Scientific Proof Not To Water in Winter

Mark Peterson

Nature provides enough moisture during the winter months to maintain plant needs during dormancy. But don’t just take our word for it — consult ET.

South Texans are pretty fortunate. We often enjoy mild winter weather with just enough precipitation to eliminate the need to water our landscapes.

Why don’t we need to water, you ask? The simple answer is that nature provides enough moisture during the winter months to maintain plant needs during dormancy. But there’s also empirical evidence why we recommend drastically cutting back on watering in winter: Evapotranspiration.

Also known as ET for short, evapotranspiration is the combination of total water used by the plant, water that evaporates from the leaves (transpiration) and water that evaporates from ground. To determine a plant’s water needs, ET is calculated by using a reference plant and comparing it to similar individual plants or groups of plants.

In Texas, our landscape reference plant is fescue grass that is 4 inches tall and growing in optimal conditions without stress. The resulting need is calculated in inches.

Through science and experience, we know that cool season grasses — Kentucky blue grass, fescue and ryegrass — do very well at 80 percent of the ET of the reference plant, otherwise known as ETo. And warm season grasses — Bermuda, St. Augustine and zoysia — do very well at 60 percent of ETo.

We then take one more step with turf. We apply a seasonal or quality coefficient to the species coefficient. This percentage varies with the seasons: winter, spring and fall is 60 percent and summer is 80 percent. So the amount of water needed in the winter is ETo x .60 x .60.

Here is the best part: nearly all South Texas native or adapted plants can grow with 20 percent to 30 percent of ETo.

Historically, November, December, January and February ETo has been 2.6 inches. That would equate to an actual plant water need of .9 inches for turf and .5-.8 inches for all other plants. But rainfall eliminates any need to water. Historic rainfall for those months averages 1.8 inches — double the average plant need. (Information may be obtained from the TexasET Network.)

Sure, some years have limited rainfall during the winter. Yet most of the time, our native plants scoff at winter drought. But if you feel you must water, use the holiday watering method. This is generally more than enough to keep your lawn healthy and weeds at bay.

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