Early morning watering is best for your lawn.
This will enable your grass blades to dry out as quickly as possible. Evening and night watering will leave the blades of grass wet for longer periods of time and make the more susceptible to disease infestation.
To determine how much water your irrigation system/sprinklers put out, place a shallow pan half way from the water source and the end of the water spray. Run the water for a set period of time to calculate how much water comes out per hour and plan your watering program accordingly.
Watering A New Lawn
When establishing a new lawn, watering is the most critical component. While getting it just right is not difficult, it does require diligence. Monitoring the moisture level in the soil is a daily, and often, a twice daily procedure. This will be greatly affected by temperatures and wind conditions.
With a newly seeded lawn, the soil must be kept evenly moist and NEVER left to dry out. New sprouts will die in a day if left to dry out. Overwatering will rot the seed and new sprouts. If you do not have an in ground irrigation system, make sure you have an adequate supply of hoses and sprinkler heads. Set them up so you will get the best possible coverage and not have to move hoses around more than necessary. Continually to have to move the hoses and sprinklers around multiple times per day will become tedious quickly.
Continue to keep soil evenly moist for the first 3 weeks until you see the seed emerge. At that point you may be able to shift to every other day watering for longer periods of time providing a deeper watering to encourage roots to go deep into the soil. The length of watering time will vary depending on the length of hose, number of hoses you uses at one time and, of course, the water pressure you have in your home.
Watering new sod is similar to watering new seed. Keep it moist until the sod begins to establish roots into the existing soil.
When the roots begin to establish, cut back watering to every other day and eventually to once or twice per week.
Watering An Established Lawn
With an established lawn, the goal is to develop a deep root system. Watering should be planned to provide 1-1 ½” water per week. This should be in no more that 1-2 applications per week. This will enable the lawn to dry out between watering to encourage roots to grow deep to reach for water. Frequent watering will encourage roots to grow shallow, which will leave your lawn more susceptible to disease and insect damage during times of stress.
If you use a lawn service and have an in-ground irrigation system, it is best to not water on the day of service AND the day after. If your service gets delayed a day due to natural rainfall, your service will likely come the day after. It would make their job impossible if your sprinklers were running the day after rain.