When the temperatures are above 70 degrees, cool season grass needs to be watered once a day. When the temperatures are above 80 degrees, cool season grass needs to be watered twice a day. We recommend watering at 3:00 AM and then again during the heat of the day. We have heard the argument against watering during the heat of the day because it can burn the lawn. This is only true if you water for 1 – 2 minutes per zone with rotor heads because you are barely getting the grass wet and the water droplets act as a magnifying lens. If you run the sprinklers for at least 10 minutes per zone with rotor heads and 3 minutes per zone of pop up heads you will cool off the lawn and give it the water it needs.
I have also hear the argument about watering less frequently, but for a longer period to make the roots go deeper. The problem with this philosophy is that it ignores the biology of our grass. Cool season grass has shorter roots when the temperature warms up. You cannot change the biology of the grass. Roots are getting shorter now and no amount of water will make them grow deeper. Watering deep can stress out the lawn more because of the extreme variances between too wet and too dry. To relate this to a human, if you are running a marathon race on an 85 degree day, would you drink 2 gallons of water the day before the race and then not drink for 2 more days because you already had enough water for the week? You would dehydrate at the same rate if you did not drink before, during, and after the race.