by Ken Gohring
The culture requirements for rhododendrons and azaleas are similar. They both require an acid soil that is moist and porous and when planted, necessary actions to insure these conditions are critical. The most critical errors are planting too deep, planting in deep shade, over fertilizing, alkaline soil and poor drainage.
The site where a rhododendron or azalea is located is going to be largely determined by the physical layout of one's home and yard. Any planting planning must consider these constraints. One factor that should be considered when locating a planting site is the plant's ultimate size. This includes both height and spread. General guidelines suggest planting broadleaf rhododendrons 5 to 6 feet apart. Most azaleas can be planted 4 to 5 feet apart, while low growing azaleas can be planted 3 to 4 feet apart. It is best to plant rhododendrons and azaleas in small groups rather than as individual plants. Grouping considerations should include bloom time, color and size.
It is best that both rhododendrons and azaleas be planted in light shade under trees with few lower branches or along tree lines. Some suggest planting under oaks and pines rather than other trees, which will have more shallow root systems and will compete with the shallow rooted rhododendrons and azaleas. However, this may not be possible because of the makeup of one's planting area. This should not be a reason to avoid planting them.
Dense shade is to be avoided. Some deciduous azaleas will not bloom in such conditions and the quality of the blooms of other azaleas and rhododendrons will suffer. Spindly growth is frequently experienced by plants growing in such areas. Full sun should be avoided as well but between the two extremes, full sun is preferable to full shade. In some cases, bright sunlight will diminish color in mature blooms. If possible, avoid south, southwest and west exposures as well as areas where winds can cause problems.
The best way to determine the condition of soil is to have it tested. Most locales have means of doing this through their local extension office. Such soil tests provide the level of acidity and the need for any necessary amendments for nutrients for the soil. Also do-it-yourself soil test kits can be used to determine the level of acidity in a soil. If no means of measuring acidity are available, one should be sure to mix natural materials such as compost or pine bark with the planting mixture used.
The standard measure of acidity or alkalinity is pH level, which is a mathematical measure of certain chemical activity of certain elements of the sample. The measure ranges from 0 to 14 with a pH of 7 considered neutral. A measurement less than 7 is considered acid and one greater than 7 is alkaline. Most plants require a pH between 5.5 and 7 but many have adapted to values outside this range. Most authorities indicate a soil with a pH between 4.5 and 5.5 is optimal for rhododendrons and azaleas. The effect of soil pH relates to how plants obtain minerals and nutrients from the soil, with some soil acidity causing these items to be more easily used by plants.
A soil's pH level can become more acid with the addition of agricultural sulfur or ferrous sulfate. Aluminum sulfate will increase soil acidity but is harmful to plants and should not be used for this purpose. County extension agents can help determine the amount of minerals necessary to revise soil acidity provided they have soil test results. They can also suggest sources for these supplements. Fortunately, most soils in the Southeast are acid in nature and no supplements are necessary in most cases.
Rhododendrons and azaleas will not grow in clay soil. If, after determining the proper location for plantings and adequate efforts are made to insure proper acidity conditions, it necessary to test the soil for proper drainage and soil fertility. To test for drainage, a hole can be dug in the ground of the planting location and filled with water. The length of time that it takes the ground to absorb the water is a measure of how well it drains. A more formal approach is recommended by Cornell University. This method consists of digging a hole one foot in depth. The hole is completely filled with water and is allowed to drain completely. The hole is then refilled and the depth is measured with a ruler. After 15 minutes have passed, the drop in inches is measured. The drop is multiplied by 4 to calculate the hourly drainage rate. A drop of less than 1 inch per hour is poor drainage and plants that do not tolerate poor drainage will suffer. A drainage greater than 6 inches per hour is excessive and only plants that can stand dry conditions should be planted in such locations.
However if poor drainage conditions do exist, other options are available. The method of extracting a large hole and lining the bottom with stones or pea gravel is used by some but others suggest that such an arrangement only provides a bowl for water to accumulate and will not provide adequate drainage. When in doubt, planting above the ground is a viable option. This method consists of using a good soil mixture that is mounded upon the earth's surface. If necessary, small walls of wood or stone can fortify the mound to prevent soil from being washed away from the plant. The plant is placed in the soil mixture with the top of the plant's root ball located at or above the mound's top surface. This is very similar to the popular raised bed method used for many ornamentals and vegetables. This raised ground method should also be used when planting in areas of alkaline soil.
It is necessary to insure the plant is located in a soil that is porous. This insures that the plant will not stand in moisture, which will result in the death of the plant. The materials used to prevent this include pine bark, leaf compost, coarse peat moss, coarse sand, perlite and other soil conditioners. If the soil in the planting area is relatively fertile and not clayey, then it should be mixed with these materials. Use of materials with small particles should be avoided because they tend to trap moisture causing difficult conditions for the plants.
If at the time of planting it is known from soil tests that the planting location is low in phosphorus, phosphorus should be combined with the planting mixture. Superphosphate or triple superphosphate can be used. Some gardeners incorporate Epsom salts at planting time. These salts provide magnesium, which is required by plants. Another mineral required by plants is iron. Lack of iron in a plant's soil will result in yellowing leaves, the same thing that happens with a shortage of magnesium. Lack of iron is caused by high pH. Sometimes this alkaline condition is caused when homes are constructed and builders are careless with mortar and it is left in the soil around a home's exterior.
Calcium is also necessary for good rhododendron and azalea growth. Some incorporate lime in soil when planting, but it is resisted by others because in raises pH. Gypsum can be used to provide calcium and it will not raise the pH. Some authorities recommend adding a cup of alfalfa meal to the planting mix. It contains a hormone that stimulates vigorous growth.
Most plants available at nurseries and stores selling plants are contained in plastic pots ranging from a quart to several gallons. Plants whose roots are wrapped in burlap or in plastic bags and bare rooted plants are also available on a smaller scale from sellers. Some recommend soaking purchased plants, whether in a pot or otherwise, prior to planting. Some soak the plants over night. This lessens the amount of moisture needed after the plant is inserted in the ground and helps it get off to a good start.
In planting a plant from a plastic pot, it is imperative that the condition of the plant's roots be examined. The roots of rhododendrons and azaleas are fibrous and small relative to other shrubs. The key to a newly planted one's success is the growth of the roots into the media surrounding the planted root ball. In some cases, potted plants are root bound and must be conditioned prior to planting. An examination of the plants roots will disclose this condition. The degree of binding is usually quite evident. One approach to rectify the problem is to use a knife to cut about an inch into the root ball from the top to the bottom at least 4 places around the root ball. Larger size plants may require more such cuts. A method used by others consists of a technique known as "butter flying." A knife is inserted into the plant's root ball all the way across about half way down from the top. It is then drawn to the bottom of the root ball. The ball is then spread out creating the butterfly effect. With either approach the tight roots are feathered out so that when planted they will be in a better condition to grow into adjacent soil. The top of the root ball should be at or slightly above the soil level.
Balled and burlaped plants should be unwrapped when planting, if possible, as rhododendron and azalea roots have difficulty penetrating the burlap. It is usually stated that it is not necessary to remove burlap when planting. This is likely because burlap will decompose over time and not be a burden to root formation. However, some of the material available today for wrapping plants has plastic content. The plastic does not decompose like the burlap previously used so one must be aware of the type of material used. Strings used to hold the burlap around the roots should be discarded.