Your home lawn, like any other plant, needs water to grow and remain healthy and in Idaho that means irrigation will be needed. Lawns that become water stressed take on a gray to purplish-green color. Foot-prints on the grass also become more obvious when grass is stressed. The amount of water to apply at any one time and how often to water depends on several factors including soil type, time of year or weather conditions and the type of grass. Remember to irrigate early in the morning to take advantage of reduced wind, reduced evaporative losses and usually reduced demand on municipal water systems.
A thorough discussion of how to properly water a lawn may be found in the UI Extension bulletin Water Home Lawns: How Much and How Often (CIS 1157).
Kentucky bluegrass and perennial ryegrass lawns may require up to 2 inches of water per week in the heat of the summer, but only about 1 inch in the cooler spring and fall. Turf-type tall fescue which uses the same or more water than Kentucky bluegrass may not need to be watered as frequently because it has deeper roots so it has a larger soil volume from which to absorb water. Buffalo grass, at the other end of the scale, uses very little water and has a deep root system, so it can get by without water for several weeks.
During the spring in April to mid May when temperatures are still cool, most cool-season lawns in Idaho will use about one inch of water each week. From about late May to mid August, lawns will use about 2 inches of water per week or slightly more. Then, from mid August to late September they use just over one inch of water. During periods when significant amounts of precipitation is received, lawn sprinklers systems should be turned off. There is no need to irrigate when the soil is already filled to capacity.
Depending on the year and the onset of winter, grasses will still use close to an inch per week in October, and it is important to keep the soil moist, not overly wet, but moist going into winter. This will help prevent winter desiccation damage.
Lawns with significant shade and wind protection will not need as much water, but remember that the grass will be competing with tree roots for water and nutrients, so extra attention needs to be given to these landscapes.
Whether you have a sandy or clay soil will have a huge influence on the watering practices for your lawn. Sandy soils do not hold very much water and, therefore, lawns growing in sandy soils will need to be watered more frequently. The grass will still use the same amount of water per week, but if the soil cannot hold very much water, you will need to irrigate more frequently. Loamy and clay soils can hold more water than sandy ones and, therefore, lawns growing in clay type soils will not need to be irrigated quite as often.
In either case, apply enough water to penetrate the soil to the depth of the grass roots. Use a shovel or soil probe to determine rooting depth. For most clay type soils it may take 1 to 1¼ inches of water to fill the soil to a depth of 12 inches. For sandy soils, only ¼ – ¾ inch of water is all that may be necessary to fill the soil to a depth of 12 inches. Use a screwdriver to check moisture depth. The screwdriver should easily penetrate the soil to the desired depth you want the water.
In sloped areas, lawns with heavy thatch, or lawns growing in clay or compacted soils, water may need to be applied in small amounts separated by one-half hour increments to allow for adequate water infiltration and to prevent run-off.
Whether you are moving hoses and sprinklers or you have an automatic irrigation system, it is important to understand how much water your system is delivering in a given time period. One simple method to determine this is to set out several catch cans or rain gauges over the area to be irrigated. Run the system for 20 minutes, or a known amount of time, and measure the amount collected. Take an average of the can measurements, but also make note of those that are way off the average. This will tell you that you either have a nozzle problem or a rotating head that is stuck, etc. The average measurement can be converted to inches of water per hour and you can use this information to determine how long to run your system on a particular day. As mentioned above, a lawn’s water needs change with the season, so you should change your automatic sprinkler timers to deliver the correct amount of water depending on the time of year. In the spring, for example, you may need to water enough to replace 1 inch of water every 6 days, but in the summer when the grass is using more water, you may need to water every 3-4 days. Adjusting the timer several times during the season will reduce water waste and give the grass exactly what it needs.
While the system is running, it is a good idea to look for problems with sprinkler heads such as clogged nozzles or rotating heads stuck in one position. Clogged nozzles can be cleaned by unscrewing the nozzle or may involve unscrewing a set screw and pulling out the nozzle with needle nose pliers. On some sprinkler heads, the orifice may be replaced. However, if you don’t have a replacement, be careful not to damage the orifice. Sprinker heads that do not rotate may simply need to be cleaned, but usually need to be replaced as damage to the internal gears could also be a problem. Check your system each spring, taking the time to see each sprinkler operates to its full range since sometimes a head will get stuck at one end of the arc and not the other. Also, be sure the sprinkler heads are in a vertical position so the water is being distributed as designed.
The University of Idaho publication Watering Home Lawns and Landscapes (CIS 1098) provides a more detailed discussion on soil variables as well as an in depth discussion on proper use of automated irrigation systems.
Additionally, a website devoted to lawn and tree water management has been developed by the University of Idaho. In this website you can find water calculation tools to help you determine how much water to apply based on your irrigation system test: www.uidaho.edu/extension/lawn