“I water the lawn for 20 minutes three times a week. Am I applying enough water?” I am asked this all the time. The answer seems easy, but in reality it is impossible. Watering plants isn’t based on length, it’s based on how much is applied in a given amount of time.
Volume vs. time
Time is how long it takes for the needed volume of water to be applied. The amount of water that comes out of the hose or sprinkler varies greatly. The pressure or rate of flow from the faucet varies from each property or water source.
The next issue is sprinkler types. All have different flow rates under pressure. In-ground sprinkler heads are measured by their rate of flow in gallons per minute, but that still does not tell us how much water is actually reaching the soil in a given amount of time. Hose sprinklers rarely measure flow rates, making it a guessing game as to how much water is being applied.
The bottom line is, until you know how much water is flowing out of your system, the amount of time it runs does not tell us anything about the amount of water actually being applied.
Measure the water volume
The only way to water by length of time is to know the flow rate of your system. This is easy to determine but will take a little effort. Set several rain gauges or straight-sided cans in the sprinkler pattern and measure the water output for a given amount of time.
Once this test is performed you know that watering for 20 minutes applies 1/8 inch, 1/4 inch, 1/2 inch, etc. of water. This then determines the amount of time a sprinkler must run to provide enough water for a healthy lawn.
How much water does a lawn need?
The next piece of the lawn-watering puzzle is how much water does it take to remain green? Years of Extension research has shown that bluegrass and tall fescue lawns need about 1 inch of water a week under cooler summer temperatures and up to 1 1/2 inches during the hottest and driest weeks of summer.
Combine this fact with your flow rate and you can then build a watering routine. Keep in mind that our clay soils can absorb on two-tenths of an inch of water in one hour. This is why many irrigation systems apply water two to three times a week.
Here is an example: Running your system for 20 minutes applies about 1/2 inch of water. Running three days a week hits the target amount. Unfortunately, those of us who drag out hoses often don’t have that much time, and we apply more water at a time to reduce frequency.
Remember, until you know your rate of application, when watering the lawn it is not about time.
Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to firstname.lastname@example.org.