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Give your lawn the water it needs but no more. This moderate approach conserves an important resource, saves money, and helps prevent grass diseases. How much water your lawn needs depends on the type of grass in the turf, the overall health of your lawn and soil, the amount of rainfall your lawn gets, and the overall climate. You may need to water only twice a year or you may need to water twice a week.
The best approach to watering grass (and most other plants) is to follow nature’s pattern of rainy periods followed by brief dry spells. Apply enough water all at once to penetrate the roots, let the soil almost dry out, and apply water again. Grass signals that it needs water by losing its spring—if you see your footprint as you walk across the grass it is time to water again. To determine how much water your lawn needs, consider the depth of its roots, the soil type, its “penetrability,” your irrigation method, and the weather.
To see how deep the grass roots are, use a shovel to remove several samples from around the yard. Average the root lengths, and add an inch to the average root depth to arrive at a target watering depth. Watering to a level substantially deeper than your lawn’s root zone is a waste of water. Root depth depends on how much time you have taken to improve your soil and on the type of grass. Some grasses, such as Tall fescues, have roots that reach 1-foot deep. Other grasses have roots that grow to only half that, in even the best conditions. As your grass develops deeper roots, adjust your watering depth target to encourage roots to go deeper.
Next, determine how much water you need to moisten the soil to the target root zone. Wait for a 4 to 5-day dry spell, and then set out some empty cans (6-ounce tuna cans work well) in various locations on the lawn. Run your sprinkler or in-ground sprinkler system and record the amount of time it takes until the cans contain 1 inch of water. Then wait a day to allow the water to penetrate the soil, and check the depth it has reached. If 1-inch of water moistens the soil to a depth beyond the root depth, try the procedure again after your soil has dried, but turn off the sprinklers sooner. Conversely, if the root depth is not reached, try running your sprinklers longer. If it rains during the week, decrease your watering by the amount of rain that fell. If it is hot and sunny or windy, increase the watering amount and frequency.
A good rule of thumb for most grasses is 1 to 2 inches per week. If you have porous soil that drains quickly, you would apply 1 inch of water twice a week. If your soil retains water well, 1-1/2 to 2-inches once a week might be enough.