Ian E. M. Donovan, Pembroke, MA
I did. I had many that died. Some still do. The critical questions are: what killed them and how do we keep it from happening again?
My approach after having thought about my dead rhododendrons for some time—no tears—was that I needed to know what the plants required in order to survive in the wild and thrive in my garden. It seemed obvious to me that once I understood what the plants needed, I could give it to them. This is my personal report of what I learned about keeping my plants growing like weeds. Many of my efforts have been trial and error. But lately I've had many more successes than failures. At least now I can usually determine why I failed with that rhododendron that died.
After twenty-five years of growing rhododendrons, I've had lots of time to observe and study them, pick the brains of my fellow ARS Members, and review the vast literature of the many who have explored and worked with the genus during the last one hundred years. If you look at pictures of rhododendrons in nature, or have seen them yourself in situ, then you know that those happy rhododendrons can just about take over their ecological niche. They become aggressive, allowing few interlopers. Many even become epiphytes if the vegetation in warmer climes attempts to shade them too much. After all, they are twenty million-year survivors.
Observations. I was shocked when I saw rhododendrons and deciduous azaleas growing in the mountains of the central and southern Korean peninsula twenty years ago. The North East Asian Continental climate is both harsh and life giving. Ernest H. Wilson in Plant Hunting, v. II, 1927, Land of the Morning Calm, reports that about two-thirds of the peninsula was deforested for firewood by the early 20th Century. A terrible war had been fought there in the early 1950s with almost every inch of land ravaged. Korean people cut down most of the few remaining trees during bitterly cold winter months for firewood, shelter, and to obtain sustenance from the bark.
But what did I see in the mountains, besides the overwhelming presence of rocks? Remember, the Korean Peninsula is mostly mountains, hills and relatively narrow valleys, with no broad plains as we know them. In spring, the dominant feature on some hillsides in latitudes near Seoul is the pink Royal Azalea, R. schlippenbachii, or the magenta R. mucronulatum. Stunning is the only way to describe an eastern slope in an early morning sun in spring, colors glistening as the heavy dew dries. The hillside takes on the coloration of the dominant plant. Equally thrilling is a northwestern slope, plants in bloom, as a setting sun illuminates them. The hillside comes alive with color.
What makes the sight so memorable is that spring arrives very quickly in southern Korea. The dominant Siberian high has generated cold and dry northwesterly winds since November. Within a few weeks during late March into early May, these winds lose their force as they shift south across China, south of the Himalayas to emanate from the warm waters of South East Asia. Bright sun, warmer winds, and a wonderful floral display explode in the landscape. Winter becomes a distant memory. Both Rhododendron species have survived another harsh winter for which they are well-adapted—R. schlippenbachii and R. mucronulatum are deciduous. The latter is a small-leafed, or lepidote, species and both are known for their hardiness. They are bathed most mornings in a life-giving fog in their mountain homes.
In May, as the winds shift their point of origin south, they pass over the Great Gobi Desert picking up fine particles of yellow dust. These particles are carried aloft by the winds and deposited by the rains to the East in China and Korea covering everything with a yellow stain. June through to September follow with intensely hot, humid days and daily rain showers, often punctuated with severe thunderstorms. The south and western coasts of the Korean Peninsula are vulnerable to the often-deadly monsoons moving in from the South China Sea.
By about the third week in August, the Dog Days begin to break and the torrential rains of August begin to diminish. Soon the southwest winds will again shift their point of origin northward as they move into Siberia for the winter. Koreans are privileged to enjoy two months of sparkling weather and fine autumn color, much as we experience here in New England and along the Allegheny and Great Smokey Mountains. Indeed, Northeast Asia is the other major geographic region of the world where deciduous tree leaves change into rich autumn colors before falling. (A small area of the Andes Mountains also experiences a similar phenomenon.)
And what of the thick layer of organic matter on the light brown, sandy soil that these beautiful mountain rhododendrons were growing in? It wasn't very thick, yet these plants were vigorous with bright green leaves and flouting a wonderful floral display. They were growing on a hillside in a thin layer of mostly decomposed granite based soil with a little organic matter. The soil seemed to be almost gravel in some areas. Water could flow rapidly through the growing medium, away from the roots. This was important during the summer monsoon season when the air temperatures often rise over ninety degrees F during the day, but a fine layer of humus maintained moisture around the roots.
Cultural Needs. I tell this climate story to illustrate my point that, for these two rhododendrons, life in the wild is good. They survive in a rigorous natural setting, far tougher than in most of our gardens.
