Winter burn is a form of damage to the leaves. Thermonasty, a natural phenomenon, is leaf movement in response to cold temperatures.
Evergreen trees and shrubs can have either needle-like or broad leaves, with examples of needle-leaved evergreens including pines, spruces, and firs, while broadleaf evergreens include boxwoods, hollies, and rhododendrons.
Winter burn - is a common problem of evergreens trees and shrubs, especially rhododendrons. Leaves do not actually burn but rather dry up. Leaves turn yellow or brown, and crispy in response to low soil moisture, freezing temperatures and blowing wind. On a windy winter day broadleaf evergreens can become water deficient in a few minutes. These evergreen plants hold on to their foliage in winter, and need tons of moisture to keep their leaves green throughout the season. Since the ground is frozen, plant roots are not able to take up enough water from the soil to replace the lost moisture. Winter injury may occur when water is transpired through plant tissue more quickly than it can be absorbed through the roots. A rapid drop in temperature after a warm and sunny winter day can also cause further injury to the plant.
Sean McNamara, owner of Redding Nursery confirms, “Yes, I’ve seen winter burn damage all over Fairfield County. The extreme drought in the fall, followed by a very cold winter, has caused significant damage to broadleaf evergreens. We definitely need more rain—hopefully, things will recover, but only time will tell. Feeding plants with Holly-tone fertilizer can help, but the most important factor is adequate water.”
Thermonasty - is a form of leaf movement that occurs as a response to changes in temperature. The movement is nondirectional, meaning quick responses that do not involve growth towards a stimulus. When rhododendron leaves become curled and droopy, it’s due to the results of thermonastic movement, a unique adaptation in rhododendrons. These are two distinct, reversible leaf movements responding to different environmental pressures : leaf curling, and changing leaf angle (drooping).
Normally, rhododendron leaves are flat and extend horizontally from the plant. During lower winter temperatures, the nastic movements of rhododendron leaves begin - The significance of winter leaf curling is to reduce the rate of re-warming following leaf freezing. Rhododendron leaves can freeze and thaw multiple times and survive, but only if the thaw is drawn out and gradual. Curling, by reducing the surface area, slows thawing down.
Changing leaf angle in rhododendrons helps to protect the leaves from damage during the winter when sunlight hits the leaves when they are cold, and the plants are dormant and not photosynthesizing.
• Above 40 degrees: leaves are horizontal, open and taking in the sunlight.
• 40 - 34 degrees: the leaves begin to hang downward and dangle, but don't really start to curl yet.
• 33 - 25 degrees: the leaves start to curl inward.
• Below 25 degrees: the leaves wrap up tightly.
Once temperatures rise, the leaves will gradually stretch right back out to their normal position.
Rhododendron care - Proper care during the growing season is a crucial part of keeping rhododendrons alive through the winter.
Soil and Moisture - Rhododendrons require soil that is moist, well-drained, acidic, and rich in organic matter. Since rhododendrons are shallow rooted plants, it is easy for them to dry out due to lack of water, or rot if over-watered. Sandy, silty, or clay soils should be amended before planting by adding peat moss or compost.
Fertilizer - Rhododendrons require little supplemental feeding, especially when the soil is rich in organic matter. Use a fertilizer designed for acid-loving plants, such as Holly-tone, and apply in late winter or early spring. Do not over-fertilize. The American Rhododendron Society advises, “Excessive fertilization can result in damaged roots and leaves, and some rhododendron varieties can be killed with fertilizer.”
Mulching - Rhododendrons have shallow, fibrous roots and benefit from a thin layer of mulch to protect the roots.
Pruning - Rhododendrons rarely require pruning, especially when planted in the proper location. Dead branches can be pruned at any time of year and if growth becomes excessive,the size can be reduced with light pruning; best done soon after flowering.
Conclusion - What is the ongoing outcome for the affected leaves on rhododendrons? Will they uncurl and reverse angle? Will they die? Will they fall off? Will they regrow new leaves?
Sean McNamara’s response, “It remains to be seen. I don’t ever remember a drought so severe in the fall. The rhododendrons continuing to stay curled now that the temperatures are warm is due to the drought. I predict that newly planted rhododendrons and those dealing with other problems may not survive. Most, if they were well established and otherwise healthy, should put out new growth in May and June. Those suffering from very bad winter burn will look unsightly until the new growth emerges. We shall see…”