Some Experiences with Rhododendrons In Southern Red Clay
By Chas. A. Dewey, Jr., Charlotte, N.C.
This article is directed principally to those gardeners who, like myself, live in areas where soil conditions and hot summer temperatures have given all rhododendrons a reputation as borderline cases or even taboo. Others, gardening in areas where many of the factors are favorable for rhododendron culture, may think we go to foolish extremes, but where some of the "cards are stacked" against us, we must carefully consider the requirements. That these requirements are so simple that they are often overlooked, I shall attempt to point out in this article. Perhaps, this article is best considered in the light of a progress report and it most certainly is not intended to convey the impression that I have learned "it all."
Although my experiences with rhododendron culture are confined to Charlotte, North Carolina, I believe that certain observations here are worthy of consideration throughout the coastal plains. This is particularly true to areas with heavy clay soils which are often near neutral in acidity and always present drainage problems.
I feel that I should mention below the popular local methods of planting rhododendrons since I unsuccessfully tried the majority of these which are:
- Clay topsoil mixed with about an equal volume of peat moss. In this instance, the top of the root ball is planted about 1 inch below the grade level to provide a shall basin to collect water.
- Same as above but using leaf mould rather than the clayey top soil.
- Same as 1) above but including "some" aluminum sulphate to acidify the soil mixture.
- After repeated failures of all the above, I tried the laborious task of removing about 24 inches of the clay, placing 6-8 inches of rocks in the bottom of the resulting pit; then filling the pit with a soil mixture consisting volumetrically of about 40% leaf mould, 40% peat moss and 20% sand. This method was expensive, a great deal of trouble and the net result was a disposal problem of an extra yard or two of very red clay, since the rhododendrons died. There may be some successful plantings in this area using this method, but I'm inclined to think that they are successful because of other factors.
While trying the above methods, I applied to certain plants almost every available fungicide, insecticide, and "torment" distributed locally.
During all of this period, while trying to succeed with rhododendrons in the ground, I had a number of plants doing quite well in large cans containing a mixture of sand and either a South Eastern peat moss, or leaf mould. Time and time again, it was obvious that the plants would do well in cans but die when planted in the ground using an identical soil mixture. The soil temperatures in the cans frequently were quite high with ambient temperatures as high as 103 degrees F. and often with the sun actually shining directly on the cans for several hours. Moisture supply to the plants in the cans was quite a variable. Often plants would wilt seriously, but would recover quickly when watered. Obviously the cooler soil temperatures and more even water supply should favor planting in the ground. The differences between rhododendron survival in a can and in the ground seemed to reduce to the following factors:
- The cans provided sharp drainage and excellent soil aeration, even after flooded with water.
- A certain immunity, or isolation, from the hostile soil conditions was provided by the cans. Perhaps, the cans provided an isolation from unfavorable bacteria which were in the soil.
Maybe the above two factors go hand-in-hand. I shall have to leave further conclusions to a more experienced mind. I have concluded that all of the locally popular methods previously listed seemed to simply create a "teacup" of favorable soil in an "ocean" of hostility. During long periods of rain, or in heavy summer thundershowers, the various types of "pits" served as collecting basins and their soil mixtures were repeatedly permeated with unfavorable water. This progressed until the intended good drainage, pH and aeration were apparently destroyed.
Based on the conclusions in the paragraph just above, the next logical step was to prepare a bed which would tend to duplicate the "can" culture. In this I have succeeded to a degree which has largely solved my problem of rhododendron survival. After having reasonable success with survival, I have found that there are numbers of rhododendrons which grow well and bloom here. My method of planting, which I am sure is not original with me, is to plant the rhododendrons "on the ground rather than in it." The technique of the various aspects of planting are as follows:
Rhododendrons are planted in raised beds, 10-12 inches above grade level in a soil mixture consisting volumetrically of about 60% coarse sawdust, 30% clayey top soil and about 10% sharp sand. For each bushel of sawdust, one quarter of a pint of ammonium sulphate is added. For a given bed, ten to twelve inches of sawdust, the required ammonium sulphate, and sand are spread evenly over the area and very thoroughly mixed with the top 3-4 inches of the existing soil. Care should be taken to carefully pulverize all clods. A powered cultivator does a fine job. The resulting bed should then be leveled off 10-12 inches above ground level.
For a single specimen plant, usually about two bushels of sawdust along with the required ammonium sulphate and sand will suffice. The ingredients are assembled on the site and, as in the case of a more extensive area, spaded into the top few inches of the soil. The result should resemble a cucumber hill. The rhododendrons are planted with the tops of the root balls at the top of the bed, or hill, level.
I prefer old rotted sawdust, but currently have plants growing in sawdust less than one year old. The sawdust is from sawmills cutting mostly pine species. There is no White Pine sawdust included. It was with a great deal of concern that I first used sawdust rather than peat and the change over was made at the same time that I began using raised beds. Peat may, or may not, be more satisfactory. At this particular time I cannot express an opinion on the use of peat moss in raised beds. In our area, sawdust is usually free for the asking, and this is a factor worthy of consideration. I do not test the mixture for pH because the red clay ingredient makes the colorimetric methods I have tried meaningless.
