Turf - Irrigating Lawns

A frequently publicized rule of thumb for irrigating lawn turf is to apply 1 to 2 inches of water per week. Although this may keep turf green during most of the summer, it may be wasteful or ineffective, particularly where thick thatch layers, heavy clay soils, or "hard pans" exist. In general, it is much more effective to water turf by need and not by rule.

The best time to irrigate the lawn is when turf just begins to show signs of wilt. This of course is not always easy to determine. However, there are two prominent symptoms exhibited by turf as it begins to wilt: (1) foot printing, and (2) turf develops a blue-green or blue-gray leaf color. After walking across drought stressed turf, leaves of the walked upon plants remain depressed for several minutes, providing the "foot printing" effect. When turf exhibits foot printing or develops a blue-gray color, it is very important that the turf be irrigated to avoid permanent wilting of leaves, and induction of summer dormancy (i.e. browning of the turf). During periods of extreme heat and drought stress dormant plants may die. Some professional managers use instruments that measure soil moisture levels, but most rely on the visual symptoms expressed by drought stressed turf, and frequent soil probing to determine moisture levels.

The lawn should be deeply watered as soon as it has been determined that turf is under drought stress. The duration and quantity of water applied should be sufficient to wet the soil to a depth of 4 to 6 inches. Deep watering encourages the roots of grass plants to grow deeper into soil. This indirectly enhances the drought resistance of turfgrasses by providing a greater reservoir of soil water for plants to draw upon.

The duration and quantity of water needed depends greatly upon soil texture and structure, and thickness of the thatch. In many housing developments, lawns are grown on sub-soils deposited during excavation that have higher clay content. This type of soil resists water penetration and the downward movement of water (percolation) through the soil. In many situations, it is extremely difficult to get water percolation to a 4 to 6 inch depth. Hence, on heavy, clay soils or where hard pans and dense layers of thatch exist, water must be applied slowly. If the rate of water applied exceeds the rate at which water infiltrates soil, water will run off and be wasted.

Sprinklers will have to be moved as water begins to run off onto sidewalks or the driveway. The sprinkler must be continually moved back and forth on rapid run-off areas at 30 to 90 minute intervals or as needed until water has penetrated to the desired depth. Installed underground systems will require zoning, turning one zone on, the turning it of and turning another zone on, back and forth until fully watered. The same principles and practices apply when irrigating turf on slopes or other rapid run-off areas. Probing the soil will enable an individual to determine soil moisture depth. This is best achieved using a probe that removes soil plugs. Probing with a screwdriver also may provide a good indication of depth of soil wetness.

Night irrigation is discouraged because it may encourage diseases, particularly when night temperatures exceed 68 F. Night irrigation, however, is a standard practice on golf courses and other large recreational turfs. Night irrigation is preferred in the aforementioned situations because it does not interfere with daily management operations or recreational activities, and it reduces the potential for soil compaction in turf areas with heavy traffic. The best time to irrigate is during the coolest part of the day when there is no wind. These conditions usually occur during early morning or late afternoon, and help conserve water by reducing evaporation. Conversely, mid-day irrigation during hot, sunny or windy days should be avoided because of increased evaporative losses of water. Furthermore, water that collects in low areas and inundates the turf for an extended period may cause scald injury during sunny, hot periods. Contrary to a popular misconception, scald will not occur if water is applied properly on hot days when turf is under heat and drought stress. Scald will only occur when turf has been inundated by water for an extended period of time on sunny and hot days.

Excessive or frequent irrigation can be just as detrimental to turf as inadequate watering. Turfgrass plants that are subjected to frequent irrigation become lush and succulent. These lush plant tissues are more susceptible to injury from heat, drought, and wear stress, and are more prone to disease injury. Frequent irrigation also discourages roots from growing deeper into soil thereby limiting the soil volume from which plants obtain water and nutrients. Excessively irrigated turf is more prone to invasion by moss and algae, and wet soils are more easily compacted by traffic. Soil compaction is common on homelawns, particularly those lawns where children play frequently. Turf growing in compacted areas will have restricted root systems, and lose vigor as a result of poor aeration. Because of the harmful effects of compaction, turf growing in these areas has a tendency to wilt more rapidly during dry periods, and normally thins-out allowing crabgrass, knotweed and other weeds to invade the area. Compaction can be minimized by not subjecting turf to traffic, and other heavy uses when soil is wet. Compaction can be alleviated by aeration, but only rototilling, adding soil amendments (if necessary), and reseeding or sodding the area will cure the problem.

Light and frequent irrigation also restricts rooting, reduces stress tolerance, enhances germination of weed seed (especially crabgrass, goosegrass and white clover seed), and encourages diseases. Many homeowners get into the habit of applying small amounts of water to their lawns on a frequent basis. These homeowners often return to a brown lawn after taking a one or two week vacation during hot, summer months. Grasses in these brown lawns are usually not dead, but in a state of dormancy. However, in extreme cases of drought coupled with a past history of light and frequent irrigations, large areas of the lawn may die. Homeowners who are likely to take vacations longer than one week in the summer are best advised to begin restrictive, deep and infrequent irrigation several weeks in advance of their vacation. Light and frequent, or excessive watering prior to leaving on vacation is likely to cause more severe injury than not applying any water at all. This is particularly true where Kentucky bluegrass or perennial ryegrass are the predominant turf species.

Adapted from "Irrigation and Water Conservation on Home Lawns", Agronomy Memo 88, from the University of Maryland