A few good soakings are better than lots of light sprinklings (but not in the evening)
Deep watering helps develop deep roots that tap into subsurface water supplies (illustration below). Light sprinklings wet only the grass and surface of the soil; this encourages shallow root growth and increases the need for more frequent watering. As a general rule, lawns require 1 to 2 in. of water per week (from you or Mother Nature), applied at three- or four-day intervals. But this varies drastically depending on the temperature, type of grass and soil conditions. Lawns in sandy soils may need twice as much water, since they drain quickly. Lawns in slow-draining clay soils may need only half as much.
When your lawn loses its bounce or resiliency, or when it wilts, exposing the dull green bottoms of the blades, it needs water. As a general game plan, water until the soil is moist 4 to 5 in. down, then wait to water again until the top 1 or 2 in. of soil dries out. To find out how much water your sprinkler delivers, set out a cake pan, turn on your sprinkler, then time how long it takes for the water to reach a depth of 1 in.
The best time of day to water is early morning. Water pressure is high, less water is lost to evaporation and your lawn has plenty of time to dry out before nightfall. Lawns that remain wet overnight are more susceptible to disease caused by moisture-loving mold and other fungi.
Good Watering Pays Off
Properly watered lawns receive an initial soaking 4 to 5 in. deep, and are then watered when the top 1 to 2 in. of soil dries out, develop deep, healthy grass roots. This usually means applying 1 to 2 in. of water per week at three- or four-day intervals. An impact sprinkler delivers water quickly, with less “hang time” for evaporation; a 3/4-in. hose delivers much more water volume than its 1/2-in. cousin.
Improperly watered lawns receive short daily waterings that promote shallow root growth. Oscillating sprinklers toss water in a high arc, so more evaporates before reaching the soil. Watering late in the evening when your lawn doesn’t have time to dry out allows disease-carrying fungi and mold to grow.