Winter Stresses on Trees and Shrubs

Winter 2000/2001 has been the coldest and harshest winter in many years. Several conifer samples showing symptoms of winter browning have come into the Iowa State University Plant Disease Clinic. Winter conditions like this year's can cause damage in several ways. Some symptoms you might see are needle browning (sometimes yellowing), cracking or death of the bark and animal feeding.

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 winter months. Damage may also appear on the windward side, which may be to the north. Dry conditions during the summer months of 1999 and 2000 may also be a contributing factor to damage we are seeing this season. 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 for 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.

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 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. If the mesh hardware cloth is to be left on all year long, it must be large enough to allow for tree growth. Chicken wire around shrubs 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.

When spring finally arrives and plants begin to resume growth, provide adequate moisture and balanced fertilization. You may need a soil test to determine if there is a nutrient deficiency. Most important of all, enjoy the season of new life!

This article originally appeared in the March 9, 2001 issue, pp. 22-23.


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