Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
What is the difference between an azalea and a rhododendron? An azalea has five stamens while a rhododendron has ten. This isn't enough of a difference to keep azaleas out of the genus Rhododendron, so scientifically speaking, an azalea is a rhododendron. The common usage of the two terms is convenient, though, when considering their use in the landscape.
Azaleas may be deciduous or evergreen and rhododendrons are all evergreen. Rhododendrons are larger than azaleas, with some exceeding 20 feet tall at maturity, while azaleas generally range from one and a half to eight feet tall. Along with their smaller stature, azaleas also tend to have smaller foliage and flowers than rhododendrons. They both require the same conditions and cultural treatment for proper growth.
Even without their flowers, azaleas and rhododendrons would have a place in the landscape. Their bold, dark green foliage provides an interesting texture in summer shrub borders and they perform admirably in foundation plantings when given the proper conditions. Rhododendrons may be used as screens and hedges. Dwarf prostrate azaleas provide an unusual and effective ground cover for shady areas. With the two groups varying in size from one and a half to twenty feet, it should be possible to select just the size wanted for a particular application.
Choose a new plant when it is in bloom in order to get the exact color desired. Pinks, purples, reds, whites, salmons, and oranges are available, and while you may find a vibrant purple that looks stunning in the nursery, consider how it will look in its planting site. If the background is a material of conflicting color, white might be a better choice. Blooming season may be anytime from early April through the end of June. Keep in mind that a plant raised in a warmer climate than Virginia's may be in bloom in the nursery earlier than it will be the following year in your garden.
Besides looking at the flowers, take a close look at the branches and leaves of any plant you are considering. Leaves should be dark green and healthy looking. Do not purchase plants with dead stems, fungal spots on the leaves, or wilting. Watch for scale insects, mites, and mealybugs. It is easier to avoid these problems at this point than to attempt to eradicate them once they are established in your yard.
These plants require moist, well drained, acid soil and grow best in light shade. Northern and eastern exposures provide protection from the hot summer sun which is a bane to azaleas and rhododendrons. Soil pH should be close to 5.5. If a soil test shows that the pH is too high, use sulfur, aluminum sulfate, or iron sulfate before planting to adjust it. Test the soil annually so that additional acidifiers can be added as needed.
Work a large amount of composted organic matter into the planting site. If the soil does not drain well (water poured into a shallow hole does not drain away within an hour), plant the shrubs in raised beds as neither azaleas nor rhododendrons will tolerate water logged roots. Mulch the soil with an organic mulch to protect shallow feeder roots from heat or cultivation damage. Water during dry spells and before the ground freezes in the winter.
Fertilize azaleas and rhododendrons annually with a preparation for acid-loving plants. Wait until blooms have faded to apply fertilizer or vegetative growth may be promoted to such an extent that the flowers are hidden. Follow the directions on the package. Organic gardeners can use cottonseed meal at the rate of two and a half pounds per hundred square feet. Sprinkle it over the mulch and water it in well. New flower buds are set in late summer, so prune after blooming to remove dead branches and faded blooms without removing next year's flowers. Removal of old blooms prevents seed production and helps assure plentiful flowering the following spring.
Insects don't cause trouble as often as do incorrect cultural practices. Rhododendrons are not commonly beset by harmful insects. The most common azalea pest is azalea lacebug, a small insect that sucks sap from the foliage leaving it bleached and silvery looking. Careful inspection of the undersides of the leaves will reveal the tiny adults and nymphs. Plants are more susceptible to lacebug infestation when grown in full sun. Several insecticides are available to control azalea lacebug; contact your county extension agent for specific recommendations. Follow all label directions carefully.
(Prepared by Ellen Silva, Extension Technician, Consumer Horticulture, Virginia Tech, Blacksburg, VA 24061-0327.)