JARS v37n2 - Growing Rhododendrons and Azaleas in the Middle Atlantic States
Growing Rhododendrons and Azaleas in the Middle Atlantic States
Sandra McDonald, Ph.D., Hampton, VA
Editors Note: Harry L. Wise states that, Sandra McDonald who is a Middle Atlantic Chapter officer and on the National Board of Directors for the ARS, wrote this article at the request of the Tuckahoe Garden Club. Although written for the Middle Atlantic area, this article is generally pertinent to all areas of the country with the exception of the three plant lists.
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, etc. Acid soil conditions exist throughout most of the Middle Atlantic, Northeast and Southeast parts 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. Rhododendron species catawbiense , maximum and 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 Alabama and Georgia to Maine. A few of these species are austrinum (Florida azalea), canescens (Piedmont azalea), speciosum (0conee 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 Middle Atlantic and Richmond, Virginia, area (USDA Zones 7 to 8). 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 which are 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. 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. 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 sub-soils are found in much of the Piedmont region of 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 on 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 20-inch deep 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 soil in the field 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. 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 2 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 most areas of the country, though spring planting is preferred in really cold areas. 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 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.
Insects and Diseases
Azaleas and rhododendrons are not troubled by many insects. Southern red spider mites and two spotted mites are sometimes found on the underside of azalea leaves and very occasionally on rhododendron leaves. Plants grown in hot sunny dry locations tend to be troubled by these mites more frequently than plants grown in more ideal conditions. The mites feed by sucking juices from the undersides of leaves producing a stippled or sometimes burnt or rusty appearance on the leaves. Spider mites can be controlled by spraying with a miticide.
Azalea leaf miner is the larval stage of a small moth which lays eggs on the leaves. The eggs hatch into small caterpillars which mine the leaf tissue leaving tunnels and blotches in the leaves. The caterpillar then comes out, folds down the tip or edge of a leaf and does further feeding. Cygon and Diazinon are insecticides currently labeled for control of azalea leaf miner.
Other insects which may occasionally trouble azaleas are aphids, lace bug, defoliators (caterpillars), leaf tier, scale insects, borers, weevils, thrips and whiteflies. Other insects which may occasionally trouble rhododendrons are aphids, borers, budworm, giant hornet, Japanese beetle, lace bug, scale insects, thrips, weevils and whiteflies. For identification of these insects and control methods, contact your state or local extension agent.
Twigs and branches of azaleas and rhododendrons can be killed by die-back diseases caused by fungi (or by borers). Dead twigs and branches should be pruned back to healthy wood and removed from the vicinity of the healthy branches or burned to avoid re-infection. Pruning shears should be sterilized between cuts by dipping in dilute household bleach (1 part bleach and 8 parts water) or 70% alcohol to prevent infection of healthy branches. Plants that have been stressed by drought tend to be more susceptible to dieback diseases than unstressed plants.
Phytophthora root rot of rhododendrons and azaleas should be prevented rather than cured by selecting healthy plants and planting them in locations with good drainage. Root rot should be suspected if the plant is wilted despite adequate water. This is generally noticed in summer when warm soil temperatures favor root rot fungi. The diseased plant will quickly die from the root upward and should be immediately removed along with the soil around the roots and disposed of where it cannot contaminate other azaleas, rhododendrons or any other plants susceptible to this root rot. A different kind of plant not susceptible to root rot can be planted in this spot. Presently, there are only temporarily preventative chemical drenches for root rot; no cures are yet available.
Petal blight can be an annoyance when azaleas and rhododendrons are blooming in a rainy spring. It is caused by a fungus which causes brown spots on the flowers and then turns the flowers to a slimy mush. The flowers will not stay fresh and pretty as long as they normally would when the disease is not present. Petal blight is more of a cosmetic problem than a health problem. A fungicide has currently been registered against Ovulinia petal blight which goes by the trade name of Bayleton®.
A few other diseases may trouble rhododendrons and azaleas, but they are not normally very serious.
