Controlling crabgrass in home lawns

Each spring we fret over how to control crabgrass in our home lawns. In the Columbia Basin, the outdoor temperatures can change so quickly that we easily miss the proper timing when apply pre-emergent to control weedy grasses. It’s easy to apply the pre-emergent too early or too late. The timing according to soil temperature is my preferred method of knowing when to apply pre- emergent.

About the middle of July most of us are confronted with our lawn showing signs of these flat clumps of light green grass, this later turns a dark green. The dreaded crab grass! Until the crab grass started sprouting we were fairly pleased about how our lawn was looking.

At that time homeowners call the WSU Master Gardeners for help. The first step in controlling crabgrass is to keep the lawn well maintained. Mow at a turf blade length of about 2.5 inches; apply fertilizer developed specifically for the purpose of creating a healthy dense lawn and water your lawn deeply in the early morning, never in the evening.

Spring is the time of year when the homeowner will want to apply a crabgrass control. Using “Plant Phenological Indicators” the crabgrass will not germinate until the change in temperature becomes suitable for the forsythia shrub to be in full bloom and forsythia flowers will wither near crabgrass germination. The pre-emergent will help with the control of other annual weeds such as purslane, oxalis, and spotted spurge, but these weeds don’t germinate until the soil temperature is at 75 degrees, which requires a separate application.

Crabgrass seed germinates when soil temperatures are at 55 degrees at a soil depth of 1-inch for a week. You will need to diligently check soil temperature to catch the optimum time to apply the pre-emergent right before the crabgrass germinates. If you apply the pre-emergent too early it will lose its effectiveness before germination. Use a soil thermometer to check soil temperature or access current soil temperature for your area at AgWeatherNet, Washington’s Agricultural Weather Network, weather.wsu.edu.

For gardening questions, contact the Master Gardeners at the WSU Grant-Adams Extension office at 754-2011, Ext. 4313 or email your gardening questions to ga.mgvolunteers@ad.wsu.edu. Visit our web page at grant-adams.wsu.edu.