Though the first major cold spell of the year has already come and gone, winter is still a few weeks away. And the latest report from the National Weather Service Southern Region Headquarters predicts below normal temperatures this winter for all of Texas. It is not too late for people to take action to protect plants from total loss.
The first step is to understand how cold temperatures impact different plants and then take steps to alter those conditions. In addition to seeking local weather reports, it is important to understand the difference between frosts and freezes, as well as topography, urban microclimates and bodies of water which can all impact a plant’s ability to withstand temperature variations.
According to Monte Nesbitt, a Texas A&M AgriLife Extension horticulturist, “Plants can get used to freezing weather if they are exposed to it consistent and gradually.” But in Texas intermittent warm periods can make it hard for plants to adjust and therefore be more vulnerable to frost or freeze damage.
A sudden, steep plunge in temperatures such as that we experienced a couple of weeks ago brought with it winds, cold air masses and precipitation over a period of days as it moved across the state. For plants, that means damage from the low temperatures as well as from the wind from the stalk to the top. When the water inside plant cells freezes, ice crystals form that can pierce and damage the cell walls, killing the cells. As temperatures rise, fluids leak out of those cells, and they begin to decay.
A frost, by contrast, happens when the sky is clear and there isn’t much wind. The amount of radiation given to the plant by the sun is lost gradually during the night to the freezing point just before sunrise. Frosts can be severely damaging as well but normally only at the top or most exposed part of the plant.
Understanding what is likely for a particular area where plants are growing can help determine the type of protection offered. Frosts are easier to protect plants from. Plants growing under tree canopies will be impacted less than those more exposed. Likewise, plants in pots can be placed under the overhang of a house for some protection. Where possible, the best protection would be from opaque covers such as sheets, blankets, cardboard boxes or trash cans, but wait until early evening so the plant can absorb as much sun radiation as possible. The idea is to slow the cooling of the plant.
Freezes are not easily deflected by the methods used for a frost. While a cover by itself would not be adequate during a freeze, adding a heat source such as a string of outdoor lights can help, because it adds heat. Plants in containers are more likely to be harmed than those in the ground because they lack the insulation that the earth provides.
Move container plants inside the home or garage. If they can’t be moved indoors, put them on a more protected side of the house. Then water them well and pile on mulch, leaves or hay and cover with a frost blanket.
Plants that are permanently set in the landscape can receive some protection from semi-permanent structures such as polyethylene film-covered structures (hoop houses, for example), windbreaks, and mounds of soil or mulch heaped around the lower trunk.
Nesbitt said the process of protecting plants usually needs to be repeated throughout the winter months since Texas weather cycles between warm and cold for months.
Wait to prune frost or freeze-damaged plants until the spring, because the dead foliage can provide a protective layer during the next cold spell and because pruning could prompt new, tender growth.