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
The most important thing you can do to minimize cold damage to your lawn and landscape is to prune and fertilize at the appropriate times of year. Cold hardiness is reduced in trees and shrubs following pruning. Trees and shrubs pruned just before a cold snap are more likely to be damaged than those pruned later in the winter when extremely cold temperatures are less likely. For this reason, it is better to wait until late February or early March to prune evergreens and summer blooming trees and shrubs such as butterflybush, Vitex, crape myrtle, and Knockout roses. Remember to wait until after they bloom to prune spring blooming shrubs such as azaleas, Indian hawthorn, and hydrangeas, to avoid removing their flower buds.
Applying nitrogen containing fertilizers to lawns and landscapes in winter can increase cold damage by encouraging growth to begin too early. To avoid cold damage, wait until March before fertilizing landscape beds and trees. Using slow release fertilizers such as Osmocote or organic fertilizers such as Plant-tone further reduces the risk of cold damage. For lawns, wait until late April before applying any fertilizer.
If a sudden drop in temperature is expected, plants that are prone to cold damage such as sago palms can be protected by being covered with a quilt, sheet, or special synthetic fabrics made for frost protection, such as Reemay garden blanket. When covering plants, be sure they are covered completely and the cover extends to the ground to trap in heat from the soil. Also be sure to remove any coverings the next morning once temperatures rise above 32. Watering plants deeply a day or two before very cold weather can help reduce damage as well, since moist soil holds more heat than dry soil. This is only necessary if soils are dry. Keeping soils too wet through the winter can increase cold damage to plants.
To learn more about caring for your garden and landscape, visit ces.ncsu.edu where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: If you live in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610.
Charlotte D. Glen is the horticulture agent with the Pender County Cooperative Extension of NC State University, College of Agriculture & Life Sciences. She can be reached via e-mail at Charlotte_Glen@ncsu.edu.