Recent cold temperatures and plants

February 8, 2019

Recent cold temperatures have some people worried about their landscape plants. In general, uniformly cold temperatures will not damage dormant and hardy plants.

Low temperature injury is more often due to extreme temperature fluctuations than to prolonged cold weather. However, if a plant is marginally hardy or highly stressed, it could be damaged.

Most of northeast Nebraska is in plant hardiness zone 4 or 5, now referred to as zones 5a and 5b. The USDA hardiness zone map is used by growers and gardeners to select trees, shrubs and perennials that will survive winter temperatures of an area.

Plants hardy to zone 5a will tolerate temperatures down to -20 degrees. Plants hardy to zone 5b tolerate temperatures down to -15 degrees. Landscape plants that are fully dormant and hardy to these zones should have no problems with recent temperatures.

If a plant is marginally hardy, such as zone 6 or higher, the plant may have been damaged by slightly below zero temperatures. If these plants were protected by a layer of winter mulch, as recommended for marginally hardy plants, they should be fine.

If you buy plants from a reputable retailer, they carry plants hardy to an area. If you buy elsewhere or order plants on line, be sure to check what hardiness zone the plant fits into. This information is typically listed on plant tags or with plant descriptions.

When an otherwise hardy plant is injured by cold temperatures, this is most likely to happen during extreme temperature fluctuations. We tend to see cold temperature injury during late fall or late winter more so than mid-winter.

For example, if autumn temperatures are warmer than average and then there is a sudden and extreme drop in temperature, when a plant is not yet fully dormant, injury can occur even on a hardy plant.

In late winter, we sometimes have unseasonably warm temperatures for an extended period. This may cause a plant to break dormancy too early and be injured by subsequent cold temperatures.

By mid-winter, most plants are fully acclimated and dormant and we do not have long periods of warm temperatures to cause a dormancy break. Hence, cold temperatures at this time of year have rarely been a problem for dormant plants hardy to zones 5a or 5b.

Frost cracks on tree trunks are something cold temperatures might lead to. These are shallow to deep vertical cracks that are most likely to appear when winter temperatures drop below 15 degrees.

Frost cracks are typically seen on the south or southwest sides of trees as this area experiences the greatest temperature fluctuations between day and night.

The sun heats the trunk during the day; then a sudden drop in temperature at night causes the outer wood layer to contract more rapidly than the inner layer, which may cause a long vertical crack at weak points on the trunk.

Once a frost crack occurs on a tree, it is likely to appear annually. Fortunately, they are not of much concern. If frost cracks are seen, nothing needs to be done.

Do not treat the area with a wound dressing or tree paint or cover it with tree wrap. As a rule, frost cracks will not harm trees, but covering a tree wound of any kind can lead to other issues such as wood decay.

Kelly Feehan is a community environment educator for Nebraska Extension-Platte County.

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