<font color = blue>FROM THE FIELD</font> Crabgrass Breakthrough – What’s Going On?

This year, a surprising number of lawn care companies are reporting crabgrass breakthrough despite having made preemergent herbicide applications in the spring. In addition to treating the crabgrass, lawn care companies should also pay attention to caring for the customer by educating them about what may be causing this atypical level of crabgrass pressure.

Weed control “failures” occur for any of a number of reasons, including:

• Heavy spring rains – Very wet conditions deplete the oxygen in the soil. When this happens, anaerobic bacteria proliferate and can prematurely break down the herbicide. Much of the Northeast and Midwest experienced wet springs this year, which may be contributing to breakthroughs in those areas right now.
• Cool spring soil temperatures – Cool soil temperatures can delay crabgrass germination, which results in weed germination as preemergent residual winds down. This, too, is a likely cause of late-summer breakthroughs this year given the cool May much of the country experienced. Unseasonably cool spring temperatures may warrant an additional preemergent (late spring) application to ensure control.
• Warm soil temperatures – Higher soil temperatures stimulate soil microorganism activity, which speeds herbicide breakdown.
• Thatch build up – An excessive thatch layer (more than ½-inch thick) will prevent proper incorporation of the herbicide barrier.
• Poor turf density – Optimum crabgrass management occurs in dense, healthy lawns that are capable of out-competing weeds for space, moisture, nutrients etc. Thin turf allows weeds to get a foothold.
• Weed Pressure – Considering that one plant is capable of producing more that 1,000 seeds, even 95 percent control will leave 50 seeds to germinate. Additionally, the more seeds present, the quicker the depletion of the herbicide, since the weed uptake is the primary reason for chemical loss under normal conditions.
• Herbicide barrier breaks – Anything that breaks the herbicide bar in the soil, such as aeration, animal activity, construction, drought cracks, or post-treatment plants provide avenues for crabgrass seedlings to emerge without contacting the herbicide.

Of course, many other factors can contribute to weed breakthrough, including poor herbicide selection, poor weed identification, improper application, and improper application timing, but those factors are relevant for any application.

Lawn care companies experiencing crabgrass breakthrough on properties they treated earlier this spring should review those properties and those applications to better understand what may be causing this problem and ensure that it does not occur next year.

The author is the Manager of LESCO’s Tech Services Department.

From the Field is a monthly column in Lawn & Landscape magazine by Toorish and Brian Kelley. Send your comments and questions to fromthefield@gie.net.