Here in southern New England we can expect six to eight weeks of hot, humid weather during the summer months, on average, often with periods of ninety degrees F for three to four days in a row. We can have very warm nights, without cooling mists unless one gardens near the shore or in the mountains. Forty inches of annual precipitation on average can be expected. Winters can be very cold. Depending on your USDA Zone, zero to minus twenty degrees F are not unusual temperatures. Lately, we've not had the good fortune of heavy snow cover to protect plants all winter and add to the water table. We have seen droughts during spring, summer, and autumn, increasing the garden management challenge.
My Korean experience got me to thinking about growing hardy rhododendrons in my garden. How could I replicate the favorable conditions in my garden that are found in the wild as I had observed them? Joining the Massachusetts Chapter's Species Study Group furthered my understanding of why these wonderful plants have been so successful. My reading established that there has been a consensus in the 20th Century about which siting and cultural elements are necessary for successful rhododendron gardening. Among these are:
Sun, for deciduous varieties, and afternoon shade for most others
Water
Rapidly draining garden soil, coarse growing medium, like much of our New England garden soil
Oxygen in the vicinity of the roots
Acid pH soil
Mulch (not peat moss)-- two to three inches over the root ball, but six inches away from the shrub's central stem.
These same elements for the garden were, in part, what I had observed in the mountains of Korea. They matched closely what I had also observed about our native plants in North Carolina, Virginia, New Hampshire, and Massachusetts. Many of these native rhododendrons, e.g., R. maximum—the Rosebay- are large-leafed or elepidote types that enjoy a woodland environment rather than full sun of the deciduous azaleas and small-leafed plants.
So what are the practical aspects of all this? Let's review the elements. The following discussion assumes that you have selected rhododendrons and azaleas that have a hardiness rating compatible with your USDA Hardiness Zone, that you have prepared your beds properly, and have done your planting as recommended by the experts. Detailed guidance and technical details can be found in the suggested readings at the end of this article.
Sun. Fairly simple - give the deciduous plants sun to encourage flowering, and the evergreen varieties less sun. This varies, in my experience, as the size of the mature leaves increases. The bigger the leaf, the less sun. My lepidotes are grown in almost full sun, while my big leafed varieties get up to fifty percent shade. They all need sun to encourage flowering, but the evergreen, large leaves transpire water too rapidly in summer and winter to withstand full sun all day.
Water. All plants need water to live. As a general rule, David Leach says that a gallon of water each week applied to the root ball should sustain an established and mulched 4-foot rhododendron or azalea for a week during the growing season. One doesn't have to pour the water onto established plants. During the hot days of summer leaves may appear to wilt. Observe those leaves in the cool of early morning. If they have not recovered their natural appearance, then they should be watered deeply before the day progresses. This stress situation indicates that the moisture level in the surface soil is too low for healthy plants. The summer of 1999 presented me with just such a challenge.
Garden Soil. What we want is a garden soil that is rapid draining, contains a modest amount of organic matter to hold moisture, has a coarse or open structure so that oxygen can get to the roots, and has a pH of 4.5 to 5.5.
Rapid draining is the primary quality for the rhododendron bed. While the plants need moisture, the soil cannot be permitted to hold water to the extent that it excludes oxygen from the roots during the growing season. Our late ARS Massachusetts Chapter member Dick Leonard told the story of the rhododendrons growing at the back of his woodland property in Taunton, MA. The land there was low and in the spring as the snow began to melt, perhaps augmented by rains, sections of the property would flood. Because the ground was still frozen and surface drainage was poor, water would stand for days at a time, covering the root balls of his large plants. A month or so later, the ground having thawed and drained, the plants would be blooming riotously. The soil temperatures would eventually soar as summer progressed.
Why weren't Dick's rhododendrons affected by Phytophthora root rot, a malady that commonly attacks ericacaeous plants in overly wet situations during hot weather? Because the temperature was close to freezing when the plants were standing in water. When the ground thawed, the surface water drained away, leaving just enough soil moisture to keep the plants thriving. There was no standing water or too high a soil moisture level to encourage the disease, which requires standing water in warm weather. Had there been an underlying layer of clay, he probably could not have been successful with his woodland garden.
Oxygen at the Roots. The shallow network of fine roots in Ericaceae needs oxygen to survive. It is the nature of the genus, and is why they are not successful in water-logged soils or in soils that contain a high percentage of clay. Usually a rapidly draining bed will have an open or sandy soil, which contributes to rapid draining.
Soil - an acid pH of 5.5 to 6.5. Rhododendrons do grow in soils with a pH level outside this range, but the ideal is pH 5.5 to pH 6.5. At this pH level the appropriate nutrients, if present in the soil, can be taken up by the plant. Most New England soils are naturally acidic. Over time irrigation water can change soil pH. If you have concerns about your garden soil pH, have it checked through your state agricultural university system. The pH measurement scale uses 7.0 as the midpoint. Measurements above pH 7.0 are considered alkaline and readings below pH 7.0 are acid. The further from the pH 7.0 neutral point, the stronger the alkali or acid reaction.
