by Joe Coleman
Evergreen azaleas form such a persistent part of our landscapes that we tend to think of them as being native to this part of the world. In actuality, they are a more recent introduction to our gardens, though a large number of cultivars (man-made introductions) were introduced early in the past century. The United States, particular the eastern portion, is the home of a large number of deciduous native azaleas. There are no native evergreen azaleas in the USA. Most evergreen species originate in eastern Asia from Japan, Ryukyu Islands, Taiwan, Korea, China, Vietnam, and the Philippines. Evergreen azaleas are classified in subgenus Tsutsusi, which includes over 70 species. They offer a wide variety of size, form, color, bloom time, habitat, and characteristics so that in defining an evergreen azalea, it is best not to deal in absolutes - you can always find exceptions to the general rules in this group.
The best definition of an evergreen azalea is the primary description of always having foliage to some degree. Actually, there are two sets of leaves: (1) spring leaves are thinner and larger, usually along the stem, turn yellow falling off in the fall, and (2) summer leaves that are smaller, thicker, more terminal and persistent over the winter, depending on the clone (ancestry) and the weather. Thus the leaves are dimorphic (two types), each type lasting just long enough to keep the evergreen appearance of the azalea in distinct contrast to the deciduous types of azaleas that lose all foliage during late fall and winter. Climatic conditions, cultural conditions, and the genetics of each plant all affect the amount of foliage retained. Each plant has its limits of what it can endure.
Azaleas Are Rhododendrons
All azaleas are now placed in the genus Rhododendron, which are in the Ericaceae or heath family. This family dates back some seventy million years, allowing for a great deal of evolutionary diversity. Rhododendron is a large genus with over 800 species. Azaleas were originally a separate genus but have been considered in and out of the Rhododendron genus since about 1800, with several hair-splitting debates over the years. For more information, see “What is an Azalea?”
Most evergreen azaleas are marketed commercially as shrubs, and this would be an apt description on original planting. In ten-years time though, depending on climatic conditions and the specific cultivar, a small shrub can grow considerably larger and in twenty or thirty years reach the size of a small tree. Kurume azaleas that look so dainty with their small leaf foliage can in time reach a height of ten feet or more. Southern Indian varieties can grow even taller and spread out even farther, so it is necessary to study the size that can be reached in ten years or more to determine just how and where to plant in a landscape. Every hybrid group contains exceptions; not every Satsuki azalea remains low and spreading in growth habit.
Fortunately, most evergreens adapt well to pruning and can be kept in place in a landscape, though if you prefer to not lose the blooms for one year, trimming should be done in late spring after flowering but before the next year's buds set. Adaptability to pruning has been proven to the extreme over the centuries by the Japanese who developed numerous cultivars and employed severe pruning of evergreen azaleas to resemble small buns of green growth that reflected images of rocks along garden pathways in classic Japanese gardening style. Many times, large overgrown azaleas can be cut back almost to the ground and come back to reach the desired size in several years - it's the root system that counts as any bonsai artist will attest! The Beltsville Dwarf series, the species Rhododendron kiusianum varieties, and the North Tisbury hybrids fill in the small shrub needs.