As for exposure to sun and wind, or planting site considerations, a great deal more time and effort will be required to clearly establish the degree of exposure individual varieties will withstand when one considers the vast number of varieties available. In my limited experience, however, it appears that many of the older "Ironclad" varieties can withstand a great deal of sunshine and wind exposure. I am inclined to think that rhododendrons can withstand sun better than the hot drying summer winds. For example, a rhododendron would likely do quite well on a southern exposure planted between two large evergreens such as Ilex cornuta burfordi. In this example, the rhododendron would likely get three or four hours full sun each day-winter and summer-including the brilliant noonday sun. If Kurume azaleas growing in an area have dark green glossy foliage and bloom well, I'd think that the average rhododendron would find the light exposure to its liking. Remember, however, the wind which may sweep over the azaleas and destroy rhododendrons, and provide screening, or wind break, shrubs.
Of course, there are many micro-climates around any home and a northern exposure with noonday shade is always good, here. Most rhododendrons here need intermittent, or broken, sunlight with protection from winds by other evergreen shrubs or hedges. It now appears that some of the newer varieties will not survive the sun required to render satisfactory performance, only time will tell. Most of my plants are growing under high pine trees.
Here, may I direct your attention to the mulches one observes around rhododendrons in their natural habitat. The mulches should be light and airy, and should not pack down to exclude air and rainfall. It is at this point I part with peat moss. I recommend that the mulch be composed of pine needles, or oak leaves, mixed with rotted twigs and broken limbs up to "broom handle" in diameter. The inclusion of the twigs and sticks tends to secure the mulch and reduce wind dispersal of mulch materials. As the twigs and pieces of small limbs decay, voids are developed and the mulch more nearly approximates a forest floor. Frankly, as the lawn is cleaned of tree litter after windy storms, the rhododendron bed makes a convenient disposal area. Obviously, I am not a very tidy gardener, but a mulch is most conveniently maintained in this fashion.
I am firmly convinced that once a rhododendron is properly planted it is best left alone. I have yet to find any spray, or chemical preparation which will satisfactorily compensate for lack of favorable planting conditions. Once the soil conditions and exposure are favorable, most of my problems with rhododendrons have been solved. If your rhododendrons grow well the first spring, and incidentally I favor fall planting, and then die of "wilt" during the hot summer, I suggest correcting the soil or exposure conditions. Of these two, I'd first consider the soil, or planting medium. I am amazed at the amount of sunshine certain rhododendrons can stand if their soil conditions are favorable.
Watering rhododendrons is recommended, particularly during their first year in a raised site, but only if drainage is excellent. Once a plant is established, I would not bother to water it unless there is a long drought. The "millions" of rhododendrons in the nearby Blue Ridge Mountains have survived without city water down through the centuries. I will readily grant that some will require coddling, but the average gardener should let nature take her course.
As for insecticides, I suggest that rhododendrons be given about the same care as a good gardener gives evergreen azaleas. You will likely have Red Spider and Lacewing Fly damage, just as on most azaleas. Insect damage to foliage is in proportion to sun exposure. Just be careful not to over do the spraying. The Society's book "Rhododendrons" covers this subject quite well.
I do not use any fertilizers other than the Ammonium Sulphate added during the initial planting. Because of what I have read, I like to think that this Ammonium Sulphate is added to serve as a reservoir of nitrogen for the decaying sawdust, but to be quite factual, I do not know exactly where it goes. I simply suggest not adding any additional fertilizer at anytime. For me, fertilization of a rhododendron, even lightly, seems to make the plant grow too rampant in the early spring to survive the hot summers and seems to prevent dormancy. I will also extend this recommendation to rhododendron seedlings and azaleas, maybe later on I will gain enough experience to again try fertilizers. It now appears that one must know exactly how much and what kind is required to start with and then later on in the season, know just what to apply in order to nullify the first. However, I must first overcome the handicap of failing to realize any good reason why I should fertilize my rhododendrons. I am only a serious amateur, and realize that a little knowledge is a dangerous thing, and it is not my intent to belittle fertilizers, but simply to state that my experiences indicate that they should not be used here. I think, too, that I can safely predict that occasionally a rhododendron will depart this life, perhaps due to the "tides of Suez," as one prominent authority suggests.
By no means do I have a trial garden, only a modest place, but I do feel free to state the following hybrids have done well for me:
'Pink Pearl' 'Cynthia' 'Vulcan' 'Unique' 'Arthur J. Ivens'
'Romany Chai' 'Betty Wormald' 'Blue Diamond' 'Bagshot Ruby' 'Blue Peter' 'May Day' (in a cool shady location)
I have about twenty other hybrid varieties and a larger number of species under trial, but I hesitate to recommend any except the above at this time. Of course, the listing above does not mean that no other hybrids will do well here. In selecting varieties, I suggest that the nurseryman's description, and the American Rhododendron Society ratings be carefully studied. A trial garden would hasten the vast appraisal program we sorely need, but, as interest grows, the Southeastern Chapter should eventually serve the purpose. I have already concluded that some of the dwarf varieties cannot survive the sun required for satisfactory growth and bloom. The method of culture outlined herein is certainly subject to constant challenge and only by challenge can we arrive at an entirely satisfactory solution.
I am certain that rhododendrons deserve a place in most every southern garden. To state that all rhododendrons will do well here is just as absurd as to state that none will. Between these two extremes there is room for many rhododendrons with handsome foliage and lovely flowers. If this article has interested others in this area to the extent that they will try, or again try, rhododendrons and share their experiences with others, then I will predict that the rhododendron will equal or surpass the camellia in popularity.