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Know the soil type in your area.
Clay and loamy soils soak up water slower than sandy soil, which will affect how much you need to water your grass. To learn more about the soil type in your area, visit: United States Department of Agriculture: State Soils.
Provide 8-12 inches deep of water.
You want each watering session to penetrate 8-12 inches into the ground, which requires different amounts of water depending on your soil type. Set up empty jars or cans while you water your lawn (mason jars work well). Mark the jars at _ inch and take note of how long it takes for the water to reach that mark. Once you know how long this takes, you can plan the length of your watering sessions according to your soil type. Sandy soil requires _ inch of water daily, loamy soil requires 1 inch, and clay requires 1.5 inches.
Water your lawn in the morning.
The prime time to water lawns is between 5 AM and 8 AM, or right before the sun rises. Never water your lawn between 10 AM and 6 PM, because the heat and sun will evaporate your water! Watering your lawn right after the sun sets is better than during the middle of the day, but keep in mind that leaving grass moist overnight promotes fungal growth.
Set sprinklers correctly.
Set your sprinklers so they automatically deliver water in the morning. If you don't have an irrigation system installed in your yard, there are programmable timers available that can be hooked up to a spigot and attached to sprinklers on garden hoses. These generally cost around $40 at your local hardware store.
Check location of each sprinkler.
This may seem obvious, but check the location of each sprinkler and make sure it's watering your grass, and not the sidewalk or driveway!
Assess the terrain of your lawn.
If some areas are sloped and some are flat, these areas will have different water needs. On your downhill slopes, water doesn't have as much time to soak in to the soil and reach roots before it runs off. Pay special attention to these areas to make sure they're getting adequate water. You may need to break out the garden hose and give your sloped areas a little extra TLC.
Water certain types of grass during winter.
In the winter, most grasses go dormant naturally, but cool-season grasses such as fescue, rye or even bluegrass don't necessarily go completely dormant. Water them every now and then on a mild day when there's no threat of a hard freeze overnight. If you're not sure what type of grass you have, use this grass indentifier tool from Miracle-Gro.