Plant Pro: Growing Azaleas and Rhododendrons

The spectacular spring flowers of azaleas and rhododendrons make them among the most popular garden shrubs. However, azaleas and rhododendrons are shrubs for all seasons. Throughout the summer and fall the leaves add a pleasing, deep-green color to the garden. Some deciduous azaleas add bright fall color before the leaves drop. In winter, some varieties stand out with large, evergreen leaves.

Although all azaleas and rhododendrons are classed as Rhododendron by plant taxonomists, the name “azalea” is commonly used for native deciduous species and many evergreen hybrid types. Generally “rhododendron” is used for those species that have large, evergreen, leathery leaves and flowers distinctly clustered in terminal groups. When a sharp division cannot be made, it is always correct to call any of them rhododendrons. Whatever name is used, the culture required for all these plants is very similar. The same cultural practices may also be applied to blueberries, pieris, heather, holly, and other plants that prefer acid, organic soils.

Location
Most azaleas and rhododendrons are at their best in fairly mild, humid climates. While varieties exist for all Rhode Island locations, selecting a good site is very important, as is selecting good quality plants.

SLOPE. A site sloping to the north or east is usually best, because it is protected from drying south and west winds. Thus, plants are less subjected to rapid temperature changes in late fall or early spring.

WIND. Always plant azaleas and rhododendrons where they get wind protection. Buildings and slopes provide good barriers. Evergreen shrubs or trees such as pine, juniper, or spruce planted to the south or west of rhododendrons protect them and make good backgrounds for showing off the flowers. Plants not given protection from the wind often develop leaf scorch or splitting of the bark on the stems. Avoid corners of buildings where wind tends to be stronger.

SHADE. 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 p.m. is satisfactory and desirable in the Rhode Island area. Plants may survive continuous shade if trees have branches pruned high. Fences, shrubbery, or screens may also give protection from afternoon sun.

Some deciduous azaleas are less sensitive to sun, and should be used if the location receives full afternoon sun. However, in full sun delicate flower colors will bleach quickly even though the plants may grow well.

Soil and Its Preparation
Proper placement alone is not enough. Azaleas and rhododendrons must have soil that is prepared carefully and thoroughly. Roots of azaleas and rhododendrons are very delicate and unable to penetrate heavy or rocky soils.

DRAINAGE. Because the delicate roots of azaleas and rhododendrons are easily destroyed, excellent drainage is important. To test drainage, dig a hole six inches deep in the bed and fill it with water. If the water has not drained from the hole in four hours, install drainage tile to carry away excess water, or build raised beds.

STARTING THE BED. Planting azaleas and rhododendrons in groups rather than individually permits more efficient use of prepared soil. Don’t place the bed close to shallow rooted trees such as maple or ash. Feeder roots rapidly move into improved soil and compete for water and plant food.

SOIL ACIDITY. Azaleas and rhododendrons must have an acid soil. Most of them thrive best at a soil pH between 4.5 and 6.0. In RI most soils have a native acid reaction. However, alluvial or river bottom soils may have a more alkaline reaction and need to be made more acid to grow azaleas and rhododendrons well.

Soils previously limed heavily for a lawn or garden may need the pH lowered. Mortar or similar building materials mixed into the soil close to foundations may increase pH. When the pH is unknown, it can be determined by taking having soil analyzed.

SETTING THE PLANTS. 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 and fall or in summer if kept well watered.

Dig the hole in the prepared bed double the diameter of the root ball or container, and the same depth. In fact, the ball can be set two inches higher than the surrounding soil to allow for settling. The exposed root ball can be covered with mulch.

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. Soak well after planting and firm the soil around the ball.

MULCH. Most azaleas and rhododendrons are shallow rooted and need heavy mulch to conserve moisture around the roots, to minimize winter injury, and to prevent injury from cultivation. Coarse materials such as partly decomposed oak leaves or pine needles are ideal. Oak shavings, hardwood chips, aged sawdust, or sphagnum peat moss may also be used satisfactorily. A two-inch depth of mulch is satisfactory.

FERTILIZATION. There is little need for fertilizing at planting time. A light application of a fertilizer formulated for acid-loving plants may be added to the surface before the mulch is applied. Use no more than 2 pounds per 100 square feet, or follow directions on the package.

Maintaining The Planting
Rhododendrons and azaleas require little care once they are properly established. Good heavy mulch will keep down weeds. No cultivation should be done because the shallow, fine roots are easily damaged.

MULCH. The thickness of the mulch must be maintained but should not be excessive. As the old mulch decomposes, add new mulch. This is best done in the late fall after plants are completely dormant. Mulch should be moved away from direct contact with stems in early fall to allow hardening during the onset of cold weather. Replace and replenish mulch before a hard freeze. If wood chips, chopped bark, or only partially decomposed sawdust are used, fertilize with about one-quarter pound of ammonium sulfate for each bushel of material used. The fertilizer is best applied in the spring.

WATERING. Over-watering in sites where drainage was faulty has killed many rhododendrons. If water does not drain away readily, drain tiles should be installed or a raised bed built.

Avoid excessive irrigation in fall. Plants kept dry in September will tend to harden off and be better prepared for the winter. If the fall has been excessively dry, watering should be done after the first killing frost. At that time watering will not reduce winter hardiness but will prepare the plant for winter. The soil should be thoroughly moist before cold weather sets in. The best time for fall watering is about Thanksgiving. During the growing season be sure that at least one inch of water (from rain or irrigation) is available per week.

PRUNING. There is little need for pruning azaleas and rhododendrons. If growth becomes excessive, reduce the size with light pruning. It is important to remove the flower stems on rhododendrons as soon as flowering is complete, although this practice is not necessary on most azaleas. Failure to do this will reduce flowering the following year. Break out only the dead flower cluster, not the young buds clustered at its base.

Azaleas sometimes branch poorly and form a loose, open shrub. The plants’ form can be improved by pinching out the soft, new shoots of vigorous growing plants. Do not pinch after July because flower buds will not have time to develop for the following year.

WINTER PROTECTION. The two winter enemies of evergreen rhododendrons and azaleas are cold wind and sun. Most damage shows up as split bark, dried leaves, or dead or damaged flower buds. If hardy types are selected and proper locations are chosen, little or no winter protection is needed. If existing varieties show winter damage, provide some protection. Don’t be alarmed when leaves curl and droop on cold days; that is normal.

Discarded Christmas trees may be used to protect plants. Branches can be anchored in the ground to shield the rhododendron from wind and sun. Screens may be built of burlap or other materials to provide shade and windbreak. Protection must remain loose and airy throughout the winter. Small plants should not be covered with large mounds of leaves. Masses of leaves may begin to decay and smother the plant beneath them. Leaves may be pulled up around the stems in late fall but should not cover the entire plant. Temporary fences made of lath or snow fencing are effective in providing necessary windbreak and light shade.