by William H. Bohl
Like most homes, the heat in my house is regulated by a thermostat. When the house cools a few degrees below a set temperature, the system turns on to warm it back up. The automatic system maintains a fairly constant temperature regardless of the outside temperature without me having to do anything once the temperature is set. Unless you’ve invested in a sprinkler controller system that monitors weather, irrigating a lawn isn’t as simple as controlling temperature in your house.
Many lawns are looking better now than just a few weeks ago likely, in part, because the amount of water being applied more closely matches what the lawn needs. Lawns use water at a low rate in April, increasing to a peak in July of about one-quarter inch per day and then tapers off as fall approaches. In September, lawns will likely use about 60 percent of that in July. With cooler weather, the number of days between irrigations needs to be increased. Apply the same amount of water each time, just less frequently.
Roots of a well established lawn should reach a depth of about 12 inches (root zone), and adequate moisture needs to be maintained to this depth. Non-sandy soils hold approximately two inches of water per foot of soil. A lawn needs to be irrigated when about half the water in the root zone has been depleted. Each time a lawn is irrigated, the root zone should be re-filled to capacity. For example, in July a lawn growing in a non-sandy soil likely needed to have about one inch of water applied every four to five days. In September, one inch of water should be applied every six to seven days. Again, the same amount of water is applied, but less often.
Each lawn is different, so it’s important to determine an irrigation schedule for your situation. An easy way to check soil moisture in your lawn is to push a small diameter (⅜ inch or so) rod into the ground. With adequate moisture the rod should easily move into the soil to a depth of 12 inches. When the rod reaches dry soil, you won’t be able to push it farther. If the rod easily moves deeper than about 12 inches, then too much water is being applied. Of course, if there are trees in the yard, deeper watering is beneficial for the trees. For lawns growing in very sandy soil, this method is not reliable because even in dry sand a rod can easily be pushed into the soil. You’ll need to dig a hole and feel how much moisture is in the soil.
There are reasons to not apply too much water. Too much water could leach nitrogen, an essential plant nutrient, below the root zone. Roots need oxygen, too wet soil excludes air. Also, if you’re irrigating with metered water, more than needed is costly.
Bohl is Extension Educator with University of Idaho located in Blackfoot. He may be contacted at 785-8060 or firstname.lastname@example.org