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 with good soil drainage, low pH and partial shade, sheltered from direct afternoon sun and winter winds. Some of the more common problems that can occur on rhododendrons and azaleas include iron deficiency, winter injury (burn), black vine weevil and Phytophthora root rot.

Iron Deficiency:

Yellowing of rhododendron leaves is typical of iron deficiency. The yellowing, which occurs between the veins, is more severe on younger leaves. This problem generally results from plants growing in soils of improper pH. Rhododendrons must be grown in acidic soil that is high in organic matter. If the pH is above 6.0, soil amendments such as sulfur, iron sulfate or ammonium sulfate must be incorporated into the root area to lower pH. It will be difficult to overcome the deficiency problem in soils high in lime or calcium, even with soil amendments. 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 suppresses root rotting organisms. If kept moist, plants will root into this mulch. Use a complete, acid fertilizer that contains iron.

Winter Burn:

Leaf drying and browning can occur on rhododendron leaves as a result of winter exposure. The leaves, even though they may be "rolled up" at times, are subject to drying out in dry winter air. The solution is to protect the plant from the drying wind. Plant rhododendrons behind buildings or other plants that can serve as wind shields. Put wind shields in place around the plants during the winter months. Mulching as described above is critical to preventing winter injury.

Black Vine Weevil:

The adult weevils feed on rhododendron leaves producing a C-shaped notching in the leaf margin. These insects can be quite damaging; most of the damage is a result of weevil larvae feeding on the roots. Affected plants lose vigor and may die eventually. See GreenShare Factsheet on the Black Vine Weevil for identification and life cycle information, as well as control recommendations.

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. Before replanting, improve the drainage and aeration of the soil. Use tile drainage or add porous materials in a layer beneath the root zone. Plant in a raised bed and do not mound the soil up around the crown. Mulching with tree bark provides biological control. The mulch must be applied to a depth of two inches and reapplied as it decomposes.

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. Caroline and English Roseum are rhododendron cultivars with some resistance to this disease.

Adapted from Harry A. J. Hoitink, Stephen Nameth and Jim Chatfield, Ohio State University Extension, 2000