This spring’s temperature has seemed to swing back and forth, which can be confusing when it comes to the timing of some routine lawn care. The key to effective crabgrass control is proper timing of the application of control products—and the time is now!
Pre-emergent herbicides have names such as Pendimethalin, Prodiamine, Benefin, and others. They are often purchased as “Crabgrass Preventer”, “Pre-emergent Crabgrass Control” and similar trade names or in combination with lawn fertilizers where we seldom pay attention to the name of the actual chemical.
There are products made to spray crabgrass after it has emerged, but they are only effective when the crabgrass is very young and tender. One of the post-emergent crabgrass chemicals is Quinclorac which is often mixed and marketed with other broadleaf weed killers. This product is most effective when applied to crabgrass seedlings in the two or three leaf stage. Pre-emergent herbicides, whether chemical or organic, must be applied and watered into the soil about two weeks before the seeds of crabgrass or other annual weeds begin to germinate. By late July and August, when crabgrass is very noticeable, these products are not very effective.
In the spring, once soil temperatures reach approximately 55 degrees, crabgrass seeds begin to germinate. Many of these products are effective for about six weeks, so if they are applied too early, they will have lost much of their effectiveness before the crabgrass seeds begin to germinate. Michigan State University has a tool to help take the guessing out of the time to apply pre-emergent herbicides. The “Crabgrass/Annual Bluegrass Preemergence Timer” will take your zip code and show you the optimal time to apply pre-emergent herbicides.
A precaution with the use of any pre-emergent crabgrass or annual weed control is that it will also kill germinating grass seeds. So, if you plan to do any reseeding, know that these chemicals will not discriminate between the grass seed and the weed seeds. You must either avoid the use of these products in the newly seeded area or use one that is clearly labeled for use on newly seeded lawns. Read and follow label directions carefully.
Regardless of your choice for crabgrass control, the timing of application is crucial. However, crabgrass does not all sprout at once, so it might still be worthwhile to apply a pre-emergent herbicide even after a little crabgrass has begun to sprout. It won’t kill the plants that have already sprouted, but will prevent more seed from sprouting. If you have some “hot spots” along curbs, sidewalks, driveways and on sunny, south-facing slopes, you might consider applying the chemical to these areas a week or 10 days earlier than the remainder of the lawn.
Residents of Stearns, Benton, Morrison, and Sherburne counties can direct questions to (quincy@umn.edu) or call (320) 255-6169 x 3.
Source: Michigan State University Crabgrass/Annual Bluegrass Preemergence Timer: https://gddtracker.msu.edu/
By Quincy Sadowski, University of Minnesota Extension, Horticulture