Many evergreen azaleas grow well in the Middle Atlantic area. Most will tolerate more sun and water than the large leaved rhododendrons. Thousands of varieties of evergreen azaleas grow well in USDA Zones 7 and 8. As winter minimum temperatures get lower farther north fewer varieties will survive the winters. Evergreen azaleas come in many shades of pink lavender, red, white, purple and combinations of these. Some suggested varieties of evergreen azaleas for the Middle Atlantic area are: 'Ambrosia', 'Angela Place', 'Aphrodite', ' Big Joe', 'Blaauw's Pink', 'Boudoir', 'Buccaneer', 'Campfire', 'Carol', 'Copperman', 'Coral Bells', 'Corsage', 'Dayspring', 'Delaware Valley White', 'Delos', 'Desiree', 'Dream', 'Elizabeth Gable', 'Elsie Lee', 'Everest', 'Fashion', 'Fedora', 'Gaiety', 'Garden State Glow', 'Geisha', 'George Tabor', 'Gigi', 'Girard's Rose', 'Girard's Scarlet', 'Glacier', 'Glamour', 'Gloria' (KBA), 'Greeting', 'Gumpo', 'Gumpo White', 'Hardy Gardenia', 'Helen Curtis', 'Herbert', 'Hershey's Bright Red', 'Hershey Pink', 'H.H. Hume', 'Hino Crimson', 'Hinodegiri', 'Hino Red', 'Hino White', 'Hot Shot', 'James Gable', kaempferi (species), 'Kathy', 'Ledifolia Alba', 'Louise Gable', 'Martha Hitchcock', 'Mary Dalton', 'Mildred Mae', 'Palestrina', 'Pleasant White', 'Polaris', poukhanense (species), 'Purple Splendor', 'Rosebud', 'Rose Greeley', 'Sherwood Red', 'Snow', 'Springtime', 'Stewartstonian', 'Teepee', Tradition', Treasure', 'White Rosebud', 'Zulu'.
Deciduous azaleas are less well known than evergreen azaleas by gardeners in the Middle Atlantic area. These azaleas lose their leaves in the winter and are thus not as popular in this area where gardeners like to see green leaves in the winter. One great advantage deciduous azaleas have over evergreen is the extensive color range of the flowers. Deciduous azaleas are available in colors from pale yellow to lemon yellow to gold and brilliant orange. There are also reds, whites, pinks and various blends and combinations. Many have fragrant flowers as an added bonus.
Native American species of deciduous azaleas are available from some sources, especially some nurseries and mail order nurseries that specialize in native plants. The flowers of these natives are usually smaller than most of the newer hybrids, but they do have a special simplicity and elegance suited to many situations.
Many deciduous azalea hybrids have been developed with large showy flowers. The Exbury hybrids are a very popular group of hybrids with others such as Ilam hybrids becoming better known every year.
Deciduous azaleas have more tolerance to sun than most rhododendrons. Some of the most popular varieties of deciduous azaleas are: atlanticum (species), 'Brazil', calendulaceum (species), canescens (species), 'Cecile', 'Gibraltar', 'Gog', 'Goldcrest', 'Homebush', 'Klondyke', 'Narcissiflora', prunifolium (species), 'Strawberry Ice', Toucan', viscosum (species)
The large leaved rhododendrons are more commonly grown in the Middle Atlantic area than the smaller scaley leaved rhododendrons. A few kinds of small leaved rhododendrons with tiny brown scales on the undersurface of the leaves can be grown in this area, but they are more difficult than either most azaleas or most large leaved rhododendrons. The small leaved types seem to grow better in a climate where summers are not excessively hot.
Many large leaved rhododendrons grow well in the Middle Atlantic area, though they require more attention to good growing conditions, especially excellent drainage, adequate moisture, a moderate amount of sunlight and good air circulation, than do evergreen and deciduous azaleas.
Some large leaved rhododendrons that do well in the Middle Atlantic area are: 'A. Bedford', 'Album Elegans', 'America', 'Cadis', 'Caroline'*, 'Catawbiense Boursault', 'Chionoides', 'County of York', decorum (species)., 'English Roseum'*, fortunei (species), 'Janet Blair', 'Maxecat'*, maximum* (species), minus (species), 'Nova Zembla', 'Roseum Elegans'*, 'Roseum Pink'*, 'Scintillation', 'Wheatley', *indicates of easiest culture.