It appears that our love affair with past mild winters is over. Near zero or colder temperatures are normal for the Kansas City area. Heavy snowfall is also common, as we were recently reminded. People have been asking about the effect of the cold temperatures on landscape plants.
The good news is that most landscape plants will tolerate the recent cold spell, and even colder temperatures. There is a greater concern when we experience rapid temperature changes in the late fall or spring. Damage is more common when the plants are not fully hardened off, prepared for winter or have begun the spring flush of growth.
For example, a sudden cold snap last fall resulted in a poor leaf drop from the trees due to the rapid change. I have to admit I am concerned that we may see dieback from this event on some plant materials that were not probably hardened for winter.
Wind chill is a human measure and means nothing to plants. Winter winds can have a drying, burning effect but not the wind chill temperature.
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Plants are rated for their absolute winter temperature tolerance based on plant hardiness zones. The USDA Plant Hardiness Zone Map classifies trees, shrubs and flowers by the lowest temperature they will survive. Hardiness zones range from 13 to 1. Zone 13 is the warm and tropical islands of Puerto Rico and Hawaii, while Zone 1 is the tundra of Alaska.
Kansas City is located in Zone 6b. Plant materials hardy in our zone will survive minimum winter lows of -10 degrees to -5 degrees. While the winter temps have been cold, we’ve stayed within our hardiness zone.
Plants with roots that are well covered or properly mulched should be acclimated to winter temperatures and have no problem. We often try to push the limits on plant materials. Two common plants grown here that might be a little out of their zone are crape myrtle and mimosa. Expect winter injury when the temperatures drop below zero. Many of these plants are root hardy, dying back to the ground and regenerating from the roots in late spring to renew the plant.
One condition that can affect winter hardiness is the moisture level in the soil. Dry soil conditions decrease winter hardiness and increase desiccation. Evergreens and broadleaf evergreens such as boxwood and holly can suffer. Continual loss of water during the winter can be serious if dry conditions prevail. Damage results in a browning or killing of the foliage. Fortunately, we have had ample moisture starting last fall and throughout this winter. It might be, in fact, that we’ve had too much of a good thing.
If you’ve landscaped using plants designated for our hardiness zone, then they should be just fine. Genetically they are prepared to survive Mother Nature’s fickle fluctuations. But then, there are no guarantees. We’ll just have to cross our fingers and wait and see.
Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to firstname.lastname@example.org.