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Growing Green: Protect Your Trees from Winter Pests

Many of my Spring calls concern trees damaged during the winter by voles.  Bark is chewed, tunnels abound and trees struggle to come back, if they are lucky.  Lawn damage caused by these varmints can be repaired, but trees and shrubs girdled by these gnawing rodents will not survive. By taking some simple steps now, you can protect and save many trees and shrubs next spring.

This has been a year of unusual environmental conditions and it is very possible that we could get season lasting snow on warm soil which will provide the voles with perfect habitat. Voles are small mouse-like rodents that are active through the winter on the surface of the ground underneath the snow and are encouraged by conditions in which there is snow on unfrozen ground.  Once they have inhabited and found food sources in a yard, they are likely to remain all winter.

Because they are food for many predators, voles spend the summer in meadows and other grassy areas where the taller grasses will provide protective cover. In the fall they move to home lawns and may enter homes and other buildings in search of food. Because they are often found in grassy areas, they are often called "meadow mice".  

The most common vole related damage is to grass and ground covers in lawns. They remain active under the snow throughout the winter and as they feed they create runway systems and nests at ground level, often killing the grass.  Voles may also feed on flower bulbs before the ground freezes and they can also cause extensive damage to trees and shrubs by chewing the bark off in a ring around the trunk near the soil line, thereby girdling the plant.  There have been situations where they have completely removed the bark on young trees up to the snowline.

To discourage voles from returning to your lawn to overwinter, keep the lawn mowed regularly until freezing and remove excessive thatch.  You might consider lowering your lawnmower slightly, but do not mow the grass shorter than 2 inches to prevent winter damage to the crowns.  Eliminate any weeds, wild grass or litter from around the lawn that may provide food and cover for voles.  If active runways are present in the lawn, trapping may be effective.  Mouse traps baited with a mixture of peanut butter and rolled oats should be placed at right angles to the runways or in areas where voles are active.

Young trees and shrubs can be protected with cylinders of hardware cloth or tree wrap.  The hardware cloth mesh should be one-fourth inch or less and, because voles can dig, should be buried an inch or two into the soil.  Wrapping the trunks of trees with a commercial tree wrap will also prevent vole damage. It is important that the end of the wrap be placed into the soil and the tree should be wrapped well above the potential snow line.  Actually, the trees should be wrapped until just below the first branch, which will also protect the tree from rabbit damage and sunscald.  It is important that the grass around fruit trees be cut short to eliminate hiding areas for voles and discourage their activity.  

By reducing ground cover with mowing, excluding the voles with ¼ inch wire cylinders around trees and shrubs, and the use of poisonous baits or trapping now, the disappointment that occurs when discovering vole damage in the spring can be minimized.

Robin Trott is a Horticulture Educator with University of Minnesota Extension.