Historically, the first evergreen azaleas to enter the U.S. came from China and Japan via Europe in the 1830's as green house plants, the thought being that the plants were tender and required protection. By 1848 R. indicum varieties were being planted at Magnolia Plantation near Charleston, SC and were flourishing outdoors. Fruitland Nursery (aka Berckman’s) in Augusta, GA was instrumental in propagating and spreading the use of the Southern Indian hybrids in the Southeast. As more varieties of evergreen azaleas reached Europe and America, more hybrid groups were developed. Green house varieties from the Belgian nursery industry became wide spread so that the name Belgian Indica was applied to tender, forcing varieties. Kurume hybrids were introduced to the U.S. in 1915 and by E. H. Wilson in 1917 with his importation of fifty selected plants (Wilson’s Fifty) from the Oishi Gardens near Kurume, Japan, and western names were added to help in their assimilation. Over time, other groups of hybrids were imported by various nurserymen, the U.S. Department of Agriculture, and a number of distinguished English plant explorers. Major additions to the growing lists of available evergreen azaleas were provided by the hybridizing efforts of Joe Gable, B. Y. Morrison, Julian Chisolm & Dr. E. A. Merritt, Orlando Pride, Tony Shammarello, Peter Girard ̶ to mention just a few ̶ all of whom worked in the 40's, 50's, and 60's. Hybridizing efforts continued to the present day through the work of Joe Klimavicz, Robert B. Stewart, Pete Vines, Sandra MacDonald, Nuccio Nurseries, James Harris, and many more still providing fresh introductions. Perhaps, the most interesting new developments follow the re-blooming hybrids of Buddy Lee with the highly successful Encore® series as well as more re-blooming developments from the Bloom-a-Thon® azaleas of Bob Head and the autumn-blooming Harris introductions from McCorkle Nursery
Flower Shapes and Sizes
Like their variety of plant sizes, evergreen azaleas also have multiple flower shapes and sizes. Flower size varies from the rather minute flower of R. serpyllifolium to the large-flowered Satsuki hybrids and Harris's 'Georgia Giant'. The most common flower form is the five-lobed single form. Yet through the metamorphosis of sepals (calyx), stamens, or even both, a number of doubling effects can occur that change the flower appearance. The shape of the petals themselves can also vary, introducing an ever expanding variety of looks, both of individual flowers and the plant itself. Flower forms include
- Single types
- Hose-in-hose (the sepals form a second set of petals behind the regular petals)
- Semi-double types (only some of the stamens form petals).
- Semi-double hose-in-hose types (the calyx is fully developed as petals such as the Pericat 'Glory' or 'Sweetheart Supreme')
- Completely double types (like Satsuki 'Balsaminaeflorum')
- Double hose-in-hose types (like the Kehr azalea 'Anna Kehr' with only a remnant of the calyx showing)
- Spider forms (such as 'Koromo Shikibu' or the Satsuki 'Kinsai' or the Holly Springs hybrid 'White Peacock')
Every once in a while, one can discover a variant in between the above types of flowers - even on the same plant. The influence of viruses or chemicals such as gibberellic acid also affects the size or form of flowers.
Flower shapes also vary from hybrid to hybrid, from the star effect of pointed petals to smooth rounded petals, to highly ruffled edges on petals, or even wavy edges. The shape of the flower tube can be tubular, funnel-formed, or bell-shaped. Combinations of various petal edges and funnel shapes along with different flower forms, produce a variety of appearances for use in a garden. In addition, flowers may appear as singles, or two or three may form a truss of blossoms at the terminal ends of branches, producing a highly floriferous look. Often the flowers are so numerous that they cover almost all the greenery.
A primary reason for planting and cultivating evergreen azaleas resides in the colors that can be added to the landscape. In describing colors of evergreen azaleas, there are several general classifications. Those that are clearly of a single color are called selfs. Blotches accent the color of the blossom with a different color on the top petal (the standard) that sometimes expands to the two adjoining petals (the wings). Blotches can be lighter or darker than the basic color. Striped flowers engender a peppermint appearance with stripes, dots, or flecks of color on a lighter base. This effect is common on Satsuki and a number of Glenn Dale azaleas. Bordered or margined flowers (picotee) are noted with a contrasting colored margin seen among Satsuki, Glenn Dale, and Back Acre hybrids such as 'Margaret Douglas' and 'Marion Lee'. Sectored flowers are those that contain a wedge of a sharply different color in a random pattern of appearance, like ‘Festive’. It is not unusual to find a self blossom or even a selfed branch of a single color on a plant otherwise noted for its sectors. It is not unusual for cuttings taken from a sectored plant, to revert to a single colored plant at some point along the propagation line. It is also possible to in time see all the variants of color when propagating such a plant, no matter what the standard color combination is at the cutting site.
In describing the colors of evergreens, we enter the world of hue, tints, and shades of colors which challenge the Royal Horticultural Society Color Chart. Names of colors can be very misleading and cannot be depended on alone. Color can change with location, fertility, weather, age, etc., so that from year to year there is usually a degree of variability. The terms light, medium and dark are used to designate degrees of value and the term 'very' just extends the range. Names of colors from color horticultural color charts to industrial paint chips to artistic names have added to the confusion in defining a particular color or shade. For instance, the term 'Strong Red' has also been called grenadine, burnt orange, orange red, vermilion, scarlet, coral rose, red, coral red, etc. Evergreen azalea colors extend through all the red, orange red, pink, purple, white shades, with the quest for a true yellow evergreen still eluding us.
