Will the Cold Temperatures Damage My Plants?

— Written By Peg Godwin and last updated by
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The USDA Plant Hardiness Zone Map is the standard guide used by most gardeners and growers to determine which plants are most likely to thrive in a location. Hardiness zones are based on the average annual extreme minimum temperature during the past 30-year period. It is not based on the lowest temperature that has ever occurred or might occur in the future.

Our area is in Zone 8a indicating that the expected minimum temperatures should range from 10 to 15 degrees Fahrenheit. Plants selected with this label are expected to survive in our recent cold snap.

Some growers enjoy plant material that are different or unusual and choose to select plants not rated for their zone. This means that they could experience a year with extreme cold temperatures and lose plants that have thrived for several years. These materials that are less cold hardy should be located in the highest part of the yard since cold air settles in the lowest area. A fence or evergreen hedge that are tall can offer some protection from cold wind. Plants that slowly freeze and thaw are damaged the least, so provide protection from direct early morning sun. Allow plant growth in late summer to naturally slow down and do not apply nitrogen fertilizer.

Most landscapes contain microclimates which are slight climate variations. These areas include heat islands caused by pavement and concrete or cool spots caused by shade or variation in terrain. A yard may be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed. No hardiness zone map can take the place of the detailed knowledge an experienced gardener picks up about their own garden.

Many plant species gradually acquire cold hardiness in the fall during the shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. Extreme cold temperatures early in the fall may injure plants because the cold hardiness has not been acquired. Likewise, warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants. These variations in temperatures are not a factor in determining a Plant Hardiness Zone.

Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, their size and health also influence their survival. Some studies indicate that in some plants the amount of summer heat affects cold hardiness.

Cold hardiness maps do not take into account other factors that affect a plant’s cold hardiness. Cold temperatures for one night is not the same to the plant as cold temperatures for several weeks. Snow cover can work to insulate perennial plants and protect roots and lower branches. Ice can make a plant colder than the ambient outdoor temperature. Growers use ice to protect during freezes in crops like strawberries but only above 24 degrees.

Cold hardiness maps that are based on past weather history are not a guarantee of the future variations in cold temperatures but they are useful tools.