How to Kill Crabgrass for Good
Unlike perennial weeds, crabgrass is an annual plant; this year’s crop will be gone for good with the first hard freeze this fall. Unfortunately, its descendants will return next year. A permanent end to all crabgrass is only possible if all of your neighbors on the continent eradicate crabgrass in their yards. Failing that unlikely solution, it is possible to kill crabgrass more or less for good from your yard by using a several strategies over a period of a few years.
The Healthy Lawn
Mow grass at a height of two-and-a-half to three inches tall, which is tall enough to shade seeds and seedlings. Mowing often to maintain this height not only deprives crabgrass of the sunshine it needs to germinate and grow it also deadheads seed spikes before they can produce seeds.
Water infrequently but deeply—crabgrass roots are shallow. Make sure that turf grass gets its inch a week to provide water down deep where turf grass can use it to grow strong roots.
Use only slow-release nitrogen fertilizers to provide a steady supply of the nutrient that contributes to turf growth. Fertilize early in the year to help grass compete successfully. Don’t fertilize during the dog days of summer when your turf grasses are semi-dormant; mid-summer fertilizer feeds weeds, not turf grasses.
Aerate lawns to reduce thatch and compaction. The Colorado State University Extension (CSUES) recommends annual aeration; check with your local extension for their recommendation for your area.
Apply crabgrass herbicides early enough to be effective; when the soil temperature is between 55 and 60 degrees Fahrenheit (12.8 to 15.6 Celsius). CSUES identifies this period as when the forsythia are blooming. After soil temperatures hit 60 degrees, turf grass seeds, which you do want to grow, begin to germinate.
Use an herbicide labeled for use on crabgrass. Prodiamine, dithiopyr, pendimethylin and a combination of benefin and trifluralin are all rated as excellent crabgrass pre-emergents by CSUES and the Missouri Botanical Garden. Herbicides should be applied at the rate noted on the package. Apply herbicide alone or in combination with fertilizer, but apply it evenly over the lawn so no uncovered patches of seeds are left to germinate.
Water pre-emergent herbicides in if there is no rain overnight. The University of Minnesota Extension recommends a quarter to a half an inch of water to carry the herbicide into the soil where it can be most effective.
Water the lawn to moisten soil before using a post-emergent control. Most of them are systemic and work best when the plant is actively growing.
Spray post-emergent killers like MSMA, quinclorac and fenoxaprop ethyl on individual crabgrass seedlings as a clean-up. Post-emergent herbicides are not as effective crabgrass controls as pre-emergent herbicides and healthy lawns.
Water the lawn again after 48 hours if the weather is dry.
Choose natural herbicide by applying 20 pounds of corn gluten meal per 1,000 square feet of lawn. The Northwest Coalition for Alternatives recommends this application, developed at Iowa State University. The meal damages roots of seedlings; it is applied in spring at forsythia time and in fall as temperatures begin to cool.
Pre-emergent killers stop germination using hormonal controls. Most are skin or eye irritants; follow precautions listed on packaging.
The Minnesota extension also recommends a second pre-emergent application if the first application is made very early; perhaps on the premise that the soil warmed after the herbicide dispersed. The recommended mid to late June follow-up application in Minnesota would be equivalent to treatment about a month after the last average freeze.