While the deciduous azaleas, particularly in the white range of colors, are noted for their varying fragrances, few evergreen azaleas show evidence of exhibiting any fragrance at all. Only a single evergreen species has a faint fragrance, and its hybrids are not noted for the trait.
Leaves are noted by their varying size and coloration with white varieties usually lighter in color and having less fall change before dropping than other more highly colored varieties. Leaves may be hairy, like R. oldhamii, or smooth and shiny as many of the Satsuki hybrids in character. Leaves of a few azaleas may curl or twist.
Recently, variegated color has become highly prized with several variegated Satsuki hybrids like 'Keisetsu' and 'Ukinishiki'; the newer introduction of 'Girard's Variegated Gem', 'Silver Streak', and 'Bordered Gem'; and the Southern Indian 'Southern Belle'. 'Red Luster' is another interesting variant. Most of the variegated foliages have to be appreciated on closer inspection, as at a distance there is only a change in general shade of the shrub.
Creating a Long Bloom Season
Individual blossoms on an evergreen azalea will last about ten days. Depending on the varieties chosen, it is possible to have evergreen azaleas blooming in a garden from late March through June, and now with the advent of fall-blooming hybrids, the season can resume in mid-August till frost. This can be achieved by careful selection of Kurume hybrids combined with Southern Indians, ‘Vittata Fortunei’, and the earliest blooming deciduous natives such as R. austrinum, R. canescens, and R. flammeum-canescens hybrids. This display would combine the larger shrubs with large flowers in the Southern Indians; the pastel to intense shades of deciduous azalea R. canescens and flammeum-canescens hybrids; the intense colors of the tiny leaves and small flowers of the Kurume flowers and the bright yellow shades of R. austrinum; and the fragrance of R. canescens to all in one brief period.
The same effect can be created later in the season with careful selection of midseason Glenn Dale, Gable, Girard, Linwood, and early Back Acre hybrids combined with deciduous R. flammeum, delicate R. vaseyi, and early R. calendulaceum hybrids.
For color late in the season, select late Robin Hill hybrids (ones with Satsuki blood), late Back Acres, late Glenn Dales, the North Tisbury R. nakaharae hybrids added to late deciduous R. calendulaceum, highly fragrant R. arborescens, and brightly colored R. cumberlandense. Once they are established and well watered through the summer, the Encore® azaleas of Buddy Lee and Bloom-a-Thon® azaleas of Bob Head provide good color in the fall through their re-budding and re-blooming inheritance. A number of other evergreen azaleas have become recognized for fall-blooming tendencies over the years and have been further hybridized to emphasize that trait. While not completely covered with blossoms, re-bloomers do present a very satisfactory effect in the fall garden.
Azaleas in the Landscape
Though they have been used as green house forcing pot plants and as bonsai, the primary use of evergreen azaleas, except in extreme cold areas, is in the landscape.
A landscape design is enhanced with the use of scale and texture in the composition. A total landscape plan invites a person into the garden presenting a harmonious surround of natural textures and forms, introducing new and exciting vistas at each turn. The size of a garden need not affect the feeling produced in advancing through each garden room. The blending of trees, shrubs, perennials, bulbs, natives, ground covers, with natural stone or man-made features enhance the gardening experience. For best performance, evergreen azaleas need high shade and protection from afternoon sun and high winds. A soil on the acid side that offers good drainage is considered essential. An amended loamy soil that breathes (allows oxygen to penetrate) is best. Only a few native deciduous azaleas enjoy boggy, wet-feet conditions ̶ R. arborescens and R. viscosum can be found along creeks and swamps, but not in them. While a little fertilization will benefit evergreens, properly planted, mulched, and watered plants will be happy for many years with only occasional pruning to keep them shaped up and in their place. Evergreen azaleas do not respond well to heavy feeding. It would be best to provide a light, long-lasting fertilizer with micro-nutrients. Fertilization should be eased off in the fall to prevent late growth that would not have time to harden off before the first hard freeze. A loss of vitality and leaf color could be the result of low magnesium which is easily remedied by the addition of Epsom salt (magnesium sulfate) to the medium. Being well sited and planted is the best treatment that an evergreen azalea can have. When attending to a bed, less is always better.
