Crabgrass is a warm-season annual weed with a wide leaf that can look unattractive to homeowners wanting a healthy lawn, says Esther McGinnis, NDSU Extension horticulturist.
Although the plants die with the first hard frost, a large soil seedbank ensures a new crop of weeds each year.
Crabgrass seeds begin to germinate in spring when the soil temperature, at a depth of 2 inches, consistently reaches 55 degrees. Seeds will continue to germinate throughout summer but the majority will germinate at soil temperatures between 60 and 70 degrees.
“With our late spring, turf soil temperatures have been slow to rise. However, the soil will warm very quickly this week with warmer air temperatures,” says McGinnis. “Now is the time to apply a crabgrass pre-emergent herbicide before we reach this germination threshold.”
To check on the turf soil temperature in your area, please consult the North Dakota Agricultural Weather Network (NDAWN) website at ndawn.ndsu.nodak.edu/soil-temps.html
Crabgrass preventers are a class of pre-emergent herbicides that are applied before the crabgrass seeds germinate. Most pre-emergent herbicides will not provide effective control after germination.
Commonly available crabgrass preventers include active ingredients such as pendimethalin, prodiamine and dithiopyr. Of these three herbicides, dithiopyr is the only one that has early post-emergent activity. Dithiopyr can control crabgrass seedlings that are in the one- to three-leaf stage.
“When applying a crabgrass preventer, follow all label instructions,” McGinnis recommends.
To be effective, it is necessary to apply 1/2 inch of water to dissolve the granules and move the herbicide into the top layer of soil. Once dissolved, the crabgrass preventer will form a barrier in the soil.
Do not apply a standard crabgrass preventer to newly seeded lawns. The pre-emergent herbicide cannot differentiate between crabgrass seeds and lawn seeds. Instead, products that contain the active ingredients mesotrione or siduron (Tuperan) can be used in that situation.