or the Azalea Society of America sponsor a garden wholly devoted to the collection and growing of evergreen azalea species. This effort would involve the active collection of these species and put them together in a specially dedicated garden that would foster interest, appreciation and study of these species. I would submit that the Southeast would be the ideal location for such a collection because the climate here is ideal for these plants. How about it, you members of the Azalea Chapter?
Genetic Base of Evergreen Azaleas
Now let us look at the genetic base of evergreen azaleas, and in particular at how many of the species in the Obtusum Subseries have been used in the development of modern day evergreen azalea cultivars. As I have already mentioned, there are by my count 54 species in total. This list was made up primarily by using those listed by Fred Galle in his book, along with those listed in the RHS Rhododendron Handbook , and any other sources I could find. Undoubtedly there are duplicate names. My list is, at best, only an attempt to put together all available information.
Whatever is the actual number of species, few have been used to develop modern-day evergreen azalea cultivars. I can count only 10 species. Of these 10 it would appear that by far the greatest bulk of the germplasm in today's evergreen azaleas comes from four species: R. simsii , R. indicum , R. kaempferi , and R. kiusianum .
I should like to offer the following evidence in support of that statement. As mentioned earlier, the hundreds of Belgian Indicas were developed largely from R. simsii . In addition the equally large group of Satsuki hybrids were developed to a significant degree from R. indica . It may come as a surprise that Wilson stated that the Southern Indica 'J. T. Lovett' is in no way of hybrid origin, but the true R. indicum . Even the famous R. obtusum , that played such an important role in the early days of evergreen azaleas, is now considered by competent authorities to be merely a hybrid of R. kaempferi and R. kiusianum . The Kurume azaleas probably originated from R. kiusianum .
As incredible as it may appear, a conservative estimate would be that 90% of today's evergreen azaleas stem from four species. Geneticists who are concerned about the general overall health of plants continually warn of the dangers to commercial crops that have a narrow germplasm baseline. They are highly subject to onslaughts of pests. Thus, the second point I wish to make is that our garden cultivars of evergreen azaleas have an extremely narrow and potentially dangerous germplasm base.
Far less germplasm comes from R. scabrum , R. macrosepalum , R. nakaharae , R. oldhamii , R. yedoense and R. tschonoskii . In fact R. tschonoskii (a plant which I have never been able to obtain) is reported to have been used in only one Gable hybrid, 'Forest Fire', (also a plant which I have never been able to obtain).
My third recommendation is that hybridizers the world over should seek out additional species and use them freely in future breeding programs. This objective would, of course, be enhanced if there were somewhere a collection of these species so that breeders could have a ready source of parental material.
A Question of Bone Hardiness in Evergreen Azaleas
This brings me to one of the most serious deficiencies in the improvement of evergreen azaleas. In most plant breeding programs plant hardiness should be a serious objective, and in many programs the most important objective.
Anyone who has conscientiously attempted to develop hardy evergreen azaleas very soon recognizes how frustrating it is to find a good reliable source of hardiness. Joe Gable used R. yedoense ( R. poukhanense ) as one primary source of hardiness. Others have used R. kaempferi quite successfully. In my experience R. nakaharae has been a fairly good source of hardiness. Despite these sources of hardiness, we still lack the truly bone hardiness in evergreen azaleas that can be found in deciduous azaleas, scaly rhododendrons, and broad-leaved rhododendrons. By bone hardy I mean something that would withstand at least -20° to -25°F.
I would dearly love to test the hardiness of R. taiwanalpinum which comes from north Taiwan at the elevation of 9,000-10,000 feet. High elevation forms often approximate the hardiness of northern forms. Let me point out at this juncture that the sources of bone hardiness in other rhododendrons and in deciduous azaleas has come entirely from species of American origin. The ironclad hardiness in American species comes about because of lucky formation of the mountain systems in North America. In North America the mountain systems run north and south. Thus with the onset of the great ice ages the plants could retreat southward in front of the ice sheets, thereby not only surviving but also undergoing a natural selection for cold tolerance. In Europe and Asia the mountain systems run predominantly east and west and hence there was not the opportunity for a southward retreat in front of an advancing ice sheet. As a result many plant species were wiped out in Europe and Asia and there was not the natural selection for cold tolerance. In evergreen azaleas this is important because all known species originate in Asia. This fact is proven when we summarize the origin of the 54 species I have listed as follows:
|Source||No. of Species *|
|*Some species occur in more than one country.|
Thus all species of evergreen azaleas originated in Asia, primarily China, Japan and Taiwan (Formosa). We often associate evergreen azaleas more closely with Japan than with China. However, 37 species come from China and Taiwan as compared to only 14 from Japan.
