Winter Injury to Trees and Shrubs

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Some of us are seeing damage to our evergreen plants. In many instances the needles or leaves have yellowing, bronzing or have turned brown and whole branches look dead. Much of this damage likely is the result of winter injury.

Since evergreens keep their foliage they continue the normal transpiration process and lose water through their leaves. On warm days during the winter, quite a bit of moisture can move out of the plant. If the plant roots can’t find moisture due to frozen ground, or just dry overall conditions – especially if it’s windy, then that water can’t be replaced, our plants dry out and desiccate as a result of tissue death caused by the water in the needles/leaves leaving faster than the plant can replace it. Winter scorched needles eventually drop off. It’s why watering your plants during the wintertime is recommended. Broad-leaved evergreens, such as hollies, often have marginal leaf burn. Reduced flowering is common during the following season. A 2 to 3 inch mulch layer will also help reduce water stress on the plant.

Did you use de-icing salt on your drive and walkways is past season? The most common de-icing product is sodium chloride—ordinary table salt or rock salt. Salt is corrosive and can not only damage concrete, but sodium and chloride are both toxic to plants in high concentrations. De-icing salt can build up on leaves, resulting in leaf scorch much like other winter damage and may kill buds and branch terminals. Salt applied to a walk or driveway will wash off and into the surrounding soil, building up over time and causing a number of problems in the soil’s ability to provide life supporting functions for plants and animals. Lawn areas, especially along walk or drive edges, die off and can become bare or weedy.

Trunk cracking is another type of winter injury that usually is seen on the southwest side of tree. Frost cracks occur when temperature fluctuations are severe. Water in the cells of the tree trunk freezes and moves out of the cells, causing the wood to shrink. Tension between the frozen and unfrozen layers of wood is so great that the wood separates, causing a crack.

Another cold weather injury to look for is sunscald. In this case the sun heats up one side of the tree on cold days. Tree bark warmed by the sun in winter can be as much as 18 degrees warmer than the air temperature. Once the sun goes down the trunk rapidly cools down and the cambium layer beneath is damaged. The tree cells can be killed. Plants with thicker bark that freezes slowly and thaws slowly will be damaged the least.

It’s still early spring and we’re not “out of the woods yet” in terms of a severe cold weather event. If cold temperatures occur later in the year (March or April) many plant parts are more susceptible to temperatures below freezing. These freezes are particularly damaging if they are preceded by a warm spell when flower buds are forming.

If you find winter injury to your plants this spring, all may not be lost. After the last chance of frost is past in mid April, fertilize the plant as it is beginning to grow new tissue. Many plants, especially if they’re well established and are of a sturdy species will fill back in damaged areas. If there is greenwood under the bark up the branch, cut the damaged wood back to the first healthy bud. and trim up dead cracked areas on the trunk. A general rule is not to prune a plant by more than a third in any given year. This summer pay special attention to managing other stresses to your plants, such as drought and heat stress by providing adequate water and ideal growing conditions.

Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.