Cold Temperatures Can Damage Plants
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
How cold can it get before plants suffer cold temperature injury? It’s important to keep in mind that a plant’s potential hardiness is genetically determined. Plants that are native to a geographic region have evolved in response to that area’s climate and weather patterns. It can be thought of as “survival of the fittest”... the hardy plants that can survive the winter weather of a region produce offspring that are also hardy. Through this natural selection process, native plants are usually able to survive the winter cold of their particular native region.
However, a hardy plant in the middle of summer can’t withstand the same amount of cold that it can in mid-winter. This is because hardiness to cold develops as a series of physiological changes within the plants. The first change to occur is a response to the shortening days of late summer and early fall. Plant hormones are produced in response to the shorter days. These tell the plant that winter is coming and plant tissues develop greater resistance to damage from freezing temperatures.
As fall proceeds and temperatures become increasingly cooler, the plants develop even greater hardiness. The rate at which plants develop hardiness varies from species to species. The rate also is dependent on the degree of cooling that occurs in the fall. If fall temperatures remain mild, plants may fail to acclimate fully to cold temperatures. Because our late fall and early winter has been so mild, it’s very possible that some plants sustained some cold temperature damage from the recent deep freeze.
At some time in the middle of winter, plants develop their ultimate mid-winter hardiness. This point is genetically determined. It’s interesting to note the part that genetics and geographics play in plant hardiness. The ultimate mid-winter hardiness can be quite variable even within the same species. For example, Douglas firs that evolved in the Rocky Mountains are hardier than Douglas firs that evolved in the Cascades. Similarly, a flowering dogwood that is native to the New York state region is hardier than one from the Florida or Georgia regions. In both cases the plants are exactly the same genus and species, they just evolved in a different climatic region.
After the plants receive their ultimate mid-winter hardiness they begin to deacclimate or lose some of their hardiness. Like the acclimation process, deacclimation is usually gradual. However, deacclimation can occur quite rapidly during an extended warm spell. Plants have the potential to acclimate to colder temperatures again and again when temperatures drop, but they lose this potential as spring approaches and growth begins.
Many gardeners ask how they can protect their plants from cold temperatures. Plants aren’t “warm blooded” creatures. Putting blankets around a tree trunk won’t keep them warm. Blankets can’t help, but other things can be done to provide a measure of protection:
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Select plants that are hardy for the local climate. Nurserymen will indicate a plant’s hardiness by noting the USDA hardiness zones for which it’s suitable. The zone for the Tri-Cities area is Zone 6. Gardeners close to the river or in the Walla Walla area might be able to push it to Zone 7. You may see “borderline hardy” plants in area landscapes that do well here for several years when the winters are relatively mild winters, but then they succumb to a particularly cold winter. That’s because they’re not truly hardy for this area.
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When planting conifers (evergreens) in the landscape, situate them to limit their exposure to sun and wind. If you live in a particularly exposed location where there is no “protected location,” plant a windbreak to protect them in winter months.
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Keep plants in a healthy condition. Healthy plants are better able to withstand the rigors of winter. Stressed plants are more susceptible to winter injury.
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Plants that are actively growing late in the season fail to acclimate as winter approaches. Avoid fertilizing, pruning, or irrigating excessively towards the end of the summer and in early fall. Keep in mind that the fall fertilizer that you put on the lawn may well be reaching tree roots and encouraging late season growth. Try to avoid fertilizing trees and shrubs in the fall, especially those that are not extremely winter hardy or ones prone to winter injury.
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While you don’t want to water plants excessively and encourage late season growth, it’s also important to note that water stressed plants are less hardy. Don’t let your plants go into winter with dry soil.
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Mulch the roots and crowns of tender plants, plants prone to winter injury, and recently planted trees, shrubs, and perennials. Use loose mulch, such as finely shredded bark, that won’t mat down or exclude air from the roots. Roses will usually benefit from an application of bark to the crown to protect them from cold and dessicating winds.
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Protect the trunks from sunscald. Sunscald damage happens when bark surfaces on the south and west sides of a tree are warmed by bright winter sun and then the temperature drops rapidly when the sun goes down. This abrupt temperature change can damage the bark and cambium underneath. Newly planted trees; trees with dark bark, such as cherries; or young trees with thin bark, such as maples, ash, crab apples, and tulip trees are particularly susceptible to this type of injury. Wrap the tree trunk with a commercial trunk wrap or paint it with an inexpensive interior white latex paint to reflect the sun and prevent damage. Bark wraps should be removed in the spring.