Q: I heard you on radio say that lawns need “one inch of water, one time a week”. Why that amount? What’s magic about one inch of water?
A: You quoted me correctly. I don’t have time to give a longer explanation on radio so let me do it here.
My recommendation is the result of two goals: healthy grass and lower water consumption.
Lawns lose water in two ways: by evaporation from the soil and by transpiring water absorbed by roots through the grass leaves. Scientists use the word “evapotranspiration” to measure how much water is lost each day.
You can see the evapotranspiration rate for your area at www.georgiaweather.net. Click on the weather station nearest your home and then click on the “31 Day Summary”.
In hot 90+ degree days, the evapotranspiration rate is around .20 inch per day. So in a week, your lawn might lose 1.4 inches of water. Your lawn needs to have that water replaced.
You might have a thundershower during the week, replacing some of the water. My feeling is that an inch of water is about average, in the hot part of summer, for what it takes to bring the lawn back to where it was, moisture-wise, when the week started.
My advice to apply the irrigation once per week is based on grass health. Grass roots need oxygen to “breathe” as they grow. If the soil is kept soggy all the time, grass health declines. A watering every seven days allows the soil to dry slightly and gets oxygen to the roots. I do not recommend short waterings every day or two.
Further, a deep watering encourages grass roots to grow deeper, making them more resistant to drought.
For most lawns, in most environments, when temperatures are above 90 degrees, applying one inch of water, one time a week will keep a lawn healthy and will use the least amount of water.