Observations on Rhododendron Gardening
in The Southeastern Coastal Region
Robert T. Hobday, Chesapeake, Virginia
To present a clear picture of rhododendron gardening in the Southeastern Coastal region, I am writing the general environmental conditions of the area with cultural experiments that are being used to correct some of the environmental deficiencies and to produce better plant health and vigor. The results of these cultural experiments are inconclusive, but I have noted that the changes and innovations appear to have caused marked improvement in plant adaptability and growth.
The climate of the Southeastern Coastal region is different and most difficult. We have unpredictably cold winters and hot, dry summers. We have wide winter and summer temperature differences. Drought, bright sunlight, prolonged growing seasons and incomplete dormancy, fluctuating winter and spring temperatures, harsh soils and alkaline soil water are but a few of the ecological forces that exert stress on a plant's ability to adapt to and thrive in the region.
Experiences in establishing rhododendrons and azaleas in this area indicate these simple measures to be helpful:
a. Select varieties bred and grown for stamina and early bloom. Studies of species and hybrids reveal that numerous sorts have in their genetic constitutions traits which enable them to do well in harsh climates. Peak blooming season from early-April through mid-May is ideal. The flower buds are not blasted by late frosts, they reach full bloom before the onset of hot weather and the new growth has time to set before the midsummer droughts.
b. Set plants in well-prepared, raised beds. To provide acceptable drainage, the beds should be raised from 10 to 18 inches above ground level. Maintain a heavy, moisture-conserving mulch throughout the year. It is well known that 10 to 12 inches of acid mulch towers the temperature of the root area as much as 15 to 20 degrees F. during the heat of the summer and lessens the shock of quick freezes during the winter and early spring. Low-growing, bushy plants tend to shade and protect their roots from drastic temperature changes.
2. Planting Sites
The home sites of the area are generally city lots. Blessed are those who have large hardwood trees growing on their properties. On city lots there is little choice of a natural site. And, because of the size of the lots, rhododendron plantings must form an integral part of the foundation landscapes. With planning and discerning choice of companion shrubs, it is possible to create delightful gardens in which rhododendrons and azaleas grow happily.
a. Use small trees and tall shrubs to provide shade and wind protection. Even picturesque fences and lath-houses will serve the purpose nicely. Dogwoods, hawthorns, some forms of junipers, dwarf firs and pines make excellent shade and wind protection for low-growing rhododendrons without competing for the nutrients. From Tidewater Virginia south, camellias have the stamina to withstand sun and heat. They grow into tall, neat shrubs and prolong the garden flower seasons with their blossoms.
b. Do not set rhododendrons against south and west walls without some form of shade. The walls reflect afternoon sun and heat. There is some question about the effects of morning sun during the winter months. I have a large planting on the east side of the house, and have experienced no leaf burn or bark splitting, even among seedlings and liners. I attribute this to air drainage, good mulch and to the stamina of the plants.
c. Maintain good air drainage throughout the year. Do not over-plant the beds with fast-growing companion shrubs. We are not overly concerned with frost pockets during the early spring, but pockets of stagnant air during hot, humid summer months can cause mildew, leaf spot and some of the more serious forms of fungus infection.
The soils of the Southeastern Coastal region are either heavy muck clay or very fine silt sand. When they are wet they compact, creating very poor drainage conditions, and they dry out slowly into adobe-like masses that throttle aeration in the root area. Preparing a suitable planting medium and setting the plant in a hole is of little consequence. The hole becomes a water-logged reservoir, shutting off the essential oxygen need by the roots; and the water becomes stagnant, promoting the growth of fungi that cause the dreaded root rot. It must be emphasized that rhododendrons, azaleas, camellias, and many other evergreen ericaceous plants will not thrive when they are set in a hole regardless of the quality of the planting medium placed at the root ball. Raised beds of well prepared planting soil are the answer and the key to successful rhododendron gardening in the Southeastern Coastal region.
a. The soils of the area are reported to be slightly acid, but the soil water is alkaline. Because of disturbances from construction, the pH. factor of most home sites ranges from 6.5 to 8.0.
b. I prefer top soil from farm land under cultivation to the commercial, black top soil from swampy woodlands. The latter in mostly fine silt and may be infected with rot fungi from decaying roots and other plant residue.
c. I lay out raised beds using old railroad ties, logs or other suitable restraining materials. I cover the area with 2 inches of coarse bark. On this I place a mixture of 50% medium loam from farm land, 40% woods litter, composted pine bark and needles, and 10% peat moss. I use this mixture to fill the bed to the depth of 18 to 24 inches. The planting mixture for the planting hole consists of 25-30% medium loam, 20-30% woods dirt and litter, 30% peat moss and 10% composted pine bark. I set the plants 2 to 3 inches above the level of the soil so that when the soil is settled the crown of the roots is not below the soil level. The bed is then mulched with 8 to 10 inches of composted oak leaves and bark; and watered. This soil mixture may seem excessively light but I am finding that by the time the soil is settled the plants have formed new roots, generally in the first few inches from the surface. They seem to experience very little shock from transplanting and adapt easily to the new conditions.
d. I set plants from Mid-September to November and from Mid-February through March. I have found it best not to fertilize new plants from nurseries. The plants are forced with special fertilizing programs and generally have ample nutrients for the first few months.
