Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
The spectacular, spring flowers of azaleas and rhododendrons place them among the most popular garden shrubs. Most azaleas and rhododendrons are at their best in fairly mild, humid climates. While varieties exist for all Virginia locations, selecting a good site is essential to success, as is selecting quality plants.
Many people think of azaleas and rhododendrons as shade lovers, yet dense shade is not satisfactory. Filtered sunlight is ideal, but morning sunlight with shade after 1 pm is satisfactory. Some deciduous azaleas are less sensitive to full sun and should be used if the location is sunny. However, in full sun, the flowers will bleach quickly, even though the plants may grow well. Always plant azaleas and rhododendrons where they are protected from wind by buildings, slopes or evergreen shrubs or trees. Plants not given wind protection often develop leaf scorch or stem bark splitting.
Proper placement alone is not enough. Azaleas and rhododendrons must have carefully prepared soil since their roots are delicate and unable to penetrate heavy or rocky soils. In addition, excellent drainage is important. The best soil in which to grow rhododendrons is good-quality, loamy topsoil. In areas with heavy clay or excess sand, good-quality topsoil can be added or used to replace the poorer soil only if the entire bed is treated that way. Amendments should not be added to individual planting holes because water movement will be impeded. If topsoil is added to the planting bed, be sure to check the pH and make any necessary adjustments.
Azaleas and rhododendrons thrive at a soil pH between 4.5 and 6. In Virginia, most native soils are acidic. However, soils that have been previously limed may be alkaline and may need to be made more acidic to grow azaleas and rhododendrons well. Take a soil sample to your Cooperative Extension Service Office for testing to be sure the pH is correct. Follow the soil test result recommendations to obtain a proper pH. If the soil pH is above 6, apply iron sulfate or agricultural sulfur to the soil surface. The amount to apply will depend on the existing pH, but in all cases apply only a small amount at any one time.
Most rhododendrons and azaleas are purchased with soil around the roots either in containers or balled and burlapped. They can be planted during the spring or fall, and even in summer, if kept well watered. Never plant azaleas or rhododendrons so deeply that the plant stem is covered deeper than it had been growing in the nursery. Planting too shallow is better than too deep. There is little need for fertilizing rhododendrons at planting time.
Azaleas and rhododendrons can be planted in groups rather than individually to permit preparation of soil with power equipment. But don't place the bed close to shallow rooted trees such as maple, ash or elm. Feeder roots of these trees will rapidly move into improved soil and compete for water and fertilizer. If drainage is poor, build a raised bed 6 to 12 inches above ground level. Plants should be spaced 3 to 4 feet apart, and at least 18 inches from the edge of the bed.
(Originally published as "Azaleas and Rhododendrons," by Bonnie Appleton, Extension Specialist, Nursery Crops, Virginia Tech, in The Virginia Gardener Newsletter, Volume 7, Number 5.)
Visit Virginia Cooperative Extension test