Protecting Trees and Shrubs Through the Cold Winter Months

Written by: Connie Carson (Nursery Yard Manager)

Cold weather can damage and even kill plants. By taking proactive steps, you can help your plants survive the frigid temperatures, desiccating winds, and heavy snow common in our area Pay particular attention to newly planted trees and shrubs for the first two years.

Critical to all trees and shrubs, especially evergreens, is making sure they are well watered before the ground freezes. The soil needs to be moist to the bottom of the root ball - at least 10-12” for shrubs and more for trees depending on the depth of the tree’s root ball. To harden plants off in preparation for winter dormancy, in mid-September begin slowly reducing the amount of water to plants. DO NOT STOP WATERING! A deep soak at the end of October/ beginning of November will help ensure your plants have sufficient water for the winter months. Moisture deficient soils can make plants more susceptible to cold damage. Moist soils hold more heat than dry soil, so frost penetration will be deeper and soil temperatures colder in dry and sandy soils.

Roots are considerably less hardy than stems; they don’t go dormant as quickly as buds, stems and branches and are much more likely to suffer cold damage. This is why watering in the late fall is so critical to plant survivability. Root damage can be further reduced by adding mulch over the root zone after a hard frost. Mulch and snow cover act as insulators for the roots by keeping soil temperatures higher and retaining soil moisture.

Evergreens

Winter sun and wind can cause excessive water loss to the foliage, resulting in desiccation and browning of the plant tissue. Bright, cold days can destroy the chlorophyll in the foliage when the temperatures drop below 28 degrees, resulting in the bleaching of foliage. Leaf damage usually occurs on the south, southwest, or windward side of the plant, but in severe cases, the whole plant can be affected. Yew, arborvitae, hemlock and boxwood are most susceptible, but browning can affect all evergreens.

New transplants or plants with late season growth are particularly sensitive. Plant these evergreens on the north or northeast sides of buildings or in areas protected from wind and winter sun. Consider using burlap wrapping as with barriers to exposed evergreens. Provide additional protection for yellow-leaved evergreens for the first three years. Keep evergreen properly watered throughout the growing season and into fall. Decrease watering slightly in September to encourage hardening off, then do a deep soak at the end of October/ beginning of November, just before the ground freezes. Do not prune after August. Wait until mid-spring before pruning out dead/insured foliage. Fertilize injured plants in the early spring.

Deciduous Shrubs

Deciduous shrubs greatly benefit from pre-winter care. Focus on fall watering and mulching. After leaf drop, clean up and discard any infected and insect-damaged leaves to prevent fungal spores and pests from overwintering in the soil. Water shrubs thoroughly during the fall until the ground freezes. Apply a 2-3 inch layer of mulch around the base of shrubs using bark mulch, soil pep, or ground-up leaves as insulation to protect plant roots. Extend the mulch to the plant’s drip line but avoid piling it next to the truck/low branches. Mulching is especially crucial for roses and an extra foot or more of loose mulch, such as pine needles and leaves, is beneficial for tender cultivars. Erect temporary windbreaks, like a burlap screen, to protect sensitive plants from strong, cold winds. Gently brush off heavy snow accumulation from branches to prevent breakage.

Prune dead or damaged branches before winter to prevent further damage from wind and snow. General pruning should wait until late winter/early spring. Plants have the best opportunity to recover from pruning wounds right before spring’s vigorous growth. Early flowering shrubs, like forsythia and lilacs, should be pruned right after they bloom.

Deciduous Trees

All plantings - new and well established - benefit from a deep soak prior to the ground freezing and may be crucial to its survival. Dry conditions make plant tissue more susceptible to cold damage.

Young trees, newly planted trees, and this-barked trees (cherry, crabapples, honey locust, katsura, linden, maple, mountain ash, plum) are all susceptible to sun-scald and frost cracking. Sunny, warm winter days cause cells of the living tissue in the tree’s trunk to lose dormancy. When temperatures drop at night, the cells freeze, burst, and die. The resulting jagged wound can take a long time to heal. Frost cracks can occur in the trunk and stems of young trees. Warm winter days cause cells to warm and expend. As the evening temperatures decrease, the outer bark cools faster and the inside of the tree remains warmer, which causes toe outer bark to split. Wrap newly planted trees using ligh commercial tree wrap or plastic tree guards for at least 2 winters and thin-barked trees for 5 winters or more. Trees with corky or deeply furrowed bark do not need to be wrapped. Tree guards should go on in November and be removes in the spring once the weather has sttled. Trunk wrapping is for seasonal use and should never be left of the tree year-round.

Reducing Animal Damage

Landscape plants are easy targets when winter food shortages force rodents, rabbits and deer to feed on bark, stems, buds, and leaves. Mice, rabbits, voles, and deer can cause severe injuries, sometimes killing trees and shrubs. Use physical barriers to prevent them from getting to your plants. Protect tree trunks with cylinders of 1/4-inch mesh hardware cloth around the trunk, but about 6 inches away from the trunk. For mice and voles, the cylinder should extend a few inches below ground. For rabbits, the cylinders should be 1.5-2 feet above the anticipated snow line. Check fenced areas frequently to make sure rabbits do not get trapped inside. For small trees, plastic tree guards can be used. For mature trees, protect trucks from antler rub with sturdy metal fencing. Deer and rabbit repellants can help though most need re-application after rain or irrigation. That said, if deer are starving, there is little that will prevent them from feeding as hunger will overcome bad taste. Well-constructed, physical barriers are always the best option.

Salt Damage

Don’t plant trees and shrubs where salt run-off collects or close to streets where salt spray can occur. salt run-off leads to build-up in soils that can damage roots and ultimately the foliage as it has been absorbed by the plant. Use the minimum amounts of salt and only where needed.

Avoid using de-icing salts on walkways and driveways by clearing snow where plants are nearby. Consider using alternative de-icers like calcium chloride or calcium magnesium acetate which are less toxic.

Additional Tips for Care

Keep plants healthy throughout the year. Regularly inspect your plants to catch potential problems early. Check soil moisture routinely to avoid under- or over-watering. Look for and address signs of damaging pest activity or disease. Check for physical damage to your tree’s physical structure and correct as soon as possible.

Maintain healthy soils to benefit the well-being of your plants. Add compost and other organic matter to improve your soil’s fertility and texture. Test your soil for pH and basic nutrients, periodically. Add fertilizers and other amendments to correct nutrient deficiencies.

Properly prune trees and shrubs to improve plant health, increase air circulation, stimulate healthier growth, manage size, improve fruti and flower production, and enhance plant appearance and structure.