The most important factors in how much damage a plant receives from cold are how hardy it is and how cold the weather gets, according to the LSU AgCenter. A surprising number of other factors, however, can play a 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 plants that might otherwise have suffered little or no damage.
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. Freezes that last eight hours or more are particularly damaging to tender plants.
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. Planting in areas covered with overhangs or tree canopies will also help to minimize cold damage.
What to Do Before a Freeze
Thoroughly water landscape plants before a freeze if the soil is dry. Strong, dry winds may cause damage by drying plants out, and watering helps to prevent this. Wetting the foliage of plants before a freeze does not, however, provide any cold protection.
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.
For plants growing in the ground, use a loose, dry material such as pine straw or leaves. Mulches will only protect what they cover and are best used to protect below-ground parts and crowns, or they may be used to completely cover low grow¬ing plants to a depth of four to six inches. Leave a complete cover of mulch on for no more than three or four days.
Smaller individual plants can be protected by covering them with various sized 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.
For severe freezes in the teens, pro¬viding a heat source under the covering helps. A safe, easy way to do this is to generously wrap or drape the plant with small outdoor Christmas lights. The lights provide heat but do not get hot enough to burn the plant or cover. Please be careful and use only outdoor extension cords and sockets.
If necessary, you may prune back a large plant to make its size more practi¬cal to cover.
What to Do After a Freeze
Unless you are keeping them inside for the rest of the winter, move con¬tainer plants back to their spots outside.
For plants that you covered, remove or vent clear plastic covers to prevent excessive heat buildup if the next day is sunny and mild. You may leave plants covered with blankets or sheets for several days without harming them, but eventually the covers will need to be removed so the plants can get light.
Do not prune anything for several days after a freeze. It often takes several days for all of the damage to become evident.
Damaged growth on herbaceous or nonwoody plants – such as can¬nas, elephant ears, birds-of-paradise, begonias, impatiens, philodendron and gingers – may be pruned back to living tissue. This pruning is optional and is done more to neaten things up than to benefit the plants. However, if the dam¬aged tissue is oozy, mushy, slimy and foul smelling, it should be removed.
Generally, it’s a good idea to delay hard pruning of woody tropical plants – such as hibiscus, tibouchina, angel trumpet, croton, ixora, schefflera, copper plant and rubber tree – until new growth begins in the spring and you can more accurately determine which parts are alive and what is dead.
Tropical and sub-tropical plants can be used effectively in the land¬scape, but they must be protected or replaced when necessary. The best idea is to plant a good combination of tender and hardy plants, so that your landscape is not totally devastated in the event of extremely cold weather.
For more information contact Alan Vaughn, county agent, Plaquemines/St. Bernard Parish at 504-433-3664 or 504-278-4234.