Winter Injury on your trees and shrubs.

Winter injury includes winter desiccation (drying), sunscald and cold-temperature damage. Springtime needle discoloration on previously healthy evergreens is often a symptom of some form of winter injury. Winter damage can adversely affect the appearance and growth of highly prized evergreens. Careful pruning will be required to restore their shape and form.

Winter damage has several symptoms, depending on the species of ever-green and the severity of the damage. Cedar leaf scales fade from green to light tan or reddish-brown. Needle tips of spruce and pine turn brown and become dry. Winter damage may occur on a few branches, at the tree top only, on one side only, or on the entire tree. Severe winter injury may cause the loss of most of the needles, and the plant can die.

Winter hardiness is influenced by a number of factors:

• plant variety
• soil drainage
• geographic location
• on-site location
• environmental conditions of the previous summer and fall 

A warm fall followed by abnormally low temperatures in October or November may result in improperly hardened plants, which are susceptible to freezing injury. Late-season nitrogen application will delay fall maturity and will increase susceptibility to winter injury.

Winter drying of needles will occur in abnormally warm winters because moisture is continuously lost and cannot be replaced as the plants are dormant. On warm, sunny winter days, radiation from the sun or reflection from snow and light-coloured buildings can increase leaf temperatures to 20C° over air temperatures. The moisture in the stems and branches becomes exhausted because of the increased loss triggered by the warmer temperatures.

Dry soils are more likely to predispose roots to damage than soils that contain a good moisture supply. Root injury may be worse during winters with little snowfall. Winter root damage may not be noticed until the following summer when the plants suddenly turn brown and die.

Reducing winter injury 

Winter injury to evergreens can be prevented or minimized by using a few precautions:

Use hardy plant varieties recommended for the specific horticultural zones of the province.

Avoid planting trees and shrubs near light-coloured or reflective structures. Damage is usually reduced in sites protected from the wind, especially in the chinook zone of southern Alberta.

Do not apply nitrogen fertilizers to woody plant material between July and the time of leaf drop of deciduous trees.

Water evergreens in the fall, after deciduous trees have lost their leaves, to ensure that the plants have sufficient moisture in the root zone to prevent freeze drying. Let a small stream of water flow under the drip line for several hours. Repeat this watering early in the spring, once the ground thaws. The importance of adequate fall watering cannot be overemphasized, since water applied in the fall is much more beneficial than any benefit of water applied in the winter.

Evergreens on the south and west side of buildings, especially under an overhang, should be well-watered in the fall because they are very susceptible to winter injury.

Erect canvas, burlap or slatted screens on the south and west sides of exposed small evergreens to prevent desiccation (drying). This protection will shade the plants and prevent excessive moisture loss by the wind. Screens should be about one foot away from the plant material.

Wrapping evergreens with burlap or plastic is not recommended because on warm sunny days throughout the winter, the internal temperature will get too high. This high temperature may cause warmed tissue to be damaged by the severe cold that follows. Plants wrapped this way may also break dormancy too early in the spring.