Evergreen rhododendrons and azaleas are constantly losing water vapor through their leaves day and night, the year round. The only way it can be replaced is by absorption of water through the roots. If it is not replenished the plants soon show their distress. In summer they wilt conspicuously, in winter they sustain leaf dehydration, bud blast, and bark splitting which, generally do not show until spring bloom and growth begin. Browning around the edges and along the mid-ribs of the leaves is nearly always a sign that more water vapor is passing out of the leaves than water is being absorbed through the roots. Newly planted rhododendron and azalea beds must be moist, either by rainfall or by irrigation. With light soil mixtures used in raised beds, it often happens that capillary attraction is not established, for a time. The plant roots may exhaust the moisture in the lower root levels even though the mulch and top soil appears moist. During the fist year the appearance of the plants is the best indication of needs for water. The water requirements for rhododendrons and azaleas are especially high:
a. When the plants are in active growth. Prodigious amounts of water must pass through the roots, up through the plant, and out through the leaves to supply the plants with nutrients and minerals to support vigorous growth.
b. When the plants bloom, tremendous amounts of water are taken in to complete the process of producing flowers. Late blooming sorts almost always need some irrigation.
When it is necessary to irrigate rhododendron plantings, they should be soaked to a depth of from 18 to 24 inches. Once the soil is soaked thoroughly additional water is injurious, and the planting should not be watered again until the roots have used all of the moisture from the lower root area. If the plants continue to wilt in the afternoon heat, and the lower root area is moist, a light sprinkling of the leaves and the mulch surface to raise the humidity should revive them. Note: The soil water of the Southeastern Coastal region is alkaline. This condition presents problems when soil water is used for irrigation.
5. Organic Matter
The requirement - organic matter - is closely related to acidity and to plant nutrition. Organic matter is peat moss, leaf-mold, decomposing wood and wood chips, decomposing bark and pine needles, and other vegetable and animal tissues. Their stages of decomposition and decomposition rate determine their importance and their relationship to the growth of plants. These substances are often fibrous and contribute, by their physical properties, to an open, spongy, water-retentive soil structure. Chemically, they are very complex and are constantly changing as they pass through successive stages of decay. They are very valuable, chemically, to the nutrition of ericaceous plants because the organic acids they liberate, among other activities, form compounds with iron which will remain in solution at a higher pH level than inorganic or metallic salts of iron. When ample amounts of acid-forming organic matter are incorporated in the planting medium, we are able to remove many of the soil deficiencies of the area.
Note: Hot summers and warm, moist winters speed up the rate of decomposition in acid-forming organic matter.
Rhododendron and azaleas respond to a feeding of a balanced acid fertilizer in the same manner as do any other plants respond. But, their requirements are different and their assimilation of nutrients is more complex. After the plants are settled in raised beds with ample moisture and covered heavily with an acid-forming mulch, the use of commercial fertilizers or other plant foods becomes a supplementary or corrective measure. It is important to note that most ericaceous plant nutrition begins in the mulch and planting medium; and that this group does not absorb the dissolved plant food directly as do some plants. With the help of beneficial bacteria and fungi, the raw nutrients are broken down, and in conjunction with organic acids, new complex compounds are formed. Then, with the aid of iron solutions, the fine roots take up their nutrients. Dependent on the stage of decomposition of the planting medium and the mulch, we must allot fertilizer for the beneficial bacteria and fungi as well as for the the plants. I find that a generous feeding of 50% acid fertilizer and 50% cottonseed meal in late February and early March is ideal. Feeding at this time of the year allows the fertilizer to dissolve and complete its initial chemical reaction before the onset of hot, dry weather. This lessens the danger of root burn and metallic precipitation. The balanced acid fertilizer and slow acting cottonseed meal generally provide sufficient nutrients for the entire growing season. About mid-September, I feed a mixture of 25% ammonium sulfate, 25% iron sulfate and 50°k super phosphate. This feeding lowers the soil pH., provides nitrogen and phosphates for flower bud formation and hardening off tender growth. There are times when special nutritional treatment is required to correct specific deficiencies in the basic cultural technique. These conditions are determined by observations of the plants' appearances and growth rates.
a. Sufficient trace elements for normal growth are usually added to commercial acid fertilizers. However, the alkaline conditions of the soil and water of the Southeastern Coastal region produce adverse effects that require some special treatment. To lower the effects of alkalinity, I dress the beds with liberal amounts of flowers of sulfur in February or March. By the time hot weather sets in, the chemical action of the sulfur has raised the soil acidity to a level where the salts produced by alkaline irrigation water are counteracted.
b. When the plants are generally pale and the growth rate is slow, generally, there is a shortage of nitrogen caused by bacterial decomposition in the growing medium and mulch. The bacteria and fungi are using nitrogen necessary for plant growth. Ammonium sulfate usually corrects this condition.
c. When plants fail to set buds or harden off properly, a feeding of super phosphate during the fall usually solves the problem. However, it takes two growing seasons to realize the improvement in bud set.