Turfgrass irrigation, along with mowing and fertilizing, is one of the basic management tools we implement when entrusted to the care of turf. Although the application of water to a turf to maintain adequate growth sounds rather simplistic, turfgrass irrigation is often done improperly leading to wasting water and turfgrass damage. When designing a conservative turfgrass irrigation program there are four questions to be answered:
- When to irrigate.
- How often should you irrigate.
- How much water to apply at any one irrigation.
- How to efficiently apply water.
Turfgrass irrigation should be considered nothing more than a supplement to rainfall. If adequate rainfall is occurring, then the turf should be fine without added irrigation. In the absence of rainfall, supplemental irrigation is needed to maintain a healthy, green turf. However, drought tolerant turfgrasses should be able to withstand drought conditions by going dormant. Once moisture is added through either rainfall or irrigation, the turf should resume normal growth patterns.
Within the state of South Carolina we grow both cool-season grasses including fescue and ryegrass as well as warm-season grasses including bermudagrass, centipedegrass, St. Augustinegrass, and zoysiagrass. There is a difference in drought tolerance among the various grasses so be sure to choose a variety that will fit with your anticipated irrigation practices. The warm-season turfgrasses are the most drought tolerant with bermudagrass and zoysiagrass being the best. St. Augustinegrass and centipedegrass are moderately drought tolerant with the cool-season grasses fescue and ryegrass being the least drought tolerant.
Two questions often arise when discussing turfgrass irrigation: how do I know when to irrigate and how often should I irrigate. Turf should be irrigated when it shows signs of moisture stress. The catch phrase that is often used in describing turfgrass irrigation is to water deep and infrequent. Apply enough water to soak the root zone down to 8 to 10 inches of soil, then wait until the turf has thoroughly dried before irrigating again.
As with many answers to questions, this often leads to several other questions: How do I know when my turf is dry and how long do I run my irrigation device to provide enough moisture to avoid moisture stress? When a turf starts to enter a phase of moisture deficit, the leaves will begin to turn a slightly darker shade of green. On turf with a broad leaf blade, you may see the leaves folding somewhat. Another good test is to walk across the lawn during the late evening and watch your footprints. If the grass in the print bounces back up then plenty of moisture is available in the plant. If the grass lays flat, you will need to water the next morning.
Keep in mind that not every blade of grass will dry at the same time. Most large areas of turf will develop localized dry spots; small areas that dry before the majority of the lawn. These areas can be hand watered when needed instead of watering large areas that show no signs of moisture stress. Persistent localized dry spots may indicate a hydrophobic condition which will inhibit water infiltration. If this is an issue, the area can be aerified or spiked. Another option is to apply a soil surfactant or penetrant. These products help break the surface tension allowing water to infiltrate.
The ideal time to water turf will be between 2am and 10am. During this time, winds are usually calm and loss of water through evaporation is minimized. The grass leaves will dry quickly from the irrigation which will lessen chances of disease. How often you irrigate will depend on environmental conditions including weather and soil types. Evapotranspiration rates (ET) can vary from day to day on turf and will be influenced by weather conditions. Cool, cloudy conditions with a somewhat high relative humidity may cause ET rates to hover around .05 inch per day. Hot, dry conditions with a low relative humidity can cause ET rates to climb to over .10 inch or more per day. Rates as high as .15 to .2 inches per day may also be seen under certain conditions, such as the extreme heat of summer. When developing an irrigation program, keep in mind that weather conditions can influence how quick a turf will dry and how often you will need to water.
When averaging ET rates for turf from various types of gauges, researchers have determined that the average weekly ET rate for turfgrass is 1 inch. Theoretically this means you should apply 1 inch of water per week to make up for the deficit. This may work well under controlled environments but may not work well in real world applications. A good, conservative irrigation program for turf would involve the following:
- Monitor your lawn daily for signs of moisture stress using the techniques outlined above. Keep in mind that there will be areas on the site that will dry quicker than others.
- Keep an eye on the weather. Hot, dry conditions will cause a turf to dry quicker than cloudy, cool, wet conditions. This will give you some insight on when to be a little more vigilant for moisture stress.
- Be sure to allow the turf to dry thoroughly before irrigating and only water those areas showing signs of moisture deficit. This may mean watering localized dry spots, certain zones that dry quicker than others, or a full run of the irrigation cycle. Target to apply water where it is needed. If it looks dry, water it. If it doesn’t look dry then do not water it.
- You will want to apply somewhere between .75 to 1 inch of water when you do irrigate those dry areas or with enough water to wet the root zone down to 8 to 10 inches. The amount of water your irrigation device is applying can be determined by placing measuring devices in each zone of your irrigation system, then turning on the zone and time how long it takes to put out one inch of water. That is the length of time to water. The same can be done for hose-end applicators. As far as how often, again, it is when the turf shows signs of moisture deficit. Irrigation intervals can vary from several times a week to several time a month depending on environmental conditions and soil types as well as turf species.
The final item to address with turfgrass irrigation is the method of water delivery. In-ground irrigation systems can be very effective and can conservatively keep your lawn well watered. However, the system needs to be properly designed and installed, ensuring that zones set up for watering turf are doing just that, watering the turf. Mixing turf irrigation with ornamental plant irrigation on the same zones can cause scheduling problems. Turfgrass is also watered overhead. Watering shrubs and ornamental plants overhead increases the likelihood of disease.
You will need to become proficient at operating the clock system that is used to set the time and frequency run times of the system. When the turfgrass can be monitored on a daily basis, the clock should be set to manual and turned on as needed. Hose-end water sprinklers can also be used to water turf and can be very conservative as you are placing the sprinkler only where you need it. The important thing to do with hose-end devices is to determine how much water they are putting out. This can be measured using the same methods outlined above for in-ground systems. There are several types of timers that can be attached to faucets that will avoid overwatering. Be sure to check all water delivery items on a regular basis to make sure they are operating as needed and there are no broken rotors, risers or pipes. Finally, install a rain shut off device on all irrigation systems to avoid watering during rain events when the system is set to water automatically.