Regional Gardening Reports :: National Gardening Association

In the Garden:
Lower South
February, 2008
Regional Report

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These PVC arches can support a frost-fighting cover during the late winter, and bird netting after the danger of frost is past.

Beating Freezes in the Garden

When winter weather wanes and spring growth emerges, gardeners get a little edgy. With each passing day our plants push forth a little more growth as blooms open and leaf buds emerge. When a freezing night is forecast, we go to great lengths to safeguard our plants from cold damage.

Keep Soil Moist
Adequate soil moisture helps plants resist cold damage. Also, moist soil absorbs more solar radiation than dry soil. This stored warmth will radiate out during the night. Combined with a cover over the plant, it can make a small but important difference. Overwatering creates waterlogged soil conditions that can be damaging to plants.

Sprinkling plants for freeze protection is a technique used by growers, but it's hard for homeowners to regulate. As long as water is being applied to the surface of the plant (or to the ice coating on the plant), the temperature of the plant tissues will not drop below 32 degrees. The reason is that as water changes from a liquid to a solid state, it gives off a tiny amount of heat. Then another drop lands on the plant and as it freezes it releases a bit of heat energy.

During an extended hard freeze, the growing ice loads can destroy plants. Irrigation systems tend to put out too much water or disperse it unevenly over the plant surface. Water must be evenly distributed and supplied in ample quantity to maintain a film of liquid water on the foliage surfaces. Sprinkling must continue until thawing is completed. If you stop sprinkling when air temperature rises above freezing but before the ice is melted, the reverse can occur. That is, as the water changes from a solid to a liquid it absorbs heat, causing the plant tissue to be supercooled.

For a practical example of this, think about that homemade ice cream you made last summer. Why add salt? Why not just ice? The salt causes the ice to melt (change from solid to liquid), which results in supercooling of the ice cream.

Cover Up
The main method of protection we have at our disposal is to cover plants before a freeze. Covers should be removed during the day to allow the sun to warm the soil, and then replaced again late in the afternoon.

Blankets keep us warm because they help contain the heat that our bodies produce. Plants do not produce heat for the cover to hold in. The heat we are trying to contain is in the soil. Therefore the covers should go over the plants and to the ground, rather than wrapped around the plant and tied around the trunk. Those "landscape lollipops" don't get much, if any, protection.

Lights Create Heat
You can provide added protection by using a heat source beneath the cover. A mechanic's light or string of outdoor Christmas lights works great. Just take the obvious precautions to avoid fire hazards and electrical shorts. Also take care not to allow a hot light bulb to contact and damage plant tissues, such as the trunk or branches. I like to use lights beneath a cover to protect valuable but marginally hardy plants like our Satsuma orange tree and kumquat bush. They can also make the difference for an in-ground bougainvillea on a really cold night.

Cold protection is an annual challenge for plant growers. However, to the determined homeowner, that prize plant may just be worth some heroic efforts at saving it.

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