Daniel Gill, Merrill, Thomas A.
Most plants in our landscapes are completely winter hardy, but some tender plants need to be protected from freezing temperatures.
Such plants – if they’re in the ground or in containers outside – are Louisiana gardeners’ primary concerns during freezes. These plants, which are native to tropical regions of the world where it never freezes, do not have the ability to protect themselves from sub-freezing temperatures.
If we want them to survive freezing temperatures during winter, we must protect them.
To start that process, identify the tender plants in your landscape that will need protection. The term tender refers to plants that are severely damaged or killed when the temperature of the cells in their bodies reaches or goes just below 32 degrees F, the freezing point of water. Plants such as hibiscus, tibouchina, firebush, bird-of-paradise, bananas, gingers, palms, ixora, philodendrons, pentas and oleander are just a few commonly used plants that are subject to freeze injury.
Then you need to decide what to do with your tender plants growing in containers outside. Basically, you have three choices.
You can leave them out and let them take their chances during a freeze. If you do this, at least take down hanging baskets and gather your container plants together under some protection such as a carport, patio cover or large tree.
Or you can bring these plants inside and keep them indoors until next spring. Make sure you put them in front of windows that receive plenty of light.
The third choice is to move these plants inside on those nights when a freeze is predicted and back out again when the freezing episode is over.
It is likely you will place some of your container plants into each category. Leave outside low value, easily replaced plants or those too big to bring in. Move those that are easily transported in and out as needed during the winter. And bring larger, more valuable plants inside to stay. These steps should be taken any time temperatures are predicted to go into the low 30s or upper 20s.
Of course, plants growing in the ground cannot be moved indoors and must be protected in place. But remember that you do not necessarily need to protect everything that is tender.
For plants growing in the ground that are readily available in nurseries and not too expensive to replace, you could just replant them in the spring if you lose them. I put plants such as pentas, lantana, Mexican heather, begonias, impatiens, blue daze and coleus in this category.
A benefit of our relatively mild winters is that it doesn’t stay cold long enough to freeze the ground. Tropicals growing in the ground that produce fleshy underground parts, such as bulbs or rhizomes, generally are very reliable about coming back even if the tops are frozen back. Cannas, agapanthus, gingers, callas, elephant ears, achimenes, philodendrons, bird-of-paradise, crinums, hymenocallis and amaryllis fall into this category.
You can ensure the survival of below ground parts or the crown of a plant by placing 4-6 inches of mulch, such as pine straw, over the soil around the base to provide additional protection. Should any of these plants suffer cold injury to their tops this winter, just cut them back as needed.
Remember, mulches protect only what they cover. Mulch may help keep the lower stems, crown and roots alive, but they won’t protect the uncovered upper part of the plant. To protect the upper part of tropical plants growing in the ground, you must cover them.
Plastic, canvas or fabric all make suitable covers.
To protect a plant properly, there must be enough material to extend all the way to the ground when the plant is covered. The goal is to trap the heat stored in the earth under the cover.
If possible, when covering, find two or three posts or stakes that are taller than the plant and drive them into the ground around the plant so they will support the cover off of the foliage. This is particularly important if the weight of the cover might damage the plant or if plastic is used – since leaves will freeze where they come into contact with the plastic cover. Secure the bottom edges of the cover with rocks, bricks or soil to keep cold air from blowing in.
Don’t forget to remove or vent covers during the day if the weather is sunny and mild.
For severe freezes or prolonged temperatures below freezing, providing a heat source under the cover improves protection. One of the safest and easiest methods I’ve come across is to wrap or drape the plant with strings of small outdoor Christmas lights. Not enough heat is generated to damage the plant, but what heat is given off by the small bulbs can make a big difference in the survival of a plant. Just make sure you use outdoor extension cords!
Wetting the foliage of plants immediately prior to a freeze does not provide any protection, but it is important that plants are not drought stressed going into a freeze. If the weather has been dry, thoroughly water plants in the ground and in containers early in the day when a freeze is predicted for that night.
Tropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. Generally, the best idea is to plant mostly hardy plants and use tropicals with restraint so that your landscape is not totally devastated if an episode of extreme cold occurs.
Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.