Pests and Disease
Fortunately, in regards to pests and disease problems, evergreens offer only a few real problems that require systemic intervention. Lace bugs can become a problem in sunny situations in which the insects suck the underside of the leaves, leaving a bleached appearance to the foliage. This requires intervention with an insecticidal soap or a good systemic insecticide to halt the spread of the damage. Except in the most severe cases, terminating the insects will allow new healthy leaf growth to replace the old foliage, which should then be raked away to prevent a reoccurrence. Whitefly can be a real nuisance to some species and cultivars and requires the repeated sprayings with a horticultural oil, pyrethrin-based product, or other insecticide to get rid of the problem. Spider mites are another under leaf problem that can drain the vitality of a plant. Repeated sprayings will be necessary to destroy both the mites and the larvae.
Perhaps the biggest problem of evergreen azaleas and currently attracting the most research concerns the flowers. Petal blight is a fungal problem that usually arises in late spring with warmer temperatures. Flowers open and soon turn to mush becoming very unsightly on the plants. The fungus Ovulinia azaleae can spread quickly and requires good hygiene as well as a consistent spraying program to be rid of the problem. Secondary spores can remain in the soil underneath the plant to be splashed back up the next spring starting the cycle all over again. Spray with an effective fungicide and replace the current mulch that contains the affected blossoms to gain control of the situation. Be aware of late-blooming Satsukis and other hybrids and watch for developing problems to intercept with treatment as needed. Azalea leaf gall is another more unsightly problem. Simply pick the galls from the plants. When really stressed by drought or infection, Phomopsis or die-back can occur. This is primarily a wound infection that can be treated by removing the dead material back to healthy tissue. Phytophthora (root rot) is primarily a fungal problem created by poor initial planting or poor drainage that encourages the fungus to proliferate. Death of the plant is the usual outcome, and the soil in the area must be aggressively treated to insure that the problem does not spread. Treat the area as contaminated and wait before replanting in the area. Check the references below for organic and other treatments for azalea pests and diseases.
When it comes to flowering shrubs, evergreen azaleas provide the charms of beauty and form to any garden. Planted high in an acidic, humus soil and provided with high shade, any azalea will perform well. With balanced watering, the shrubs will flower profusely, and with moderate weather conditions, they will bloom dependably year after year. Their brilliant colors radiate throughout the garden giving a highlight to spring each year. By careful design, the flowers will help direct the eye on other garden elements that need to be highlighted. Evergreen azaleas can form a colorful background, be a special specimen plant to attract attention and be admired, or function as a garden border. The memory of their beauty will be a lasting impression of the garden.
Douglas, Sharon M. (2011). Common problems of rhododendron and azalea (revised). Connecticut Agricultural Experiment Station. PDF available at plant_pathology_and_ecology/common_problems_of_rhododendron_and_azalea_03-11-11r.pdf. Accessed 12/6/2013.
Galle, Fred C. (1987). Azaleas (revised ed.). Portland, OR: Timber Press.
Lee, Frederic P. (1965). The azalea book (2nd ed.). Princeton, NJ: D. Van Nostrand Company.
Moorman, Gary W. (). Azalea and rhododendron diseases—fact sheet. Penn State College of Agricultural Sciences. Available online at http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/azalea_rhododendron-diseases. Accessed 12/6/2013.
Royal Botanic Gardens and Domain Trust (Australia), Pests and diseases of azaleas and rhododendrons—Fact Sheet. http://www.rbgsyd.nsw.gov.au/plant_info/pests_diseases/fact_sheets/pests__and__diseases_of_azaleas__and__rhododendrons. Accessed 12/06/2013.
University of California Agriculture & Natural Resources. Pests in gardens and landscapes: Azalea-Rhododendron spp. Family Ericaceae (Heath family). http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/azalea.html and associated web pages. Accessed 12/6/2013.
Wade, Gary L., S. Kristine Braman and Jean Williams Woodward. (2010). “Selecting and growing azaleas”. University of Georgia College of Agricultural and Environmental Sciences publication B 670. Available online and PDF download at http://www.caes.uga.edu/publications/pubDetail.cfm?pk_id=7732. Accessed 12/06/2013.