Winter can be tough on trees, both newly planted and established. Winter conditions can cause damage in several ways. Conifers and other evergreen plants are particularly prone to damage over the winter. Often the damage that occurs over winter does not become apparent until the following spring. Some symptoms you might see are needle browning (sometimes yellowing), cracking or death of the bark, and animal feeding.
Winter Browning of Conifers
Winter browning is often referred to as desiccation injury. On sunny, windy days during the winter when temperatures are above freezing, needles lose (transpire) water. If the soil is frozen, the transpired water cannot be replaced and the tree suffers desiccation stress (needle browning and tissue death). However, water loss alone in midwinter is not the only cause of needle browning. A combination of environmental stresses can cause needle browning on conifers. Winter browning may result from the interaction of extreme low temperatures and frequent fluctuations between freezing and thawing. These conditions are usually found on the south or southwest side of conifers since those areas generally receive most of the sunlight during the winter months. Damage may also appear on the windward side, which may be to the north. Dry conditions during the previous summer months may also be a contributing factor to damage because trees went into winter already stressed.
Avoid pruning browned, burned areas from evergreen trees and shrubs in the early spring since these branches may still have viable buds that will produce new foliage when growth resumes. If the buds did not survive, prune dead branches back to living tissue. Some plants may appear to be fine for a while, and then begin to decline when conditions become stressful in the summer. This may be evidence that there was some root damage.
Proper placement of winter injury sensitive plants, such as yew or arborvitae, can reduce the chance of winter damage. Avoid planting them on the south or southwest side of buildings or in exposed areas. Constructing a physical barrier can also protect the plant from the elements.
Sunscald injury may occur on the bark of trees. It is characterized by an elongated sunken, dried, or cracked area of dead bark, usually on the southwest side of a tree. On cold days the sun can heat the bark to the point where that area breaks dormancy. When the sun sets or is shaded by clouds, the active tissue freezes and dies. Young trees, newly planted trees, and trees with thin bark are most susceptible to sunscald. Trunks can be wrapped in the fall with tree wrap or plastic tree guards to prevent sunscald. Remove the protective covering in the spring.
If sunscald damage occurs, treating exposed areas of wood with wound dressings or paints is not advised or recommended. Proper and timely supplemental irrigation remains the best recourse and remedy for trees that have suffered injury to important bark and vascular tissues.
Besides environmental stresses, mice, rabbits, and deer can cause severe damage to plants in the winter. Animals can girdle trees and shrubs and eat shrubs down to the ground when food supplies are low during a harsh winter. Placing a mesh cylinder made of hardware cloth around the trunk can protect trees. The cylinder should extend 2 - 3 inches below the soil line for mice and 18 - 24 inches above the anticipated snow line for rabbit protection. Be sure the mesh cylinder is in place before the first snowfall and remove the cylinder in spring. Do not leave the mesh cylinder on all year. Leaving the metal mesh wrapped around the tree can restrict growth or cause girdling. Chicken wire around trees will protect them from rabbit damage. Deer can be excluded by fencing, but it must be well constructed and high. Repellants may also be used to give woody material an undesirable taste or smell. Animals may not be deterred by repellents if feeding pressure is high. Supplying animals with supplemental food may be effective in preventing damage to plants. However, be aware that this method may attract more animals to the area.
Do not treat areas on trees damaged by animals with wound dressing or paints. Provide adequate moisture through the spring and summer and monitor closely as these areas are more likely to see secondary damage from diseases or insects. If the animal damage was extensive enough, especially if the damage fully circles and girdles the tree, it will likely not survive.
Updated from an article that originally appeared in the March 9, 2001 issue of Horticulture and Home Pest News, pp. 22-23.