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Crabgrass: annual weed, perennial problem

Crabgrass may be an annual grassy weed, but it certainly seems to be perennial problem for homeowners. Preventing it every spring can be done, but without knowing more, that becomes the normal expectation and need every year.

Conditions for crabgrass

As a warm season annual, crabgrass will not show itself until weather conditions allow for germination. Crabgrass germinates once soil temperatures are greater than 55 to 60 degrees Fahrenheit for seven to 10 consecutive days and continues until soils reach 95 degrees. In comparison, other annual grasses germinate as soils get warmer than 60 degrees. (Last week, for example, soil temperatures were in the low to mid 40s, so still way too cold to promote any kind of crabgrass germination.)

Other conditions that favor crabgrass in the lawn are mowing lower than 2½ inches, as this opens up the soil to sunlight, something crabgrass seed needs to germinate. Bare areas or thinned turf also promote germination. This includes areas along sidewalks and driveways where deicing salt was used.

Plan for prevention

The better strategy is to improve conditions and promote lawn health, which will naturally help prevent crabgrass. Reducing soil compaction, over-seeding to increase the number of grass plants per square foot, topdressing with a quality black dirt or compost, and proper mowing, can all make a great difference in whether or not you have crabgrass issues. Light frequent watering can promote crabgrass seed germination.

If the decision is to treat with a crabgrass preventer – which can control other annual grassy weeds – timing will be critical. Most preventers have a limited time to protect the lawn. Applied too early, the protection runs out before the crabgrass seed finishes its germination period. If applied after the germination begins, you will still see some crabgrass.

If our summer temperatures are more moderate and remain below 95 degrees, there can be germination throughout the summer, long after any product can be effective. The bright spot is since crabgrass is an annual, every plant will die with the first frost and you get to start all over fresh next year.

After crabgrass germinates, its immediate need is to grow up and get “on top” of the lawn. Early on, it is very easy to get rid of if you only have a small area to deal with. It has a weak singular root and only two to four small leaves. Once it is up in the sunshine, it begins to form a rosette of leaves, smothering the lawn grass below. As it continues to develop, roots are formed at nodes along the stems and dig down into the soil, making any kind of mechanical removal just about impossible. If you attempt to pull it then, it comes apart in pieces.

Before you treat

While I am writing about crabgrass this week, please note, we are several weeks away from applying any kind of preventative product. However, you can go out now, look for places that you might expect crabgrass to show up, and consider how to better the lawn to help naturally prevent crabgrass.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.