Turfgrass Establishment Series – Irrigation
Irrigation Practices During Turfgrass Establishment from Seed
In the last edition of the Turfgrass Establishment Series, Dr. Ed Nangle outlined the steps to planting and mulching seed. In order for a seed to germinate, it must first absorb water – a process termed “imbibition”. Once the seed has undergone imbibition, the germination process is irreversible. This means the seed must have sufficient moisture to ensure adequate germination and subsequent growth and development. Irrigation is our way of managing moisture during turfgrass establishment from seed.
In general, we do not want the seed to dry out. Typical irrigation recommendations for mature turf involves deep and infrequent cycles, resulting in water available for root update. This process is not needed for turfgrass seed since the seed lies at the soil surface. Also, overirrigation can increase incidence for disease activity and can lead to seed washing out. The mulching techniques listed in Turfgrass Planting & Mulching can help limit movement of seed. To keep the seed moist during establishment, light and frequent irrigation cycles are recommended.
"How many times per day and for how many minutes?"
I wish it were that simple. Research I was a part of at the University of Arkansas found that irrigation replacing 125% of the previous day’s evapotranspiration (ET), which is the combination of water lost from the soil surface (evaporation) and through plant processes (transpiration), was sufficient for tall fescue establishment from seed. Evapotranspiration accounts for temperature, wind speed, relative humidity, solar radiation, and day of year. We would expect hot, windy days with low humidity to have greater ET loss than a hot, humid day with no wind.
ET values are expressed in inches of water lost over a period, typically one day or one week. For example, an ET value of 1.13 inches is forecasted for the upcoming week (September 4-10, 2023) in Columbus, OH.
To replace 125% of 1.13 inches this week, we must apply 1.41 inches of water (1.13 inches × 125%). This equates to 0.2 inches of water per day. How many minutes it takes to apply 0.2 inches of water will depend on your irrigation system output. The output of your irrigation system is referred to as "precipitation rate", which is expressed in inches per hour. The best way to determine precipitation rate is through an irrigation audit.
However, the precipitation rate for a given sprinkler head can be found in the manufacturer’s catalog. It is important to note the nozzle, arc pressure, and head spacing for your system to ensure accurate precipitation rates.
For example, let’s say your irrigation system has a precipitation rate of 1.3 inches per hour, accounting for distribution uniformity (which will not be covered here). That equates to approximately 0.02 inches per minute (1.3 inches per hour / 60 minutes per hour). To apply 0.2 inches of water each day, the irrigation system must run for 10 minutes (0.2 inches ET per day / 0.02 inches irrigation per minute). As stated earlier, applying all this water in one cycle would promote standing water and seed movement. I typically recommend irrigation at least four times during the daytime hours, but this will be dependent on the capabilities of your irrigation system. In this scenario, I would recommend five irrigation events throughout the day with two-minute run times. Adjust your irrigation run times as ET changes.
Equation for calculating irrigation run times during establishment:
Once the turfgrass seed begins germinating and producing leaves, reduce the frequency of irrigation to promote deeper wetting of the soil. Prior to the first mowing, irrigation once daily in the early morning hours is sufficient. Mature cool-season turfgrass stands require irrigation replacing approximately 80% of ET and can be irrigated 2-3 times per week.
Want to catch up on other turfgrass establishment steps? Check out the Turfgrass Establishment Series.