I’ve been working in public horticulture for over 30 years now, and the battle with crabgrass is always a consideration when it comes to maintaining healthy lawns. The common, hairy crabgrass (Digitaria sanguinalis), a warm-season annual, can be challenging to manage successfully without a preemptive strike and the knowledge that you have a tight window for maximum control. The years that I’ve forgotten this “control window” is when I see a nice, dark green lawn become speckled with lighter green, wider-bladed grass that takes advantage of the warming soil and any thin spots. But with a battle plan in place, you’ll mitigate your crabgrass issues before they even begin.
How crabgrass spreads
The good news about crabgrass: Because it’s an annual, in fall all crabgrass plants will die. However, the real danger is in its prolific reseeding (below mower height) that sets the table for the resurgence of another generation that will germinate when soil temperatures warm in spring. Light green leaves emerging among your turf in mid to late June are common indicators of crabgrass infiltration. The prostrate stems, featuring wide blades with hairs, have a bit of purple to them. The stems also tend to trail and root at the nodes. Your local Extension service or agent can likely help you positively identify crabgrass, but again, in the Midwest its germination is in spring after the soil reaches 55°F to 60°F for several days. This grass loves hot, humid weather, populates in weaker turf areas and openings, and revels in frequently irrigated areas.
Mitigate germination with your mower height
There are two very common approaches to turf care that will help mitigate the establishment and spread of crabgrass. This includes maintaining a thicker lawn and committing to a 3-inch elevated mower height. Longer grass will help shade out crabgrass, although that mowing height can be a challenging sight for those that prefer a shorter cut.
Properly apply pre-emergent herbicide
There are also chemical controls to consider, the most effective being a well-timed application of a selective pre-emergent herbicide for crabgrass control that inhibits seed germination, thereby thwarting the looming population boom.
There are many chemicals on the market for this control and as a rule of thumb, application timing universally coincides with the forsythia (Forsythia sp., Zones 6–9) bloom window in April. Make sure you can identify a forsythia, of course, and keep an eye on that bloom time, which could have wide variability depending on how spring arrives. It is also usually recommended that you apply before an impending rainfall or supplement your own irrigation for activation.
Some of these pre-emergent products also have fertilizer, although I personally don’t find that component essential, as the timing is quite early for turf in general. I handle fertilization on a separate schedule (June and October). Keep in mind that a pre-emergent chemical for crabgrass will adversely affect the germination of other grass seed you’ve spread in that area.
Crabgrass is so difficult to control due to the length of the germination season, hence the need for that timely pre-emergent application. Hand-removal of smaller patches is certainly an option, but addressing the existing turf conditions of vigor and mowing height, as mentioned, will further help with successful control efforts that include that well-timed pre-emergent application.
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Mark Dwyer is the garden manager for the Edgerton Hospital Healing Garden in Edgerton, Wisconsin, and he operates Landscape Prescriptions by MD.
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