When to Put Down Crabgrass Killer

The presence of crabgrass in a lawn can produce the homeowner's lament. The ubiquitous annual weed appears in spring and dies only after the first hard frost. Timing the application of a crabgrass-killing herbicide is vital to controlling this pest.

Pre-emergent

The effectiveness of a pre-emergent herbicide requires application before soil temperatures climb to 55 degrees Fahrenheit. Crabgrass is a warm-season weed, and the seeds generally germinate when the soil warms to between 55 and 60 degrees and remains warm for three to seven days. The ideal application window may range from February to May, depending on your climate and weather.

Post-emergent

Controlling growing crabgrass is more difficult. Apply a post-emergent granular herbicide as soon as possible, or in accordance with the manufacturer's recommendations. Caution is required when selecting herbicides: Some products target certain lawn grasses as well as crabgrass. After the crabgrass plant has three or more tillers, or branches, granular herbicides are much less effective. Options thereafter include spot treatment with a wide-spectrum herbicide or waiting for frost to kill the annual.

Tips

When forsythia blooms, crabgrass season near.

Keep and eye out for the bright yellow spring blooms if your landscape or neighborhood has a forsythia shrub. Forsythia is one of the earliest plants to bloom in spring, and its bloom time generally occurs just before crabgrass seeds germinate. While not scientifically accurate, it is a visual wake-up call for you to apply pre-emergent crabgrass killer.

References

• University of Rhode Island Landscape Horticulture Program: Turf Weeds - Crabgrass
• University of Tennessee Extension: Crabgrass Species Control in Turfgrass
• Virginia Cooperative Extension: Crabgrass - A Real Pain in the Grass

Resources

• Cornell University Department of Horticulture: Managing Lawn Weeds