Rhododendrons and Azaleas
Rhododendrons and azaleas, which are closely related, are both popular flowering shrubs in the Northeast. These shrubs thrive in many urban environments, with relatively few serious health problems once they are established. They do, however, have some special requirements that must be met in order to ensure good health. Essentially, these require matching the environmental conditions to those of the areas to which these shrubs are native. Rhododendrons and azaleas grow on forest floors in many parts of the world, in shaded habitats with acidic soils rich in organic matter. Soils are often covered with a surface layer of decaying leaf litter. Matching these conditions where native rhododendrons and azaleas thrive is the key to their good health in the landscape. Plant in areas where soil has good drainage, is high in organic matter, and is acidic, that is, pH is low (6.0 or below). Partial shade and shelter from direct afternoon sun and winter winds are also recommended.
Some of the more common problems that can occur on rhododendrons and azaleas are:
Yellow Foliage (Chlorosis)
Iron is necessary for the formation of chlorophyll, which is the green pigment in plants. At high soil pH (greater than pH 7.5), iron is not soluble and cannot be absorbed by plant roots. In order to make iron available to the plant, soil conditions must change. Typically, when there is iron deficiency leaves gradually yellow except for the veins, which remain green. The yellowing is more severe on younger leaves. Eventually the leaves die and weak leaves or no leaves appear in spring twigs, and finally entire plants die.
Physical causes of chlorosis are: incorrect cultivation of the soil, which destroys some of the feeding roots; insufficient mulch, which allows the soil to heat and dry out extensively and injures feeding roots; sandy soil with too little organic matter to retain moisture or poorly drained soil that has too little oxygen. Chemical causes include too much lime, either alone or in relation to other soluble salts, or an actual deficiency of iron in the soil
If soil pH is 6.0 or higher, sulfur, iron sulfate, or ammonium sulfate must be incorporated into the root area to lower pH. However, it is difficult to overcome iron deficiency in soils high in lime or calcium even with soil amendments, so these amendments should be avoided.. In such cases, mulch the plant heavily with a good grade of sphagnum peat. Bark mulch mixed with the peat provides a mulch with good aeration and drainage, as well as suppressing root rotting organisms. Also, use a complete, acid fertilizer that contains iron.
Browned Foliage (Scorch and Burning)
Because broadleaf evergreens carry their leaves all winter they are constantly losing water to the air. The leaves, even though they may be "rolled up" at times, are subject to drying out in dry winter air. If these plants are exposed to strong winds in winter when the soil is cold and the roots are inactive water is lost faster than the roots can replace it. The result is called winter burn. A similar situation is likely to occur again in the first few hot days of spring when soil is still frozen. The symptoms are a browning or scorching of the leaves at the tip of the leaf blade. The affected parts tend to be reddish brown and the wood may be shriveled. Excessive drying beyond normal wilting (drooping and slight curling of the foliage) may be followed by the same browning or scorching of the leaves in summer.
To prevent such difficulties, do the following:
• Plant broadleaf evergreens in spots protected from wind and sun; avoid southern exposures.
• Provide evergreen plants with ample moisture, both before the ground freezes and during the growing season. In late fall, water the ground heavily to prevent its drying out during winter.
• Mulch evergreens when growth has hardened in the fall (after non-evergreen plants have dropped their leaves) in order to insulate the soil from sudden temperature changes, reduce the depth to which freezing takes place, and reduce the time that the ground is frozen. Root growth continues at 45o F or more.
• Protect sensitive evergreen plants with a windbreak or sunshade.
Black Vine Weevil
The black vine weevil is a pest of over 100 landscape plants. Larval damage by the black vine weevil is the most severe. The larvae feed on the roots and underground stems, sometimes girdling the root crown. Adults feed from the outer margin of the leaf inward, creating characteristic C-shaped notches, and these notches can be used as an early indicator of potential larvae in the soil. Adults cut notches on the margins only, they never create holes on the center of the leaf. Affected plants lose vigor and may die eventually.
Phytophthora Root Rot (Rhododendron Wilt)
This disease is caused by a soil-borne fungus (Phytophthora spp.). It is usually a problem where wet (saturated) soil conditions occur frequently. Early symptoms of the disease consist of retarded growth, drooping of foliage (perhaps on one or two branches only), and yellowing of leaves. Infected roots appear dark and "mushy." As the disease progresses, a browning discoloration of the wood may extend upward from the base on affected branches. Plants in poorly drained soils are subject to waterlogging, which makes them highly susceptible to this disease, and plants may die quickly.
Although infected plants cannot be cured, root rot may be tolerated by the plant if improvements in soil drainage and aeration are made as soon as possible. Young plants can be lifted and replanted. If plants cannot be lifted and replanted try to improve drainage and lessen the occurrence of over watering by redirecting rain runoff, placing drain tiles, or changing irrigation programs. If the plants die from root rot, it would be unwise to replant another rhododendron in the site without considerable improvement in the soil conditions.
Snow Damage
Ericaceous plants, particularly the evergreens, suffer from heavy snowfall that bends plants and breaks stems and branches. Plants may be protected in late fall by providing a protective cover such as snow fence placed on 2x2 posts. Simply brushing the snow off the plants is just as effective. However, a moderate amount of snow is desirable as winter protection, in fact, light, dry snow may be carefully shoveled against these plants to provide insulation against winter damage from low temperatures.
Failure to Bloom
Non-blooming may result from one or a combination of the following:
• The plant is too young.
• Winter injury and cold injury in other seasons may have killed flower buds. When plants harden-off incompletely in early autumn, early fall frosts may kill the buds. In this condition, the buds will also be killed by sudden cold spells in November and December. During the first warm days of spring, temperature hardiness is quickly lost by flower buds as they break dormancy. A severe frost following this loss of dormancy can cause the flower or parts of the flower to die.
• The plant has insufficient sunlight.
• Vegetative growth is excessive.
• A normal off year occurs in some plants; some bloom heavily one year, lightly the next, and heavily again a third year.
• Seeds were allowed to develop the previous year. Seed heads should be removed after flowering.
• Pruning was done after flower buds formed in summer. The shrub should be pruned within four to six weeks after flowering.
Adapted from: Ohio State University Extension, 2000; Cornell Cooperative Extension, 1999
Useful Websites
Azalea and Rhododendron Diseases (Clemson Extension)
http://hgic.clemson.edu/factsheets/HGIC2050.htm
Growing Azaleas and Rhododendrons (University of Missouri Extension)
http://extension.missouri.edu/explore/agguides/hort/g06825.htm
Growing Azaleas and Rhododendrons (Virginia Cooperative Extension)
http://www.ext.vt.edu/pubs/envirohort/426-602/426-602.html
Maintaining Healthy Rhododendrons and Azaleas in the Landscape (The Ohio State University Extension)
