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Preventing and Correcting Winter Injury to Trees and Shrubs

Winter is fast approaching and you should start thinking about preparing your trees, shrubs and gardens to survive the winter. The past two winters were very cold and I notice a lot of damage to ornamental plantings in various yards.

Extreme cold, drying winds, road salts, or a sudden drop in temperature are common causes of winter injury to trees and shrubs. The severity of winter damage is determined by a number of factors including the plant species and where the plant is located when the worst winter conditions occur during the period of dormancy.

Buds and branches that die is usually a symptom of low temperatures. A tree or shrub that leafs out too early can also be damaged or killed by cold weather. Late frosts can happen and this can affect tender leaves and flowers. Evergreens that have needles that turn brown, especially on the windward side, probably developed winter desiccation or drying out. Damaged and broken limbs is a sign of snow and ice breakage. The base of the tree or shrub can have the bark stripped off or damaged. This is a sign of animal damage. Another cause of this damage is using a weed whacker too close to the trunk. I have seen this a lot. Don't strip off the bark when you are using these machines to control other undesirable growth. Trees and shrubs can get sunburned also, especially on the southwest side of the trunk.

To help recognize the typical symptoms and likely cause(s) of winter injury, read on:

Fall Protective Measures

Proper Planning

When you choose which ornamental trees to plant, make sure that they are zoned to grow in your area. In northern Maine, there is a difference from one end of the county to the other. Choose the right area to plant in and plant according to instructions provided with the tree or shrub. Don't plant something calling for full sunshine in a shady spot. Make sure the soil type is the type the shrub prefers. Mulch the shrub and be sure to protect or screen the shrub if necessary. Dig the hole deep enough but not too deep so that you bury the trunk. I like to add good soil or miracle grow potting soil mixed in the with native soil to help improve it.

Low Temperature Injury

This type of damage occurs with any of the following winter conditions

1. Winter temperatures are much below normal or temperatures are below normal in the early fall or late spring

2. Winter temperatures fluctuate during the dormant period so that dormancy is broken and plant tissues are damaged.

To Prevent Low Temperature Injury:

1. Select trees and shrubs that are hardy in your area. Many varieties of ornamental shrubs sold around here are marginal in winter hardiness and may be subject to winter damage in Maine.

2. Harden off plants early in the year so that they are ready for winter. This refers to the tree or shrub developing buds it will use next year. Avoid late summer fertilization or pruning which may stimulate new growth. Do not fertilize trees or shrubs after mid-July.

3. Avoid planting most conifers and evergreen broad leaf plants such as rhododendron in protected southern exposures or areas where they are exposed to winter winds.

4. Water trees and shrubs during dry periods in the summer and before freeze-up in the fall or early winter if the soil is dry. Trees and shrubs withstand severe winter temperatures much better if soil moisture is adequate. A mulch of wood chips, sawdust, leaves or straw applied several inches thick over the root zone after soil temperatures are below 40 to 45 degrees F will conserve soil moisture and help protect the root system from winter damage.

Winter Desiccation (drying)

Dormant trees and shrubs, especially evergreens, continue to lose moisture even in the coldest winter periods. Evergreens continue to photosynthesize later into the year and start earlier than shrubs and trees with leaves. The rate of water loss increases with fluctuating temperatures, drying winds, and full sunlight. When the ground is excessively dry or frozen, water is lost faster from the buds, young bark, or foliage of evergreens than it can be replaced from the roots. As a result, drying or desiccation of tissues occurs with the death of foliage, buds, or even the cambium. Damage is usually more severe with plants in exposed sites or in areas where temperatures fluctuate during the winter. The south facing wall of your house probably experiences this fluctuating temperature.

Apply water and mulch when necessary. For small trees or shrubs, place screens made of canvas, burlap, plastic or wood slats on the exposed south and west sides to reduce winter desiccation. The screens should be at least 2 feet from the tree or shrub and anchored securely.

Winter Sunburn or Sunscald

Newly planted, thin-barked trees, such as the maples, ash, and crabapples may be damaged in winter when the bark surfaces are warmed by a bright winter sun and then chilled rapidly when temperatures drop sharply at night. This type of injury characteristically occurs on the south and west sides of the trunk.

Winter sunburn injuries can be prevented by wrapping the trunk of newly transplanted trees with burlap or other tree wrapping materials. The wrap should be kept in place for at least 2 years. You may also want to spray the bark with an insecticide to prevent damage by bark borers.

Snow and Ice Damage

Evergreens and other shrubs with multiple stems are often damaged by heavy accumulations of snow or ice. I had some of this damage appear on my shrubs this year. They are getting larger and taller and less capable of supporting the snow. Wet heavy snow can stick to the shrub and weigh it down.

Evergreens and shrubs can be protected from snow and ice by tying the branches together with strong rope or twine. You can also prop up susceptible branches that you think may break. If ice accumulates on the shrubs, don't try to break it off or you can cause more damage.

If severe branch breakage occurs because of heavy ice or snow, the branch stubs should be pruned back to the main stem to promote rapid healing before growth resumes in the spring. Just don't do it too early and don't prune so close that you damage the trunk.

Animal Damage

Mice and rabbits often damage young trees in the winter by feeding on the bark and girdling the tree. Damage from these rodents occurs most commonly in winters when there is prolonged heavy snow cover and food is scarce. Rabbits feed on the bark above the snow, while mice tunnel under the snow and feed near the ground level. Mouse damage is usually more severe when the trees are surrounded by heavy grass, weed cover or heavy mulch.

The most effective deterrent to girdling by mice or rabbits is to wrap the trunk and low branches of young trees with screen wire or hardware cloth. Be certain the wire wrap is buried below the ground line and extends high enough above the possible snow line to prevent rabbits from reaching to the trunk or branches. To help control mouse damage, maintain an area free of grass or weeds for a 1- to 2-foot radius around the base of the tree.

Damage Control

If winter injury occurs, the following steps will reduce permanent damage to the trees or shrubs involved:

1. Wait until the threat of late spring freezes are over before pruning damaged branches. Don't damage the trunk. Prune at the branch collar. See the information of the main part of this site for proper pruning techniques.

2. Give the trees or shrubs a boost by fertilizing them. Use a complete fertilizer like 10-6-4 and spread under the drip edge. The drip edge is the furthest extent of the where the branches are growing. Follow manufacturers directions for proper application rate.

3. If the tree or shrub was damaged, water it in the dry periods of summer. Apply enough water so that is can soak into the ground deeply and water all sections of the roots.

4. If some branches die after the tree or shrub starts to grow, prune these branches out.

Next week (8/23/15): Selective Timber Harvests: What you need to know.

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