Most grass plants have their roots at a depth of 4-6 inches. The soil should be moistened to this depth. Soil composition is the biggest factor when figuring out how much water is required to achieve this depth. Although it will vary from lawn to lawn, a good general rule to follow is; loam and clay soils require 1-1 ½ inches of water, while sandy soil requires ½ to ¾ of an inch of water to moisten to the same depth. Soils differ in the time it takes to achieve these levels of moisture. Clay soil is very compacted and water takes a long time to penetrate such a soil. Sandy soils on the other hand are very loosely compacted and require very little time to achieve the desired depth.
The time in which you water is on of the most important aspects of your success with your watering schedule. Mornings are the best time for watering. The grass has a chance to utilize the water before evaporating, and the leaf blade dries out during the day. Evenings are your 2nd best time if you are unable to water in the mornings. One of the drawbacks to watering in the evening is the leaf blade has a good chance of remaining wet throughout the night. Given the right conditions, this can lead to diseases such as Red Thread or Leaf Spot. Watering during the day is the worst time to water. Most of the water put down is lost to evaporation and the droplets can act as a magnifying glass and cause damage to the leaf blade. If this is the only time you can water, it is better than nothing, if your lawn is showing signs of drought stress.
Most lawns require 1-2 inches of water per week during the growing season. This will vary with local weather conditions. Hot, dry, windy weather creates the need for greater moisture in your lawn. An excessive thatch layer acts like a sponge and traps much of the moisture you put down. Aerating your lawn will improve water transfer to your grass’s root system. Trees in your lawn will compete aggressively for any water you put down. These areas require more moisture than other areas of your lawn. Gradually lessen your watering schedule as fall approaches. This tells your lawn to “harden off” and prepare for winter. Watering up till the first heavy frost will freeze your leaf blades causing massive damage to the cell structure of the plant. This will cause many problems come springtime.