Shallow, frequent sprinkling to add a little water each day is the worst way to water a lawn. It only encourages shallow, weak roots, crabgrass and disease development. Before sunrise, irrigate to full depth of the root system. Then wait until the supply is nearly exhausted before watering again. But, do not let the grass undergo drought stress. The key here is duration, not frequency. It is better to irrigate for longer periods of time and less frequently than vice-versa.
Bluegrass and red fescue roots may not reach depths greater than 4 to 6 inches during the summer. About one inch of water (620 gallons per 1,000 square feet) can be stored in an average Missouri soil to this depth, and this should last about a week. A reasonable rule for summer lawn irrigation is to apply enough water in addition to natural rainfall to total one inch per week. On sandy soils that cannot store this much, greater frequency with lesser amounts may be required. During the heat of summer, it may be necessary to irrigate an extra day, to compensate for the lack of rainfall and extreme heat.
Don’t guess at how much water must be applied to reach desirable wetting depth. Place tall, straight-sided cans in the sprinkler pattern. Measure water depth in the cans when the root zone is thoroughly wetted, that is, when puddles and runoff begin to form. Thrust a small probe (screwdriver) into the soil. Decreased resistance to the probe in wetted soil can help gauge depth of wetting.
Most sprinklers apply water faster than soil can absorb it. Few established lawn soils in Missouri can absorb 1/2 inch per hour; many absorb much less. To prevent waste, move sprinklers frequently. Properly engineered permanent irrigation systems with timing controls for “interval watering” do the best job. A soaker hose is also an excellent choice.
Steep slopes, hard spots and hot areas require special attention. Mechanical aeration, extra slow watering and use of wetting agents may help water infiltration.
Keep these things in mind and it will help you minimize the damage a St. Louis summer can do to your lawn.