by Allen Owings, LSU AgCenter horticulturist
HAMMOND, La. – Winter is here, and this time of year brings questions about protecting landscape plants during the cold months of December, January and February. The best approach to protecting plants in winter is to pay attention to the weather forecasts and try to know a couple days ahead of time when a freeze is approaching.
This year's gradual decrease in temperatures this fall without major frosts and freezes has led to attractive foliage color. We have noticed many trees in the woods and in the landscape with improved fall color this year when compared to a normal Louisiana fall.
Now, it's time to prepare for colder temperatures of late fall and winter, keeping in mind frosts and freezes are different. First frost dates in Louisiana vary from mid-October to mid-November while first freeze dates vary from around mid-November until mid-December. Of course, these dates are highly variable year to year, and some areas in south Louisiana have even gone through winters without a significant freeze.
Weather conditions prior to a freeze or frost also play a role in how these temperatures affect plants. If the first cold event is a freeze, landscape plants will suffer more damage. If several cold fronts move through the state and produce several frosts prior to a freeze, we will see less negative impact to plants that normally show damage.
To prepare plants for a freeze, thoroughly water them if the soil is dry. This is especially important for container-grown plants. Shrubs in landscape beds also can be helped with irrigation prior to a freeze.
It's best, however, to make sure your shrubs received adequate irrigation or rain throughout fall. Strong, dry winds that frequently accompany cold fronts may cause damage by drying plants out, and watering helps to prevent this. Wetting plant leaves 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, mulch them with 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-growing plants to a depth of 4 to 6 inches. But don't leave a complete cover of mulch on for more than three or four days. Many folks heavily mulch their tropical hibiscus in landscape beds using this method. Smaller individual plants can be protected by covering them with various sizes of cardboard or plastic foam boxes.
Larger plants can be protected by creating a simple structure and covering it with fabric or plastic. The structure keeps the cover from touching the foliage, preventing broken branches and improving cold protection. It need be nothing more elaborate than driving into the ground three stakes slightly taller than the plant. 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 when temperatures dip into the teens, providing 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. But be careful and use only outdoor extension cords and sockets. If necessary, you may prune a large plant to make its size more practical to cover.
Keep in mind that your cool-season bedding plants are adapted to the normal cold winter temperatures, so cold protection typically is not needed.