Master Gardener: Temperature ups and downs wear on plants

Gardener

Winter temperatures can cause evergreens to become discolored, but wait until March to fertilize to avoid cold damage.

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Published: Thursday, January 9, 2014 at 18:31 PM.

Cold damage is not uncommon in eastern North Carolina lawns and landscapes, even though our climate is relatively mild. Symptoms of cold damage include brown leaves on evergreens, dead patches in lawns, twig dieback on trees and shrubs, and in extreme cases, complete death of a plant. Most years, extremely cold temperatures are not the cause of plant injury during our winters. Instead, it is usually a combination of fluctuating temperatures along with factors related to plant care. While little can be done to moderate temperature changes, there are things we can do to minimize their effects on our lawns and landscapes.

Understanding cold hardiness

All plant species have a genetic ability to survive a certain degree of cold. For example, camellias can be killed at 0 degrees Fahrenheit, while dogwoods can survive temperatures as low as negative 20 degrees. To help gardeners choose plants tolerant of their area’s winter temperatures the US Department of Agriculture developed the Plant Hardiness Zone Map, which divides the country into numbered zones based on the average winter minimum temperature for that area. Zones range from 1, the absolute coldest with winter temperatures below negative 50, to 11, the warmest zone, where temperatures stay above 40.

Most of eastern N.C. falls in zone 8a, meaning we can expect average winter minimum temperatures between 10 and 15 degrees. When purchasing plants look for the Hardiness Zone rating listed on their tag. Plants rated zone 8 or lower should survive our winter temperatures without damage. One thing to keep in mind about the zones is that they are based on average minimums, not extremes. During true extremes, when temperatures fall below the average, even plants rated as hardy to our zone may be damaged or killed.

Sometimes, plants that are rated as perfectly hardy in our area will experience winter injury because they were not completely prepared when cold weather arrived. For plants to tolerate cold temperatures, they must adjust to temperature change over a period of time. The sudden onset of cold weather in fall, such as experienced this past November, can result in more cold damage than usual. In addition, abrupt changes in temperature, especially when several mild winter days in the 60’s or 70’s are followed by a sharp drop into the 20’s, can catch plants unprepared and result in cold weather damage.

Avoiding cold damage



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