Mulch. If all other siting and cultural elements are in place, then mulch is the frosting on the cake of a well planted rhododendron. Mulch is a coarse organic material which is applied on the surface of the root ball 2" to 3" deep. Its function is to even out the daily temperature swings in the soil while slowing evaporation of moisture from the soil. As it breaks down in the course of a season or over several years, the decaying mulch becomes available as that mysterious thing called organic matter in soil composition. Pine needles and bark, wood chips, and oak leaves are particularly good mulches. Many other readily available, economical materials can be recycled as an attractive mulch, but peat moss is not one of them.
Peat moss dries rapidly when exposed to air and becomes almost impossible to re-wet. Peat moss is very expensive per cubic foot when compared to other readily available materials. Most peat moss at the garden center today is so fine that it is easily blown about by the wind when dry. Finally, peat moss is a non-renewable resource in our lifetime. If you choose to use peat moss as a mulch or soil amendment, take precautions when handling it to protect your hands and to not inhale the fine particles. Adverse health-related reactions may occur. As a soil amendment, peat moss can carry Phytophthora root rot disease spores, which can be fatal to rhododendrons.
Summary. Growing rhododendrons successfully in the garden is like owning and driving a car. The first thing is to learn the rules for operation and maintenance. Practice them, and observe what is happening through the seasons. Add mulch and water when needed, but keep your eyes on the plant. It will tell you what is happening, usually in time so that you can make adjustments safely. At least if it dies, the plant is easily replaced. But find out why it died before planting another rhododendron in the same hole.
Recommended Reading. Several recent books are available through the ARS Massachusetts Chapter's Book Table currently run by Barbara and Henry Wrightington. Used copies of the older works can be found at reasonable prices on the World Wide Web at such sites as: www.bibliofind.com, www.bibliocity.com, and www.abebooks.com. Just search on "rhododendron".
Bowers, Clement Gray. Rhododendrons and Azaleas: Their Origins, Cultivation and Development. 2nd ed. New York: Macmillan, 1960.
Excellent discussion of siting, culture, and selection. Recommended varieties are obsolete for the most part. Readable discussion of hybrid history, selected species, and how to hybridize.
Clarke, J. Harold. Getting Started With Rhododendrons and Azaleas. Portland, OR:Timber Press, 1982.
Paperback reprint of the classic book first published in 1960. Good information for the beginner by an American rhododendron pioneer.
Cox, Kenneth. Rhododendrons: A Care Manual. San Diego: Laurel Glen, 1998.
Large format, superb color photographs, easy to read, authoritative, and not too expensive. The best hard cover book for beginners on the market today.
Cox, Peter A. The Cultivation of Rhododendrons. London: Batsford, 1993.
Written for the serious gardener of the Genus Rhododendron by one of the world's top experts. Extremely well done, answering most questions. Clear, not too technical, and fully illustrated. No better book for the medium to advanced aficionado.
Galle, Fred C. Azaleas. Revised and Enlarged Edition. Portland, OR: Timber Press, 1987
Everything your ever wanted to know about azaleas, almost. Clear, complete cultural guide, and more. Look for this revised edition, which corrects several errors in the 1985 first edition of the late Fred Galle's masterwork.
Greer, Harold E. Rhododendron Basics: Growing Healthy Plants.
This slim paperback is very economical (less than $5.00 including shipping) and well illustrated. Seventeen pages and forty-nine color photographs extracted from Greer's Guidebook To Available Rhododendrons: Species & Hybrids. 3rd. ed. Eugene, OR.: Offshoot, 1996. Both hard cover and paperbound. Either edition contains the same concise, accurate facts and "how to" cultural instructions.
Leach, David G. Rhododendrons of the World. NY: Charles Scribner's Sons, 1961.
The late David Leach's classic remains among the best comprehensive discussions of rhododendron and azalea culture in the garden. While some details of chemical nutrients and pesticides are obsolete, the overall information about how to grow is valid. Excellent discussion of 150 important garden species. Readily available used at about $40.00 plus. Be sure that the copy you purchase has the two loose inserts: a large folded color phylogenetic chart depicting the putative evolutionary relationships in the Genus Rhododendron and the black and white drawing "Rhododendron Regions in Europe and Asia".
Reiley, H. Edward. Success with Rhododendrons and Azaleas. Portland, OR: Timber Press, 1992.
Excellent technical information written by a professional horticulturist and ARS officer. Extensive "Good Doer" lists and up-to-date technical facts. Perhaps more about nursery production and propagation than the beginner requires. Available in paperback at a reasonable price, as well as used.
