Winterizing trees is the proactive process of preparing them to withstand the environmental stresses of the cold season, ensuring their survival and health into the next growing cycle. This preparation focuses on protecting the tree from desiccation, extreme cold, wind damage, and sudden temperature fluctuations. Both deciduous and evergreen trees are susceptible to winter injury, especially when young or newly planted. While mature trees have greater natural resistance, younger trees with thinner bark and less established root systems often require seasonal intervention to prevent long-term damage.
Preparing the Root Zone and Soil
The first step in winter protection focuses on the root zone, which is particularly vulnerable to freezing and drying out. Tree roots require consistent moisture until the ground completely freezes, as they do not enter the same deep dormancy as the above-ground portions of the plant. If the fall season has been dry, deep watering until the soil is saturated is important before the first hard freeze, a practice sometimes called dormant watering. This deep hydration prevents root desiccation, which occurs when roots cannot draw water from frozen soil.
Properly applied mulch serves as a protective insulating layer that helps regulate soil temperature and conserve moisture. A layer of organic mulch, such as shredded wood chips or bark, should be spread 2 to 4 inches deep over the root zone, extending out to the tree’s drip line if possible. It is important to maintain a gap of 2 to 3 inches of bare soil directly around the base of the trunk to prevent moisture retention against the bark. Piling mulch directly against the trunk, often referred to as a “mulch volcano,” can encourage pests, disease, and bark rot.
Managing the tree’s growth cycle through fertilization is also important. Applying nitrogen-rich fertilizer late in the season can stimulate tender new growth that lacks the necessary hardiness to survive winter temperatures. To ensure the tree is fully dormant, avoid late-season fertilization, which directs the tree’s energy toward soft tissue development instead of winter preparation. The insulating effect of mulch also prevents frost heaving, a process where repeated freezing and thawing cycles push shallow roots out of the soil, causing damage.
Protecting Trunks and Limbs from Damage
Once the root zone is addressed, physical protection shifts to the trunk and above-ground structure, guarding against thermal, animal, and weather-related damage. Sunscald is a common winter injury that affects the bark of young, thin-barked trees, especially on the south or southwest side. On sunny winter days, the dark bark absorbs solar radiation, which can warm the underlying dormant cells enough to become active. When the sun sets or is blocked by clouds, the temperature rapidly drops below freezing, killing the active cells and causing the bark to crack and split vertically.
Prevention involves wrapping the trunk with a light-colored material, such as commercial white tree wrap or plastic tree guards, which reflect sunlight and keep the bark temperature stable. White wraps are preferred over dark materials. The wrap should be applied from the base of the tree up to the lowest branches, overlapping slightly with each turn. For newly planted or susceptible thin-barked species, this protection may be needed for two to five winters until the bark develops a rougher texture.
Physical barriers are necessary to deter rodents and other wildlife from damaging the tree by eating the bark, a process known as girdling. A cylinder made of wire mesh or hardware cloth can be placed around the base of the trunk, extending a few inches below the soil line and up to 18 to 24 inches above the anticipated snow line. For trees with multiple leaders or thin branches, heavy snow and ice can cause structural failure or branch breakage. Gently tying together multi-stemmed evergreens with a flexible material provides support and prevents the branches from splaying under weight.
Seasonal Timing and Removal of Protection
The effectiveness of winterizing measures depends on proper timing for both application and removal. Winter preparation should not begin until the tree has entered full dormancy, which typically occurs after the leaves have dropped and the first hard frost has occurred, usually in late fall. Applying protection too early can trap moisture and heat, which may encourage pest activity or fungal issues. Waiting until the ground is fully frozen before applying mulch is advisable, as this prevents rodents from nesting in the warm, newly applied layer.
Removing the protective measures in the spring is just as important as applying them, as leaving them in place can severely harm the tree. Tree wraps and guards should be taken off when the threat of the last hard frost has passed and sustained spring temperatures arrive, typically in mid-April. If wraps are left on too long, they can inhibit the natural expansion of the trunk during the growing season, potentially leading to girdling damage. The warm, moist environment trapped between the guard and the bark also creates an ideal habitat for damaging insects and disease pathogens.
After the protection is removed, a brief post-winter inspection of the tree is beneficial. This check should look for signs of damage that might have occurred despite the winterizing efforts, such as broken branches or areas of missing bark. Clearly broken or damaged limbs should be pruned cleanly, which helps the tree compartmentalize the wound and begin the healing process. The mulch layer, which has served its purpose as an insulator, should be checked to ensure it is not too deep or piled against the trunk as the season progresses.