Grass seed sprouts and grows best in soil that's moist on the surface and to a depth of 1 to 2 inches, but watering a new lawn begins even before sowing the seed. Moisture deep in the soil helps prevent the soil at the surface from drying out. Grass seedlings don't survive drought. Soggy soil rots their roots, however, so frequent, light watering is needed for the first 30 days after sowing to maintain medium moisture levels.
Sandy soil drains freely and needs more frequent watering than clay soil, which holds on to water and quickly becomes waterlogged.
Watering Before Sowing
Soil that's moist only at the surface is at risk of quickly drying out. To provide the best growing conditions for grass seed, water the lawn area three or four days before sowing.
Apply water until the soil is moist to a depth of 6 to 8 inches. When the soil surface dries out, it's ready for sowing.
Sowing grass seed at the best time of year for the particular variety helps provide conditions allowing the seeds to use the available water efficiently. Cool-season grasses such as Kentucky bluegrass (Poa pratensis) and tall fescue (Festuca arundinacea) should be sown in early fall when rainfall often increases and the cool temperatures encourage strong growth. Bermuda grass (Cynodon spp.), buffalo grass (Buchloe dactyloides) and zoysia grass (Zoysia spp.) are warm-season grasses that should be sown in early summer or late spring, at the beginning of their growing season.
Kentucky bluegrass is hardy in U.S. Department of Agriculture plant hardiness zones 2 through 6, and tall fescue is hardy in USDA zones 4 through 7. Bermuda grass is hardy in USDA zones 7 through 10; buffalo grass is hardy in USDA zones 5 through 8; and zoysia grass is hardy in USDA zones 5 through 10.
Kentucky bluegrass and tall fescue can be watered in the evening in fall. Diseases can develop in soil that's moist overnight, but the cool fall temperatures reduce the risk of diseases.
Watering New Lawns
New lawns need water at least once per day, and sometimes up to four times per day. Water the grass seed so that the soil stays evenly moist to a depth of 1 to 2 inches. If you're unsure whether the soil is moist to the correct depth, push a screwdriver into the soil and gently move it back and forward to create a narrow hole. Pull the screwdriver out and look for moist soil clinging to the metal, which shows the depth of moist soil beneath the surface.
When the grass seedlings are 2 inches tall, water the lawn less often -- perhaps every other day. When the soil surface is dry, apply enough water to moisten the soil to the depth of the grass roots.
Spread clean straw thinly over the grass seed so that the soil is still visible, to help conserve soil moisture.
Don't water new lawns so heavily that the seed washes away.