April: Winter Injury

Temp changeThis past winter’s weather was a roller coaster ride of seasonably cold temperatures in December and January, followed by record warm stretches in February, followed by cold temperatures again for much of March. These extreme temperature fluctuations can injury many woody landscape plants.

The Most Typical Forms of Winter Weather-related Plant Injury

Cold injury

Cold injury is a physical damage to plants due to cold temperatures.  Fruit trees (e.g., apples, pears, cherries, plums and especially peach and apricot) are particularly prone to this problem.  Plants can suffer from cold injury in the fall if temperatures stay relatively warm and then precipitously plummet, not allowing plants enough time to properly harden off for the winter.  Cold injury can also occur in the middle of the winter if there are extended warm periods where plants break dormancy, followed by cold temperatures that kill actively growing tissue.  Finally cold injury can occur in the spring if plants leaf out and then are exposed to an unexpected frost.  Trees and shrubs with cold injury typically have scattered branches that never leaf out, or scattered branches where leaves emerge and then shrivel, dry up and die.  In extreme cases, cold injury can kill an entire tree or shrub.  Cold injury in apples and pears is often misdiagnosed as fire blight.

Winter burn

Winter burn occurs when trees or shrubs (e.g., yew, Alberta spruce and boxwood) run out of internally stored water and dry out.  Winter burn can occur during the winter, but most often is noticeable as trees and shrubs break dormancy.  Browning of needles (e.g., on yew) or bleaching of leaves and stems (e.g., on boxwood) are characteristic symptoms.  Winter burn can affect just a few branches (which in some cases eventually recover) or an entire plant (which in some cases may die).

How to Minimize the Effects of Winter Injury

Choose hardy plants

Consult the USDA plant hardiness zone map to determine your hardiness zone and select plants that are rated to survive in your climate.

Plant marginally hardy plants in protected areas

You can still try to grow your favorite marginally hardy trees and shrubs, but plant these near your house, particularly in sheltered areas that may be slightly warmer than the rest of your yard.

Plant at the right time of year

Plant in the spring or late summer (August or September) to allow for root growth before the onset of winter and to avoid hot mid-summer temperatures that can be stressful on new transplants.

Mulch properly

Mulch around trees and shrubs out to their driplines, keeping mulch at least three inches from the bases of plants.  Use one to two inches of a high quality mulch (e.g., shredded oak bark mulch) on heavier, clay soils and three to four inches on lighter, sandier soils.  Proper mulching can help insulate roots over the winter.

Water properly

It is particularly important to water evergreens into the fall up until the ground freezes or there is a significant snowfall.  This will help prevent winter burn.  Established trees and shrubs require approximately one inch of water per week; newly transplanted trees and shrubs requite up to two inches of water per week.  If supplemental watering is needed, use a soaker or drip hose; Do not use a sprinkler.

Fertilize properly

Do not fertilize trees and shrubs in late summer and fall.  Fertilizing late stimulates growth that is more likely to be injured as temperatures drop in the fall.

Prune properly

Avoid pruning in later summer and fall.  Late pruning on some trees and shrubs can stimulate growth that is more likely to be injured by cold fall temperatures.

Protect plant for the winter

Build barriers (e.g., of burlap or canvas) around sensitive plants to deflect and reduce the drying effects of winter winds.

For more information on winter injury/winter burn and additional specifics on how to prevent winter injury/winter burn from being a problem, see the University of Wisconsin Garden Facts “Winter Burn” available at https://pddc.wisc.edu/fact-sheet-listing-all/.