Tender shrubs can be covered with blankets for several days. Weight covers to the ground so heat released from the soil can’t escape on cold nights. (Photo by Bob Souvestre/LSU AgCenter)

A large navel orange tree is protected with a blue tarpaulin that is draped directly over the tree. Remove cover after the freeze threat passes so the plant can get sunlight. (Photo by Bob Souvestre/LSU AgCenter)

A simple structure – here a tomato cage – can be used to hold the cover off a plant. (Photo by Dan Gill/LSU AgCenter)

News Release Distributed 11/13/14

BATON ROUGE, La. – Early freezing temperatures this year are causing concern among Louisiana homeowners and gardeners.

“If you look around at some of our landscapes, particularly in south Louisiana, you would think we live in the tropics,” said LSU AgCenter horticulturist Dan Gill. “Indeed, some winters the temperature never does dip much below the mid- to upper 20s, allowing tropical plants to survive.”

This year, however, the looming freeze means people have to take action.

“All it takes is one night of temperatures in the low 20s or teens to severely damage or kill many tropicals,” Gill said.

Plants come in different degrees of hardiness, Gill said. A plant that will tolerate a temperature of 15 degrees is hardier than one that will be killed at temperatures below 25 degrees.

“Our commonly used landscape plants, including trees, shrubs, ground covers, lawns and vines, are hardy to at least 10 or 15 degrees and will not be damaged by typical winter weather,” he said.

“Basically, a plant is considered hardy if it can reliably survive winter temperatures where you garden with no protection or, at most, some mulch,” Gill said. “On the other hand, a plant is considered tender when it will not reliably survive winter temperatures where you garden without extensive protection.”

Freezes can be characterized as radiational or advective.

Radiational freezes or frosts occur on calm, clear nights when heat radiates from surfaces of objects. These freezes are generally considered light and primarily damage the foliage of tropicals.

Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plants by covering them.

Advective freezes occur when cold air masses move down from northern regions causing a drastic drop in temperature. The conditions are quite different from a radiational freeze. Temperatures tend to be much lower and are liable to last longer during advective freezes, and protecting plants is more difficult.

A gradual decrease in temperature over a period of time will harden off plants allowing them to withstand freezing temperatures better, Gill said. “A sudden drop to below-freezing temperatures as we’re having now may cause damage even to hardy plants that might otherwise have suffered little or no damage.”

This is not true for especially tender tropical plants, though, because they can’t tolerate freezing temperatures in any circumstances.

Finally, plants do not “feel” wind chill. “Do not focus on wind chill temperatures – it is the actual temperature you need to pay attention to,” Gill said.

Three ways to protect plants from freeze damage include moving them inside, mulching them or covering them, Gill said.

Move all tender plants in containers and hanging baskets into buildings where the temperature will stay above freezing. If this is not possible, group all container plants in a protected area – such as the inside corner of a covered patio – and cover them with plastic.

For plants growing in the ground, mulches can help protect them. “Use a loose, dry material such as pine straw or leaves,” Gill said. Because mulches will only protect what they cover, they are best used to protect below-ground parts or crowns or to completely cover low-growing plants to a depth of 4 inches.

“Leave mulch that completely covers plants in place no more than three or four days,” he said. “Mulch at the base of a plant can remain in place all winter.”

If they are not too large, individual plants can be protected by covering them with various sized cardboard or plastic foam boxes. Larger plants can be protected by covering them with fabric or plastic.

Covers will work best for radiational freezes by preventing or blocking heat loss. Even with protection, many plants may still die during extreme, prolonged cold that occurs during advective freezes.

“If you are growing vegetables, harvest any broccoli, cauliflower, fava beans or peas that are ready,” Gill said. Freezing temperatures will not hurt the plants but can damage the heads, pods and flowers. Also, harvest any ripe citrus fruit prior to a hard freeze.

When it comes to citrus, protection becomes important when severe freezes occur. Determining what to do depends on the type and age of the tree, the expected low temperature and the expected duration of the freeze, Gill said.

Satsumas, for example, do not need protection until the temperatures approach 20 degrees. Lemons, limes and oranges generally need to be protected when the temperature dips below 26 degrees.

Any of these trees, however, may be killed or damaged at these temperatures if they are not sufficiently hardened, Gill said Hardening occurs when trees are gradually exposed to cooler temperatures and become more tolerant to freezing temperatures. So chilly but above-freezing nights during fall and early winter before major freezes make the trees more cold-tolerant.

The length of time citrus trees are exposed to sub-freezing temperatures is also significant. “If the temperature is below freezing for 24 to 36 hours, damage can usually be expected,” Gill said.

All ripe fruit should be harvested if a significant freeze is expected. Temperatures cold enough to damage the tree will also ruin the fruit. It takes temperatures in the mid- to low 20s five to 10 hours to freeze the fruit.

Protect single, smaller trees by constructing a simple frame over them and covering it with one or two layers of translucent plastic. “This is generally most practical for smaller trees,” Gill said. In southeast Louisiana, such an extreme practice would be needed only on a few severely cold nights.