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Journal American Rhododendron Society

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Volume 12, Number 3
July 1958

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An Outline of the Commoner Azaleas as to Their Source and Makeup
C. H. Phetteplace, M.D.
An illustrated lecture delivered at the fourteenth Annual Meeting of the Society at Portland, Oregon

        Azaleas botanically, as is generally known, belong to the heath family, which is a large family including a number of such plants as huckleberry, mountain laurel, gaultheria, Labrador teas, and others. In the first well known classification of these plants, which was by Linnaeus in 1753, the azaleas were regarded as a separate and distinctly different genus than the rhododendrons proper. The American azalea species typified the group. They were all deciduous and all had five stamens, for example, while the typical rhododendron was evergreen and had ten or more stamens. With the introduction of Asiatic species, however, certain azaleas with evergreen foliage began to appear, as well as certain borderline forms, such as R. dauricum, which looked like some of the azaleas, was semi-deciduous, and had ten stamens and would hybridize with either rhododendrons or azaleas. There were other inconsistencies to confound the taxonomist.
        At one time a solution was offered that would divide these plants into about twenty different genera. George Don in 1834 lumped them all under the genus Rhododendron and most botanists have followed this practice ever since. There is still need for more study and work in the field of classification among the species of the entire genus, but the arrangement of Don offers a good working classification and has stood the test of over a century of time.
        The azalea group thereby becomes simply a series in the genus rhododendron, just as the triflorum or the Fortunei series, for example, and like the other major rhododendron series, the azalea series becomes divided into a number of subseries, as we shall see. It then becomes scientifically proper to refer to these various azalea species as rhododendrons; for example, Rhododendron occidentale or simply R. occidentale, rather than Azalea or A. occidentale. However, it is quite likely that we, as gardeners rather than botanists, will continue to call them azaleas and it is quite proper that we should.
        As in the grouping of all of the rhododendron species series, it is the custom to use the one rather typical species in each group to designate the name of each subseries. For example, R. obtusum is rather typical of the Asiatic evergreen subseries; hence their designation subseries obtusum, just as R. fortunei is used to typify the fortunei subseries of the so-called true rhododendrons. The following, then, is a list of the entire azalea series rather simply divided into its several subseries.

Classification of Azalea Species

  1. Subseries obtusum.  Entirely Asiatic.  Source of all so-called evergreen azaleas. Have anthocyanin pigments only.
  2. Subseries luteum. Consists of all the North American species. One from Europe, two from Asia, the remainder all from North America. Have both anthocyanin and yellow pigments.
  3. Subseries schlippenbachii. From eastern Asia. Carry no yellow pigments. Are all deciduous.
  4. Subseries canadense. Two from eastern U.S.A., two from Japan. Have no yellow pigment.
  5. Subseries tashiroi. A single, little-known species.
  6. Subseries nipponicum. A single species not cultivated appreciably.

        Under this list then are grouped all of the natural growing azaleas that have been discovered in the world thus far. Since the last two subseries listed have not been of any horticultural importance, it will be seen that the first four subseries listed actually constitute the source of essentially all of the azaleas that we know today. And since there has been no hybridization done of consequence from subseries schlippenbachii nor subseries canadense, it can be seen that all of our hybrid forms come from these two interesting subseries; namely, obtusum, which is Asiatic, more or less evergreen and devoid of yellow pigment and subseries luteum, which is largely occidentale (except for R. molle and R. japonicum), all of which are deciduous and contain all pigments. Disregarding the two exceptions noted above, one might paraphrase Kipling and say that obtusum is East and luteum is West and ne'er the two shall hybridize. Some attempts at intercrossing these subseries have been made, but there has been no noteworthy success. It is well known that the evergreen rhododendrons will cross quite readily with the luteum azaleas to give us azaleodendrons, but I have no knowledge of such breeding with the members of the obtusum or evergreen azaleas, nor in fact, with the canadense nor the schlippenbachii subseries.

I. Subseries obtusum These species are predominantly from Japan, but some are from Korea, China and Formosa. They have long been used extensively for hybridization in Japan, Europe and America. The list contains about thirty native species, the best known of which are:

  1. R. indicum
  2. R. kaempferi
  3. R. obtusum
  4. R. macropetalum
  5. R. poukhanense
  6. R. scabrum
  7. R. pulchrum
  8. R. simsii
  9. R. mucronatum
  10. R. oldhamii

These species are the source of a tremendous amount of very enjoyable material from tender greenhouse varieties to some of our hardier garden plants. Almost all are extremely floriferous and many are the low, compact shrubs so useful in home and garden. 

II. Subseries luteum:

  1. They are all deciduous.
  2. One member comes from Japan, one from China, one from the Black Sea region, and about fifteen from North America.
  3. They account for all of the yellow-orange shades found in azaleas.

