Trees Need Attention to Prevent Winter Injury
Winter injury to trees and shrubs is caused by extreme cold temperatures, temperature oscillations, wind, accumulating snow and ice, road salt, and wildlife. Damage may not be visible until spring or even months to years after the injury occurred. Minimizing winter injury is important for overall tree health.
Desiccation, or the drying-out of plant tissues, occurs frequently on evergreen trees and shrubs in Utah. During warm winter periods, evergreen plants will transpire through their leaves which requires available water in the soil. If the soil is dry or frozen, the water demands of the plant will not be met and leaves become scorched. Desiccation is worse in wind and intense sunlight, which increase transpiration. Leaf scorch is most prevalent on sides of the plant that face prevailing winds or on south and southwest sides of the plant.
To minimize winter desiccation injury on evergreens, use the following techniques:
- shield plants from wind and intense light with burlap or fencing
- do not plant trees close to highly reflective surfaces like light-colored walls
- water plants properly (click here for more information), including during warm spells in winter
- anti-desiccant sprays are available, but may not be very effective and need frequent re-application
Trees and shrubs can be injured by exposure to extreme temperatures in spring, fall, or winter. Low-temperature injury to plants includes injury or death of flower and terminal buds, leaves/needles, cambial tissue, exposed roots, or plant parts that have not adequately hardened off in fall. Severe damage to cambial tissue becomes apparent in late spring, where the plant leafs out normally, but dies after stored sugars are depleted. Tissues damaged by cold temperatures appear brown to black, water soaked, or dehydrated. If cold injury is suspected, wait until late spring to prune dead wood so the extent of damage can be accurately assessed.
The following steps can minimize cold injury:
- use plants rated for your hardiness zone and particular microclimate
- avoid fertilizing plants after mid to late summer
- avoid late-season pruning that promotes succulent growth
- water plants properly in the summer, fall, and winter to minimize drought stress
- mulch around plants that have tender or shallow root systems
Perhaps the most common winter injury that occurs to tree bark in Utah is sunscald. Sunscald is bark death caused by exposure to direct and snow-reflected light, typically on the southwest side of trunk and scaffold branches. On sunny days, bark and cambial cells heat up and break dormancy, and are then frozen and killed after a night of severe cold. The dead tissue may not flake off for a few months to a year later, exposing the wound and surrounding callous tissues. Young and thin-barked trees are most susceptible to this kind of damage.
The best prevention for sunscald injury is to wrap the tree trunk from the lower limbs to the ground with white tree wrap from December to April, or use full strength or a 1:1 ratio of latex paint and water.
Freeze and thaw cycles can cause new transplants or plants with poorly developed root systems to heave, exposing roots to drying and cold exposure. Mulch around plants to buffer soil temperatures from the air and replant heaved plants as soon as possible in the spring.
Snow and Ice Breakage
Accumulations of snow and ice on tree and shrub branches can lead to breakage. This damage is most pronounced in the fall when leaves are still on the plants. Plants located partially under roofs, leaky gutters, and other plants can receive extra loads of snow and ice. In rare instances, highly saturated soil coupled with a heavy snow or ice load can cause plants to uproot and fall over.
To prevent breakage, prune to reduce surface area where ice and snow can accumulate. Prop up branches that have heavy loads or that may be susceptible to breaking. Tie up branches of smaller trees and shrubs for the winter so that snow will slide off. Clear snow off limbs by pushing up gently on the branches with a broom. If breakage has occurred, remove damaged branches to allow proper healing.
A rapid drop in temperatures can sometimes cause tree xylem to crack, creating vertical splits in the tree stem. This is caused from a temperature differential in the inner and outer layers of the xylem. The outer layers become cold much faster than the inner layers and contract, causing the xylem to split. This is especially true of trees that already have wounds or had a frost crack previously. Because this damage occurs from low temperatures it is difficult to manage.
The following can help reduce the chance of frost cracking:
- create a mulch zone around trees and shrubs to avoid injury from mowers and weed trimmers
- brace large frost cracks in winter to allow healing
- monitor cracks for the presence of pathogens and/or insects
- choose hardy plant species
One commonly overlooked problem is damage caused by animals, in particular mice, voles, and rabbits. Damage most frequently occurs where snowpack forms around tree stems providing protection for rodents who feed on tender bark tissues, girdling the plant. Rabbit feeding damage usually occurs above the snow line. Rodent feeding may be increased where there is excessive mulch or plant material around the trunk.
To minimize animal injury:
- wrap trunk and low branches with screen wire, hardware cloth, or plastic tubing, and bury the barrier below soil level and extend to above rabbit height and estimated snow line
- create a turf and weed free barrier around trees; reduce mulch thickness
- place rodenticides in rodent runways
- trap and remove rodents and rabbits
-Ryan Davis, Arthropod Diagnostician
Winter Injury. 2012. Cornell University, Department of Plant Pathology and Plant-Microbe Biology, Plant Disease Diagnostic Clinic.
Pecknold, P.C. 2001. Winter Injury of Ornamentals. Purdue University, Cooperative Extension Service. BP-2-W.
Beckerman, J. 2001. Winter Injury on Trees. University of Minnesota, Yard & Garden Brief.