If we superimpose a map of Asia on a map of North America, we can compare the latitude of the above three areas to North America. Thus a line through the middle of China would strike our southernmost states in the U.S. The 40th parallel which runs through Philadelphia, Pennsylvania and Eureka, California also runs through Beijing, China, northern Japan and North Korea. Although I do not know the exact areas where the northern-most Obtusum species are found, I am quite positive they are south of this 40th parallel. Philadelphia is hardiness zone about 6, 0° to -10°F, and Eureka about 7-8, 20° to 0°F.
Thus the third point of this talk is to say that the evergreen azalea species known today are found entirely in the warmer climates of southeast Asia and we have no species from extremely cold areas. Recommendation number four is that our Society encourage and in so far as feasible sponsor plant exploration on the northern Japanese island of Hokkaido, and in the islands and the humid coastal areas of northeastern Asia. These are the land areas adjacent to and north of those areas where species of evergreen azaleas are known to exist. It is very probable that unknown species and hardier forms of known species exist in these more northerly limits of the distribution of evergreen azaleas and, if found, could have profound effects on improvement of hardiness of future evergreen azaleas.
I will summarize my in-depth look at evergreen azaleas by repeating the three points made in this talk.
(1). Evergreen azaleas are not well known, not well appreciated, nor well classified.
(2). Evergreen azalea cultivars today have a dangerously narrow germplasm base because they are made up of genes primarily from only 4 species.
(3). There are no iron-clad, bone hardy evergreen azalea species known today, but it is possible they exist in the northerly limits of their distribution in Asia.
In addition I have made four recommendations:
(1). That the American Rhododendron Society encourage and sponsor a revision of the Obtusum Subseries azaleas.
(2). That the American Rhododendron Society encourage and sponsor an evergreen azalea collection and garden.
(3). That hybridizers make a special effort to broaden the germplasm base in evergreen azalea cultivars by using additional species in hybridizing.
(4). That exploration be made north of the presently-known areas of distribution for evergreen azalea species in order to find sources of increased hardiness.
Accompanying this article are three charts.
The first chart lists evergreen azalea species which are misplaced, questionable, or illegitimate. There will likely be a disagreement on some of my placements. I hope so because often disagreements lead to action, and actions are what I solicit.
Table 1. Some Evergreen Azalea Species which are either questionable, misplaced or illegitimate.
(Based on RHS Handbook, Wilson and Rehder, Galle)
R. amoenum (Lindl) Planch = R. kiusianum Makino
R. eriocarpum Nakai = R. simsii Planch var. eriocarpum
R. komiyamae Makino = R. tosaense Makino
R. linearifolium Sieb. et Zucc. = R. macrosepalum Maxim
R. macrantha G. Don = R. indicum (L) Sweet
R. mucronatum G. Don = hybrid of uncertain origin
R. narcissiflorum Planch = double flowered form of R. mucronatum
R. obtusum (Lindl.) Planch = possible hybrid of R. kaempferi Planch and R. kiusianum Makino
R. oomurasaki Makino = plant of garden origin
R. phoenicium G. Don = plant of garden origin
R. poukhanense Levl. = R. yedoense Maxim var. poukhanense
R. pulchrum Sweet = plant of garden origin
R. sataense Nakai = perhaps R. kiusianum Makino
R. sublanceolatum Miq. = R. scabrum G. Don
R. tashiroi Maxim = based on breeding behavior belongs to Subseries Farrerae
R. techtum Koidz. = hybrid of R. kaempferi Planch and R. macrosepalum Maxim
R. vittatum (Fort) Planch = R. simsii Planch
R. yodogawa (Grignan) Kunert = R. yedoense Maxim
The second chart lists the species which are reportedly introduced into western gardens. There are many so rare that none of us have yet seen them.
