Winter injury of rhododendron is a common problem. It is due to a combination of moisture, stress and cold temperatures. It is more common in winters when the weather is warm. The plants lose water through the leaves when the air is warm or there is high wind. The plant is not able to replace lost water because the soil water is frozen. Many rhododendron species protect themselves by turning their leaves downward and rolling them parallel to the midvein when it is extremely cold. This protects them by decreasing the area exposed to the sun, which keeps the leaves from warming up and losing water.
Newly planted young plants are more susceptible to winter injury, as they may not have been completely acclimated, dormant, or cold-tolerant before transplanting. Any roots injured during planting will not have had a chance to regenerate before the cold temperatures occurred. Also, if the potting soil used is more porous than the natural soil, it will dry out faster. If the plant is root bound at the time of transplanting, the roots will have a difficult time growing away from the root ball.
The most common symptom of winter injury is bud kill. From the outside, the buds look normal, but if the buds are cut open, they are brown or black inside. Leaf scorch is another common and much more noticeable symptom. The leaves have brown areas, usually either along edges, or a single area that covers the entire tip end of the leaf. It can affect the whole leaf. Often, fungi will grow in the dead parts of the leaf.
In years with a mild winter that is followed by cold snap in early spring, the plant is not completely dormant. In this case, bark splitting can occur. If this is severe, it can partially or completely girdle the plant. If this happens, the affected branch or branches wilt and die in the following season. This injury also predisposes the plants to attack by fungi, such as Botrytis cinerea or Botryosphaeria dothidea.
Container grown plants should not be pot-bound at the time of planting. Plant only in acidic (pH 5.0-5.5) soil. Do not fertilize later than June, as excessive growth later in the season encourages the plant to delay dormancy. Water during summer and fall dry periods to keep the plant healthy. Plant in loose, well-drained soil to encourage root regeneration after any planting injury. It is important to select varieties adapted to the area. Many hybrids are from crosses between tropical and semi-tropical species and are not as cold tolerant as other species.
Shear, G.M. Winter Injury. 1986. in Compendium of Rhododendron and Azalea Diseases. pp. 45-46. D. L. Coyier and M. K. Roane, eds. APS Press, St. Paul, MN.
Sinclair, W. A., H. H. Lyon, W. T. Johnson. 1987. Diseases of Trees and Shrubs. Comstock Publishing Associates, Ithaca, New York.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998
Reviewed by: Mary Concklin, IPM. 2011
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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