Most Montana lawns are composed of Kentucky bluegrass and fine fescue (Both are Cool Season Grasses). These need frequent watering during the hot summer months to maintain their green color. However, too many people water too much, too shallow, or too frequently to the detriment of the soil and the plants. Such watering keeps the upper layers of soil near the saturation point, encouraging shallow rooting and production of weak turf that is highly susceptible to damage from drought, traffic, and pets. It also encourages the growth of weedy grasses, such as annual bluegrass, that prefer moist conditions.
Water no faster than the soil can absorb the water. Much of Montana has heavy soils that must receive water slowly for it to soak in. Even a very thin layer of heavily compacted soil substantially reduces the rate of water infiltration.
How much water to apply depends upon the soil’s water-holding capacity, how wet the soil was when you began irrigation, and the soil’s drainage. In general, apply a slow, steady stream of water until the soil is moist to a depth of 6 inches.
How often you water depends upon the grass species, humidity, rainfall, wind, the slope of the terrain, and soil type. In most cases, the rule of thumb is to water just as the grass begins to wilt. The exception to this is, with newly seeded lawns that must never be allowed to dry out.
Water deeply 2-3 times per week, rather than daily. Watering daily will give lawns a shallow root system that will dry out during the heat of the summer. Daily watering may force the bluegrass and fescue into a summer dormancy as a last resort to protect itself from death due to the shallow root system being constantly dried out. Shallow root systems dry out faster and weaken your turf. Watering less frequently and deeply will encourage the lawn’s root system to push deeper in the soil to seek water making the lawn stronger and more drought-resistant.
Early Spring – Apply about 1 inch of water per week to the lawn.
Late Spring – Apply about 1.5 inches of water per week to the lawn.
Mid-Summer – Apply about 2 inches of water per week to the lawn.
Fall – Apply about 1 inch of water per week to the lawn.
How long should you water to get one inch? We can’t tell you that, because we don’t know what kind of sprinkler system, how many gallons per minute are being delivered, or what the water pressure is for each yard. But you can measure that for yourself. Place empty tuna cans (empty tuna cans have a depth of 1 inch) and place them in the area you plan on watering. Run the sprinkler system and see how long it takes to get a half-inch of water in the can (Half Full) – just like a rain gauge. A rule of thumb for underground sprinkler systems are pop up spray heads deliver twice as much water to its area than a rotor driven or impact sprinkler head in the same time frames.
Water only when the temperature is rising, for example, in the early morning. This ensures the lawn will dry completely before nightfall. Also, it’s cooler and less windy in the morning, so you have less evaporation, which will save money on your water bill and take less time to get the water on the lawn. Watering during the heat of the day wastes water through excess evaporation. Watering at night can increase the chances of disease in the lawn and a wet lawn at night, on a regular basis, can lead to fungus and disease issues. If morning watering is impossible, watering any time is always better than not watering at all!
Avoid watering so long that it runs off the lawn and down the street. If water starts to run down the street before the lawn receives the half-inch to one inch of water per session, turn off the water and wait for it to soak in, then resume watering. This might happen if the lawn hasn’t had water for a while or if the soil is compacted to the point where the water cannot penetrate. Compaction occurs in heavy clay soils and in heavy traffic areas. Aeration may remedy this problem, but usually, a change in the condition causing the compaction is one strategy for long term relief. Annual aeration is another solution to remedy compaction.
Special Thanks to "Montana Master Gardener Handbook" Fifth Edition 2012 Pg 246 - Water