Daniel Gill | 4/5/2005 1:38:31 AM
If you look around at some of our landscapes, you would think we live in the tropics. Indeed, some winters the temperature never dips below freezing in south Louisiana.
We all know, however, that winter freezes are entirely possible throughout the state. And, on occasion, severe freezes in the teens in south Louisiana and below 10 degrees F in north Louisiana can be devastating.
Two terms are used when it comes to the ability of a plant to tolerate cold. If a plant will endure temperatures 32° or below with little or no damage, it is termed hardy. There are degrees of hardiness. A plant that will tolerate a temperature of 15° is hardier than one that will be killed at temperatures below 25°. Most of the trees, shrubs and cool-season annuals we grow are completely hardy. Many tropicals have some degree of hardiness and will tolerate temperatures in the upper 20s.
The term tender refers to plants that are killed or severely damaged by temperatures of 32° or below. Many tropical plants fall into this category as well as warm-season annuals. Sometimes tender plats will survive a hard freeze by coming back from their roots. Since the ground here does not freeze, the roots, bulbs, rhizomes and other below-ground parts often survive.
When freezes do come, they can be characterized as radiational or advective. Radiational freezes or frosts occur on calm, clear nights when heat radiates from surfaces or objects into the environment. Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces.
Advective freezes occur when cold air masses move down from northern regions, causing a drastic drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss also occurs during an advective freeze, the conditions are quite different from a radiational freeze. The low temperatures are liable to last much longer during advective freezes, and plant protection is more difficult.
FACTORS INFLUENCING COLD INJURY
The most important factors in how much damage a plant receives from cold are how hardy it is and how cold it gets. A surprising number of other factors can play a big role in how much cold injury occurs.
A sudden drop to below freezing temperatures from a period of relatively mild weather may cause damage even to hardy plants that might otherwise have suffered little or no damage.
A gradual decrease in temperature will harden off plants, allowing them to withstand freezing temperatures better. This is not true for tender plants, because they will not tolerate freezing temperatures regardless of the preceding temperatures.
The longer below freezing temperatures persist, the more likely damage is to occur. This is because as time goes by, heat stored in plants, soil, walls, etc. that initially moderates temperatures around the plant is lost.
Healthy plants withstand freezing temperatures better than sick ones. Make sure good care is given to your landscape during the summer growing season. Pruning and fertilizing trees, shrubs and ground covers should be avoided after September; it can stimulate late growth that is not as cold hardy and may lead to freeze injury.
Finally, where a plant is located in the landscape can make a big difference on how much damage occurs. The micro-climate of a location is the environmental conditions of the specific spot. There are many micro-climates in your landscape. The careful placement of tender or less hardy plants in sheltered areas that block cold north winds and trap the heat of the sun can help them survive freezes. Areas covered with overhangs or tree canopies are also a good choice because heat is prevented from radiating out into the atmosphere.
WHAT TO DO BEFORE A FREEZE
Thoroughly watering landscape plants before a freeze may reduce the chance of freeze damage. Many times cold weather is accompanied by strong, dry winds that may cause damage by drying plants out. Watering helps to prevent this. Wetting the foliage of plants before a freeze does not, however, provide any cold protection. A well-watered soil will also absorb more solar radiation than dry soil and will re-radiate the heat during the night.
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 (like the inside corner of a covered patio) and cover them with plastic. If plants are kept inside for extended periods, make sure they receive as much light as possible.
For plants growing in the ground, mulches can help protect them. Use a loose, dry material such as pine straw or leaves. You should be aware that mulches will protect only what they cover. A mulch at the base of a bird-of-paradise will help the roots, but will provide no added protection to the leaves. Mulches, then, are best used to protect below-ground parts, crowns or may be used to cover low-growing plants to a depth of 4 inches. Leave complete cover on no more than three or four days.
If they are not too large, individual plants can be protected by covering them with various sizes of cardboard or Styrofoam boxes.
Larger plants can be protected by creating a simple structure and covering it with sheets, quilts or plastic. The structure holds the covering off the foliage, preventing branch breakage and improving cold protection. It need be nothing more elaborate than three stakes slightly taller than the plant driven into the ground. The cover should extend to the ground and be sealed with soil, stones or bricks. Plastic covers should be vented or removed on sunny, warm days.
The covers will work best for radiational freezes by preventing or blocking heat loss. The extreme, prolonged cold that occurs during advective freezes is not so easily dealt with. Many plants will still die, even with protection. This can be helped by providing a heat source under the covering. An excellent technique is to wrap the plant with small, outdoor Christmas lights. The lights provide gentle heat throughout the plant area but do not get hot enough to burn the plant or cover. Be sure to use outdoor extension cords.
If necessary, you may prune back a large plant, like a hibiscus, to make its size more practical to cover. For trees, such as citrus, that are too large to cover, you may at least want to wrap the trunk with an insulating material such as foam rubber or blankets. Even if the top dies, you may be able to regrow the tree from the surviving trunk.
If you are growing vegetables, harvest any broccoli, cauliflower, fava beans or peas that are ready. Freezing temperatures will not hurt the plants, but can damage the heads, pods and flowers. Also, any citrus fruit should be harvested from the tree before a hard freeze.
WHAT TO DO AFTER A FREEZE
After a freeze is over, check the water needs of plants in containers and in the ground. Unless you are keeping them inside for the rest of the winter, move container plants back to their spots outside.
Remove or vent plastic covers to prevent excessive heat buildup if the day is sunny. Pull back mulch that completely covered low plants.
Damaged growth on non-woody plants should be pruned away, and dead leaves on all plants can be picked off. Delay hard pruning of woody plants until new growth begins in the spring, and you can more accurately determine which parts are alive and what is dead. Don’t be too quick to dig up and remove plants that appear to be dead. On occasion, they may eventually resprout from the roots in April or May.
Tropical and sub-tropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. The best idea is to plant a good combination of tender and hardy plants.