With Calgary’s variable climate it can sometimes be a challenge to establish and maintain trees and shrubs in the landscape. During the winter months temperature fluctuations can cause winter injuries. Winter injury is a term used to describe several types of plant damage caused by environmental conditions that are prevalent during late fall, winter or spring. Here are some of the more common winter injuries and management practices to help prevent them.
Desiccation - Drying out
Desiccation can cause significant damage especially on evergreens. This is a condition where water leaves the plant (transpirational loss) quicker then it can be taken up by the roots. Many factors can influence desiccation.
- If it has been a dry fall, there may be insufficient moisture in the soil to supply the roots with adequate water.
- With severe cold weather, the ground may freeze to a depth beyond the root system’s reach thus cutting off the supply of water.
- During windy periods and sunny, mild weather water loss is the greatest. The heat from the sun can cause the stomata (pore or opening in the leaf through which gas exchange can occur) to open increasing transpiration (water evaporation through the leaf).
Often you will see discoloured, burned needles or leaves.
Proper watering practices are critical to minimize damage from desiccation. In the fall plants need to be deeply watered - soaking the soil surrounding the root system - prior to the ground freezing. During warm spells in the winter check to see if the ground around trees and shrubs is moist and water if necessary. Applying a layer of mulch will help reduce water loss and help maintain uniform soil moisture around the roots. Placing a protective barrier of burlap over and around plants will help to protect plants from drying winds and heat from the sun. An application of an antidesiccant compound in late fall and mid-winter can also be helpful in the prevention of damage.
Freezing - low temperature injury
- New growth in the early fall produced from late summer fertilization or pruning may not have had time to harden off sufficiently to survive sudden drops to below freezing. Ice crystals rupture cell walls - damage shows up as dead branch tips or branchlets.
- Late spring frosts can injure metabolically active tissue. Death of dormant buds, especially expanding flower buds, can result. Usually leaf buds survive.
Avoid late summer or early fall fertilization as this stimulates growth which is easily killed by cold. Make sure you are pruning correctly and at the correct time of the year. Make sure plants are properly fertilized and watered throughout the growing season. Proper plant care can help minimize injuries from freezing.
Sunscald - sun burn
This occurs when the sun warms the tree bark during the day and then the bark rapidly cools after sunset. These abrupt fluctuations are most common on the south or southwest side of the trunk and branches which can result in killing the inner bark of those areas. Young and thin barked trees are the most susceptible to this type of injury.
Wrap trunks of susceptible trees with protective tree wrap. Planting around the base of susceptible trees will shade the trunk and help prevent injury.
Frost Cracks - southwest disease
In the winter during the day the sun warms the trunk, on the south and southwest side, causing the tissue to expand. Once the sun sets, the temperature drops abruptly causing the tissue to contract. The outer part of the trunk cools faster causing the outer tissue to contract faster then the inner tissues. The difference in contraction rates causes the outer trunk to crack. These cracks may become wider or narrower with temperature fluctuations throughout the winter months. These cracks often close and callus over during the summer months only to open again in subsequent winters. This callusing and recracking may lead to the formation of large frost ribs (seams) on the sides of affected trees. This type of injury is more prevalent in trees that are suffering from drought stress.
Wrap trunks of trees with a tree wrap in the late fall. Large frost ribs (seams) can be braced to prevent reopening during the winter thus facilitating callusing and healing. Ensure trees are thoroughly watered prior to ground freezing in the fall. Try to avoid injuring the bark of trees when they are young. These cracks are an ideal site for the entrance of rot or canker diseases. Affected trees should be checked regularly to ensure they are free of disease.
Snow and Ice Breakage
Heavy snow or ice on weak branches can result in breakage. Careless snow removal and high winds can also lead to broken branches.
Properly timed pruning will effectively reduce the amount of damage. It is very important to remove any weak or narrow angle crotches and any dead or diseased branches. When removing snow from trees or shrubs use a broom. When the branches are frozen (covered in an ice layer) and brittle avoid disturbing them.
Winter injuries can cause severe damage to trees and shrubs. However, by planning ahead damage can be avoided. Here are a few things to keep in mind:
- Select Hardy Plants - grow plant material that is known to be winter hardy in your area.
- Select an Appropriate Site - plants that are susceptible to winter injuries should be sited where they are protected from prevailing winds and intense winter sun.
- Avoid Low Spots and Roof Overhangs. - Low spots create frost pockets which experience rapid fluctuations in temperatures. Heavy snow loads can break branches so plant trees away from roof eaves.
- Promote Healthy Plants - the healthier and happier your plants are the less susceptible they are to winter injuries!
- Use Pruning Paint - protects cuts and breaks from infections and promotes rapid healing.