GROWING RHODODENDRONS AND AZALEAS
THE MIDDLE ATLANTIC STATES
Sandra McDonald, Ph.D.
Rhododendrons and azaleas belong to the genus Rhododendron which is in the heath family (Ericaceae). Rhododendrons and evergreen and deciduous azaleas in general require a rather acid soil and good drainage as do most of the other members of this family which includes Kalmia, blueberries, cranberries, heaths, heather, and others. Acid soil conditions exist throughout most of the Middle Atlantic, Northeast, and Southeast regions of the United States. Some varieties of azaleas or rhododendrons should be found suitable for nearly any garden in the Middle Atlantic area, i.e. Virginia, West Virginia, Maryland, New Jersey and eastern Pennsylvania. The species Rhododendron catawbiense, R. maximum, and R. minus or carolinianum can be found growing wild in certain mountainous parts of the eastern U.S. Many different species of deciduous azaleas are found in the wild from Florida, Alabama and Georgia to Maine. Some of these species are R. austrinum (Florida azalea), canescens (Piedmont azalea), flammeum (Oconee azalea), periclymenoides (pinxterbloom azalea), atlanticum (coastal azalea), and calendulaceum (flame azalea). Since so many rhododendrons and deciduous azaleas grow wild in the eastern U.S. we might expect many of these plants and various hybrids of them to do well in our Middle Atlantic gardens, and they do!
Many other species of azaleas and rhododendrons are native to China and Japan and other areas in Asia, especially southeast Asia; a few are found in Europe.
More than ten thousand hybrids of azaleas and rhododendrons have been named. These hybrids consist of various combinations of Asian, North American and European species. The lovely evergreen azaleas which do so well in the Middle Atlantic region and along the Gulf Coast and other areas of the South and Southeast and even up north to Long Island and Boston are mainly derived from Japanese plants. Many kinds of plants from Japan besides azaleas and rhododendrons have been found to be particularly well suited to the Middle Atlantic region due to similarities in climate.
Healthy looking plants should be purchased from reputable local nurseries, garden centers, mail order nurseries, or even discount stores or supermarkets. Rhododendron and azalea winter hardiness is important and only plants that are hardy to at least -5 degrees F should be selected for use in the eastern part of the Middle Atlantic area westward to the Richmond, Virginia area (USDA Zones 7 to 8). West of Richmond, especially from the mountains of Virginia westward and on throughout West Virginia, plants should be selected that are hardy for USDA Zone 6, and in certain areas USDA Zone 5. Occasionally plants that are not adequately hardy for this area are found for sale, but these should be avoided except by the experienced gardener who wants to experiment. Starting out with proven and standard varieties will provide the greatest chance of success.
In selecting healthy plants look for plants with good green foliage free of fungus leaf spots and dead stems. The plants should not look wilted.
Rhododendrons and azaleas are healthiest in light shade, especially under oaks and pines with the lower branches trimmed up. Do not choose a location near maples, elms, ashes or other trees with shallow competitive root systems. Some varieties of azaleas and rhododendrons may survive in full sun, but avoid planting them in a south, southwest or west exposure, especially if heat and light are reflected on the plants from a nearby building. In dense shade the plants tend to grow spindly and do not bloom profusely. Protection from winter's cold drying winds is also desirable.
Planting soil should be acid with pH between 4.0 and 6.0. Avoid areas with old builder's debris, particularly mortar which can raise the soil pH above the desirable range. Rhododendrons and azaleas require a well drained soil, free of any standing water. Do not plant rhododendrons under down spouts or at the edges of sidewalks and driveways because poor drainage, lime or salts may kill them. Do not plant in places where other rhododendrons have wilted and died because the site may still be contaminated with disease organisms.
Rhododendrons and azaleas have fibrous and usually shallow root systems that require much oxygen and moisture during the summer. Tight clay subsoils are found in much of the Piedmont region of Virginia as well as in other areas of Virginia and West Virginia. In summers of heavy rainfall, root rot diseases can kill the plants if they have not been properly planted. A hole dug for the plant in this type soil can quickly become a disease bathtub because water does not drain away fast enough. Planting in raised beds is the best way to avoid this situation. Beds should be built up with 12 to 18 inches of organic material such as oak leaf mold, other shredded acid type compost, pine bark, coarse peat moss or decomposed pine needles. In heavy clay soil the plants should at least be placed on top of the ground and the root balls covered with some of the above organic materials.
If the garden topsoil is not heavy clay, but is a loose loam or sandy loam containing much humus, the plant can be set in a wide but rather shallow hole with at least the top inch or two of root ball above the soil surface. The bottom of the hole and area around the root ball should be filled with a mixture of equal parts loam and some of the above-mentioned organic materials. Plants that have been grown in field soil will establish themselves more quickly than plants which have been grown in containers in a light-weight mix. Container grown plants may have roots that encircle the plant. In this case the outer roots should be cut from top of the root ball to the bottom at several places around the circumference of the root ball and loosened up to stimulate production of new roots and to prevent the roots from continuing in a circular growth pattern which would eventually strangle the plant.
After planting, cover the planting area with a mulch of coarse pine bark or pine needles about two inches deep to keep the shallow roots cool. Water the plant well.
Fall or early spring are the best planting times for rhododendrons and azaleas in the Middle Atlantic area. Plantings made in late spring and summer will require more frequent watering than plantings made in fall or early spring. It is not advisable to plant material in active growth.
Rhododendrons and azaleas do not tolerate as much fertilizer as many other plants. Over fertilizing can burn the roots and can encourage serious root disease. If planting soil is very poor and sandy or if fresh sawdust has been added to the soil, it is advisable to fertilize. If the leaves are light green or yellow, but not yellow with green veins, the plant should respond to a light application of fertilizer. About one-fourth to one-half the rate recommended for other kinds of plants should be adequate. A split application, one-half in March or early April and one-half in mid-May is most desirable. Scatter the fertilizer lightly around the outer edge of the root ball. Do not apply fertilizer in late summer or fall as the plant may be stimulated into growth and be killed during the winter. Special rhododendron and azalea fertilizers are available. If in doubt, do not fertilize. Many more rhododendrons and azaleas die from too much fertilizer than from none at all.
Occasionally plants may have yellow leaves with green veins. This is chlorosis and is usually caused by lack of available iron, possibly because the soil is not acid enough. Chelated iron sprays on the foliage will help, but if alkaline soil is the problem, it should be corrected. A soil test is helpful in diagnosing this situation. Lime from a building foundation, building debris, excessive fertilization, excessive dryness or excessive wetness are some possible causes of chlorosis.
Newly planted azaleas and rhododendrons will require extra watering for one or two years until their roots have become established in their new environment. Summer watering should be done carefully and not excessively to avoid saturation of the soil with water. These conditions are conducive to the root rot disease. Do not allow plants to wilt from lack of water during active growth. Older, well-established plants will tolerate some drought. Watering plants during October and November may be necessary if these months are dry. The plants need to go into winter with adequate water so they can replace water lost from leaves during the windy and cold weather. This helps prevent winter leaf damage. Azaleas and rhododendrons grow well where the soil is moist and well aerated, but definitely not excessively wet or saturated.