|Table 2. 27 Evergreen Azalea Species Believed to be in Cultivation. (Based on RHS Handbook and Galle)|
|R. boninense Nakai||- Bonin Island, Japan|
|R. breviperulatum Hayata||- Taiwan|
|R. indicum (L) Sweet||- Southern Japan|
|R. kaempferi Planch||- Central and Northern Japan, Korea|
|R. kanehirae Wils.||- Taiwan|
|R. kiusianum Makino||- Japan|
|R. lasiostylum Hayata||- Taiwan|
|R. macrogemmum Nahai||- Japan|
|R. macrosepalum Maxim||- Central and Southern Japan|
|R. microphyton Franch||- Yunnan, China|
|R. nakaharae Hayata||- Taiwan|
|R. noriakianum T. Suzuki||- Taiwan|
|R. oldhamii Maxim||- Taiwan|
|R. otakumii Yamazaki||- Japan|
|R. piananense||- Taiwan|
|R. ripense Makino||- Southern Japan|
|R. rubropilosum Hayata||- Taiwan|
|R. rufohirtum Hand-Mazz.||- Yunnan, China|
|R. scabrum G. Don||- Liukiu Archipelago, Japan|
|R. serpyllifolium Miq||- Central and Southern Japan|
|R. sikayotaizanense Masam||- Taiwan|
|R. simsii Planch||- China, Taiwan|
|R. subsessile Rendle||- Philippine Islands|
|R. tamurae Masam||- Japan (Yakushima)|
|R. tosaense Makino||- Southern Japan|
|R. tschonoskii Maxim||- Korea, Japan|
|R. yedoense Maxim||- Korea|
The third chart lists species that, so far as I can determine, have never been introduced into cultivation. This list most certainly contains many synonyms. Only by a serious revision of the Subseries can this determination be made.
|Table 3. 27 Evergreen Azalea Species Not Yet Introduced.|
|(Based on RHS Handbook and Galle)|
|R. annamense Redd.||- Central Vietnam|
|R. atrovirens Franch||- Yunnan, China|
|R. chrysocalyx Levl. et Vant||- Western Hubei, China|
|R. chunii Fang||- Guangdong, China|
|R. flumineum Fang et M.Y. He||- Yunnan, China|
|R. glandulostylum Fang et M.Y. He||- Guiangxi, China|
|R. hainaneuse Merr.||- Hainan, China|
|R. jinpingense Fang et M.Y. He||- Yunnan, China|
|R. jinxiuense Fang et M.Y. He||- Guiangxi, China|
|R. kwangsiense Hu||- Guiangxi and Guangdong, China|
|R. kwangtungense Merr. et Chun||- Guangdong, China|
|R. lingii Chun||- Guangdong, China|
|R. longiperulatum Hayata||- Northern Taiwan|
|R. mariae Hance||- Guangdong, Jiangxi, Hunan, China|
|R. minutiflorum He||- Guangdong, Guiangxi, Hunan|
|R. miyazawae Nakai et Hara||- Japan|
|R. myrsinifolium Ching||- Jiangxi, China|
|R. naamkwanense Merr.||- Guangdong, China|
|R. ovatosepalum Yaman||- Taiwan|
|R. rivulare Hand.-Mazz.||- Hunan; Jiangxi, Guizhou, Sichuan, China|
|R. saisiuense Nakai||- Cheju, Korea|
|R. sasakii Wils.||- Taiwan|
|R. saxicolum Sleumer||- China|
|R. seniavinii Maxim.||- Fujian, Hunan, Guangdong, China|
|R. taiwanalpinum Ohwi||- Northern Taiwan|
|R. tenue Ching||- Guiangxi, China|
|R. tsoi Merr.||- Guiangxi and Guandong, China|
Dr. Kehr has devoted much of his life to plant research, with a special interest in evergreen azaleas. He currently serves on the ARS Research Committee and was formerly Chairman of that group. He has written many articles for the ARS Journal.