Minnesota winters may be as stressful on trees and shrubs as they are on humans. Plants often suffer from root or stem damage, frost cracks, winter browning, die-back, bud death and occasionally, complete death. This message explains what to look for, what to do once a plant has winter damage, and what to do to prevent damage.

Root and stem damage may be noticed by normal leafing out followed by sudden leaf and shoot death soon after warm weather occurs. Occasionally stems never leaf out. Shrubs might be renewed through new stems coming up from the base. Specifically to reduce root damage, keep plants healthy throughout the growing season, then mulch over their root system with 3 to 4 inches of wood chips to help prevent soil from getting too cold and freezing too deep.

Stem frost cankers (a.k.a., sunscald) and bark cracks can occur anywhere on the stems but most often on the south, west and southwest sides of smooth barked trees and shrubs. Young, recently transplanted and drought-stressed trees are most vulnerable. With sunscald, extreme temperature changes to the bark in late winter (March, April) can result in death of the cambium layer (the thin layer of living tissue right under the bark), followed by a sloughing off of outer bark. Water-stressed trees exposed to extremely cold temperatures in mid-winter may suffer vertical frost cracks. Vertical frost cracks (cracks that go beyond the bark and into the wood) almost always start at old wounds from "flush-cut" pruning, damage from critters (e.g., rabbits) or mechanical equipment such as lawnmowers.

Protection

Don't rely on snow to insulate the ground from cold temperatures; embrace organic mulches in autumn. Avoid planting sensitive plants in full southern or northwestern exposures. These exposures can either "trick" plants into coming out of dormancy too early in the spring, or they suffer from drying, cold, winter winds. Stem protection such as wrapping young trees in the fall may help some plants. Promptly remove this in spring. Light-colored, loose stem protectants are preferred. This is strictly a physical protection from thrown snow, sand, or ice. No research definitively shows that wrapping prevents frost cankers, frost cracks or critter damage in Minnesota. Stem protection during growing seasons helps reduce frost cracks by reducing accidental stem wounds (e.g., lawnmowers or string trimmers hitting the stems).

Keep plants healthy - in particular, avoid drought stress - during the growing season, right up to late autumn. Plants that are stressed from drought, deicing salts, diseases, insect pests or from being grossly removed from their native hardiness regions always suffer more winter damage than other plants. Mulch and irrigate through late summer, and correctly prune and control pests during plants' formative growing years.

Browning of evergreens, such as arborvitae and yews, may not show up until May or June. Tolerate brown foliage until new growth occurs. Then minimally prune plants. Follow the above plant health care practices to reduce browning. Excess snow and ice affects plants in different ways. Trees bent over from heavy snow should return to their normal shape, so leave snow alone or carefully remove it with a broom. Do not try to remove ice; branches may be trimmed back if the ice breaks them. Neither snow nor ice normally kills plants; they only temporarily disfigure them. Restorative pruning in the spring eventually allows plants to regain their characteristic forms. To reduce risks of winter injury, plant healthy, hardy trees and shrubs in well-prepared sites. For more information visit Forest Resources Extension web site.

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