        It is difficult to state exactly the number of North American species that are separate and distinct that we have. The reason for saying above "about fifteen" is that it is difficult to classify some of them as being a separate species from others somewhat like them. The reason for this is probably two-fold: One is that in some species there may be a number of forms or variances within the same species. Secondly, geographically some of these plants grow so that it is possible that some natural hybridization may have taken place. It appears there is still much work to be done to accurately identify and classify these American species. Mr. C. D. Beadle, Superintendent of the Biltmore State Gardens, during his lifetime collected some 3,000 native plants, seeking to demonstrate different forms. Before his death, he destroyed all the notes he had taken, probably due to a feeling that there were shortcomings about them that made them worthless. Dr. Henry Skinner, who is presently the head of the National Arboretum, has traveled some 27,000 miles studying these plants in different parts of eastern and southern United States. A condensed account of some of these expeditions well worth reading can be found in the Year Book of the Royal Horticultural Society for 1957.
        Very accurate information as to distribution of these species, coupled with more information to be derived from chromosome studies, may be necessary to clear up some uncertainties that exist. The following is a list of all of the species in subseries luteum:

1. R. japonicum 7.  R. austrinum 13. R. atlanticum
2. R. molle (sinense) 8. R. canescens 14. R. viscosum
3. R. luteum 9. R. bakeri (cumberlandense) 15. R. serrulatum
4. R. occidentale (Fig. 30) 10. R. alabamense 16. R. oblongifolium
5. R. calendulaceum 11. R. nudiflorum 17. R. arborescens
6. R. speciosum 12. R. roseum 18. R. prunifolium

 

R. occidentale
Fig.  30  R. occidentale
Gifford photo

III. Subseries schlippenbachii:

  1. All come from eastern Asia.
  2. Series distinguished by whorled, deciduous leaves, leaf shoots and flowers coming out of the same terminal buds.
  3. Color range from white to pink, red, rose-purple; i.e., anthocyanin pigments only.
  4. All are deciduous.

The species in this group are as follows:

1. R. quinquefolium, pure white. 5. R. farrerae, rose, not hardy.
2. R. reticulatum, rose-purple. 6. R. mariesii, rose-purple, tender.
3. R. schlippenbachii, (Fig. 31) pale to rose-pink. 7. R. sanctum, rose, not in cultivation.
4. R. weyrichii Brick red. 8. R. amagianum, brick red, hardy.

 

R. schlippenbachii
Fig. 31.  R. schlippenbachii

IV.  Subseries canadense:

  1. Two come from Japan, one from Canada and one from North Carolina.
  2. Some similarity between this subseries and subseries schlippenbachii.
  3. Flower and leaf branches do not spring from the same bud.
  4. All deciduous.

Canadense azaleas: The first three are especially beautiful garden plants and are relatively hardy.

1. R. albrechtii, (Fig. 32) red-purple. 3. R. vaseyi, white to rose-pink.
2. R. pentaphyllum, rose-pink. 4. R. canadense, lilac to magenta-rose.

 

R. albrechtii
Fig. 32.  R. albrechtii
R. Henny photo

AZALEA HYBRIDS

A meticulous listing and description of all of the known groups of azalea hybrids would become quite tiresome in a presentation of this kind. Therefore only those groups which have had wide usage or seem to have some particular interest will be mentioned.

I. American Hybrids:

     There has been some sentiment to the effect that American species by proper selection and breeding could be the source of a race of azaleas much better for our needs than any so far developed. It is pointed out that the European and English hybridizers had a relatively small number of our American species that are now known and available. Perhaps it might be worthwhile to make a fresh start. Several hybridizers have already started work along these lines. Among them are E. E. J. Kraus at Oregon State College, John Creech of Glendale, Maryland, and Henry Skinner of the National Arboretum, Washington, D.C. R. calendulaceum and R. occidentale, with their many forms and wide color range, are especially promising as parents. Eastern growers who must be so concerned about hardiness feel that if the European species luteum were eliminated that this breeding could result in a hardier race.

II. Kurumes:
1.  These are a mixture of clones from selected obtusum species and hybrids.
2.  All are importations from Japan over the past forty years over 300 clones.
3.  Are one of the best known azaleas to the American public.
4.  Examples:

     a. 'Hino-degrii'

     b. 'Coral Bells'

     c. 'Snow'

     d. 'Christmas Cheer'

