If you set and forget your lawn sprinkler timing, you will have the correct irrigation timing twice a year: one time in the spring and again in the fall.
The rate of water use on your lawn varies from week to week from April to October. July water use is the peak of that period, with more than 3 inches of watering needed per week. This rate drops to 2 inches per week by mid-August.
Most websites and publications I have read advise the home gardener to place several straight-sided cans — tuna cans work well — around the area to be sprinkled and see how long it takes to fill to an average of 1 inch. That part is good advice, as it shows you two aspects of your irrigation sprinklers. First is rate of application, important to know. How long did it take to reach that inch? Secondly, it will show you how evenly your sprinklers apply water, which, all too often, is not at all even.
If some of the tuna cans catch 1 inch of water and others half as much, don’t be surprised. The spots that get half of the water the rest get will dry out in half the time, and you may waste water by irrigating much more frequently just to prevent those dry spots from going brown. Most sprinklers are designed to overlap 50 percent so that water from one sprinkling device will reach the adjacent similar sprinkler head.
If the dry spots in the lawn are caused by uneven water application, and you set your water schedule by that occurrence, you will overwater the rest in order to keep the dry spots happy.
If you finally get your irrigation rate evened out, and you still have areas of the lawn that dry out first, perhaps you have soil texture variability, compaction or tree root competition. A large tree can transpire 70-80 gallons of water per day in July, so it may dry the lawn nearby.
Soil texture is very variable in North Central Washington, what with all the flooding. Sand and gravel bars are randomly scattered all over. Most often, your house is sited on land that was leveled before construction. This may have left shallow spots or sub-soil layers that provide varying conditions for the plants growing in that area of the yard. You may have sand or clay — pity the gardener who has both — or any texture in-between. This makes a difference in frequency of irrigation, but not total amount of water that plants need.
Finer-textured loams or sandy loams hold twice as much water as sand, therefore, can go twice as long between irrigations.
If the dry spots are caused by uneven soil texture or depth, and you apply just enough frequency and amount to keep those dry areas happy, you will be closer to the proper amount of water on the entire yard.
Remember, the frequency of irrigation must change if you put the same amount of water on with each irrigation. Applying the same water timing each irrigation will lead to over-irrigation in the spring, under-irrigation in the heat of July through about mid-August, then over-irrigation in the fall.
We can’t always be certain of the daily weather, but it’s a pretty good bet to say it will be hot and dry starting shortly after the Fourth of July. It would be best to stay on top of your watering of trees and lawns during this peak-use time of year.
A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World.
Tim Smith is one of four columnists featured.