By Denise Attaway, College of Agriculture, Forestry and Life Sciences; Public Service and Agriculture
Bob Polomski, Clemson University Extension horticulturist, said protecting plants doesn’t take much time or effort and can prove to be beneficial in the long term, especially for container plants.
“Freezing temperatures can damage plants by rupturing plant cells as temperatures drop and ice crystals form on leaves and stems,” Polomski said.
Some signs to look for to determine if plants have been damaged by the cold are wilted, blackened, or drooping leaves; split stems or bark near base of plants; long, deep narrow cracks running up and down tree trunks; and drying out of plants, especially evergreen plants.
Desiccation or drying resulting in marginal leaf “scorch” or tip “burn” is a common problem of broad-leaved evergreen and semi-evergreen plants. When temperatures drop below freezing for a period of time, water freezes in the stems of shaded, interior trunks and branches. When sited in a location receiving direct sun, particularly in the morning, leaves warmed by sunlight transpire and lose water. Because water cannot move up the frozen stems to replace water that is lost, the affected leaves dry out and turn brown along the margins and at the tips. Often these drought-like conditions cause the leaves to droop and the entire plant to look wilted. Winter drying may also lead to twig die back. The full extent of injury may not be evident until late winter or early spring. Windy, sunny conditions worsen this problem.
To reduce the incidence of winter drying, screen tender plants with burlap barriers to protect them from northwest winds and sun. Place sensitive plants in locations that receive morning shade during the winter months.
Low or fluctuating temperatures also can harm flower and leaf buds. This may result in few, if any, blooms on spring-flowering shrubs.
“Unseasonably warm weather during winter or early spring can cause plants to come out of dormancy,” Polomski said. “This problem also can exist in the fall when an early frost occurs before plants have stopped growing and properly acclimated to cold weather.”
“Only broken or damaged limbs should be removed at this time,” he said. “At the end of winter and in early spring, evaluate all shrubs and trees. Before pruning out a limb that looks dead, scrape the bark and look at the underlying tissue. A green layer indicates living tissue. Sometimes it’s best to postpone any pruning until new growth emerges. At that time, you can easily tell the difference between healthy and dead wood. We need to respect the resiliency of shrubs and trees and know that with judicious pruning, mulching and watering during the growing season, they will recover.”
The best way to avoid cold damage to plants is to select hardy plants. Polomski advises growers to use the USDA Plant Hardiness Zone map to select plants for their specific locations. An interactive version of the map can be found at http://bit.ly/PlantHardinessZones.
“The USDA cold hardiness zone rating that has been assigned to a plant is the average minimum temperature at which the roots or the crown in some species, which are at or just below the soil line, are killed,” he said, adding it is also important to know the plant will survive summer heat and humidity as well.
In addition to proper plant selection, Polomski also said it is important to consider the micro climates in landscapes. Micro climates are climates of very small or restricted spaces influenced by sun exposure, existing plants and nearby structures.
“These little pockets may vary by several degrees — a difference that can mean life or death in cold weather,” Polomiski said. “Take note of the different micro climates in landscapes. Generally, minimum winter temperatures occur on the north and northwest parts of your property and in low areas where cold air settles. In the summer, the hottest areas are in unshaded western exposures.”
Polomski also advises people to take advantage of tree canopies that can protect neighboring plants from radiant heat loss.
“In winter, the micro climate beneath tree branches may be several degrees warmer than the surrounding air,” he said. “This small increase in temperature could protect some plants. The shade provided by the tree in early morning can reduce the amount of cold damage to some plants.”
Overhangs, arbors and fences may provide similar protection.
For potted plants, pack plants close together and cover with a plastic sheet that does not touch the plants. Mulch or mound soil around pots. Protect plants in exposed locations by wrapping burlap or building a lathe structure around them.