III. Mucronatum:

1. Many are believed to be descendants of R. indicum and R. mucronatum.
2. Double forms of white, lilac and purple exist as named varieties.
3. Ledifolia alba best known.
IV. Indian azaleas:
1. The usual commercial potted greenhouse plants have little R. indicum ancestry, but more:
     a. R. pulchrum
     b. R. mucronatum
     c. R. simsii
2. True R. indicum blooms late and not amenable to forcing.
3. Most Indian azaleas are double and variegated forms.
V. Mollis Hybrids:
1. Fundamentally are strictly crosses between R. japonicum and R. molle.
2. More recently some Ghent blood may have been introduced.
3. Koster of Holland and Waterer of Knap Hill of England the greatest propagators.
4. Are outstanding for mass color effect.
VI. Ghent Hybrids:
1. Historically were crosses between:
     a. R. calendulaceum (yellow)
     b. R. nudiflorum (pink)
     c. R. viscosum (white, fragrant)
2. About 1793 R. luteum from Black Sea region was introduced and soon added (Montier, 1885, at Ghent).
3. R. occidentale was later introduced.
4. Double and semi double forms not uncommon.
5. Have wide color range; calendulaceum a factor.
VII. Occidentale Hybrids:
1. About 1870 Anthony Waterer crossed R. occidentale with R. molle and certain mollis hybrids.
2. Later crosses of occidentale with mollis hybrids in England and Holland gave rise to a number of named hybrids. The best known are:
     a. 'Exquisite'
     b. 'Graceosa'
     c. 'Magnifica'
     d. 'Irene Koster'
3. Ben Lancaster of Camas, Washington, has introduced a number of occidentale hybrids crossed with Ghent hybrids which have large scented flowers.
VIII. Knap Hill Group:
1. Represents selective breeding carried on for many generations.
2. Begun by Anthony Waterer about 1870 using mainly molle, calendulaceum, occidentale and arborescens.
3. Subsequently Slocock, Edgar Stead and the late Lionel de Rothschild all participated in carrying this work forward.
4. The entire group has been referred to as the Knap Hill Group:
     a. Waterer's Knaphill
     b. Ilam group
     c. Slocock's
     d. Exbury's hybrids.
5. All outstanding; new ones of great merit continually appearing.
6. Have large, more open, flat flowers, remarkable trusses and wide color range.
7. Fertility maintained well; ultimate may not yet be reached.
IX. Gable Hybrids:
1. Breeding begun by Joseph Gable, Stewartstown, Pennsylvania, 1927.
2. Object: Extremely hardy evergreen azaleas have been attained.
3. Used mainly R. poukenense, R. kaempferi, R. mucronatum.
4. Wide range of flower type and habit.
5. Anthocyanin pigments of obtusum subseries only.
6. Many hose-in-hose and semi double forms.
X. Glendale Hybrids:
1. Work of B. Y. Morrison of U.S. Department of Agriculture, starting in 1939.
2. He also used obtusum's species entirely.
3. Object: Large flowered, hardy azaleas covering entire blooming season from April to June; work not yet completed.
4. Has introduced 400 clones selected from 70,000 seedlings.

        In conclusion, it may be said that essentially all of our azaleas of today come from four subseries. For all practical purposes, it may be said that each subseries remains aloof and to itself so far as hybridization with other azalea subseries is concerned, although some members of subseries luteum and its hybrids will hybridize quite readily with other rhododendron series to give azaleodendrons. Although subseries canadense and subseries schlippenbachii contain a number of species that are beautiful garden plants in themselves, neither of these subseries have produced useful hybrids.
        We then have left only the two great subseries, (1) the more or less evergreen obtusums and (2) the deciduous luteums, both of which have given us an unending list of hybrids, nor have their possibilities yet been exhausted.
        The obtusums are much the older group horticulturally and although they have the limitation of only anthocyanin pigments and most of them have no fragrance, they have the virtues among the various types of being adaptable to a wide variety of conditions from greenhouse and pot culture to outdoor growing in some fairly cold climates. They are more or less evergreen and many remain low and compact and will bloom abundantly year after year with little attention. Almost no garden in our climate is without some of these very colorful and useful plants.
        The luteums are younger and probably still hold for us some mysteries. Although deciduous and their habit does not lend themselves so well to the rockery and low garden, they have the eye appeal of both red-blue and yellow-orange pigments. Many are delightfully fragrant. It would seem that we are still a long way from achieving all that can be obtained from carefully planned and comprehensive hybridizing of these species.
        We do not need more names of azaleas. There are already thousands of names. There is a need for good azaleas of all types that will do well in a much wider range of climate than we now have. Such work that is already in progress, especially with the native American species, is going to be interesting to observe.
        It has been said that the genus Rhododendron makes up the greatest of all ornamental shrubs known to mankind. It is equally true that no series in the genus has contributed more to our enjoyment than the azalea series and none offers more fascinating prospects for the future.
        The following list of material has been referred to, and it is recommended for those who have further interest.

Lee's "The Azalea Book" is new and a real source book of information.
"Rhododendrons and Azaleas," by Clement G. Bowers. published by The Macmillan Company, New York City, 1936.
"The Azalea Book," Frederic P. Lee, published by D. Van Norstrand, Princeton, New Jersey.
"The Azalea Handbook" of The National Horticultural Society, 32nd and Elm Avenue, Baltimore, Maryland.
"Rhododendrons Occidentale," Leonard Frisbie and Dr. Edward P. Brakey.
"Rhododendron and Camellia Year Book", Royal Horticultural Society, 1955.
"Search of Native Azaleas" by Dr. Henry T. Skinner.
"The Rhododendron and Camellia Year Book", Royal Horticultural Society, 1957.
"Deciduous Species of Azaleas," James P. Russell.
"The Rhododendron and Camellia Year Book", Royal Horticultural Society, 1954.

Volume 12, Number 3
July 1958

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