If you’ve dealt with crabgrass in your lawn before, your answer might be a resounding “yes” in answer to the title of this article. If you’ve only begun to see crabgrass popping up in your lawn and landscape, perhaps I can help you avoid becoming crabby about crabgrass.
Crabgrass is a summer annual grassy weed, which means it germinates from seed every spring, completes its life cycle, and dies with the first killing frost. It has high survival and reproductive rates, so it is probably unrealistic to expect a crabgrass-free lawn. Large crabgrass and smooth crabgrass are the two most common species, and we occasionally find southern crabgrass in Indiana. Plants will tiller and form small clumps. When mature, all crabgrass species produce a seed head with finger-like spikes (racemes) having multiple branches clustered at the top of long, erect stems (see photo below).
Good cultural practices like mowing high and encouraging dense turf are the most effective crabgrass prevention strategies, but herbicides may be necessary in some cases.
Homeowners who had a crabgrass problem last year should consider some type of chemical crabgrass preventer application before very long. These pre-emergence herbicides must be applied early in the spring to be effective (roughly from mid-March to about mid-April in northern Indiana) – at least a week or two before germination of crabgrass. Purdue research has shown that these herbicides can be applied as early as March 1 and still be effective.
According to Dr. Aaron Patton, Purdue Extension turfgrass specialist, “The preemergence herbicides (crabgrass preventers) that are applied in spring to prevent the emergence of crabgrass seedlings work only as long as they remain in the soil.” He said that generally, these products last in the soil about 60-120 days depending on a number of factors (rate applied, ingredient, turf cover, temperature, moisture, etc.). Data show the average historical crabgrass germination date to be about April 29 in Fort Wayne. Of course, this will vary based on your location; the local weather and site conditions will determine that date from year to year. Crabgrass germinates when soil temperatures are approximately 60 degrees F for 3-5 days at the one-quarter inch level.
A forecasting model developed by Michigan State University can help Midwest homeowners and professionals fine-tune the timing of their crabgrass preventer product applications and offer average crabgrass germination dates based on heat unit accumulations. You can stay up-to-date with the forecasts by visiting the model at https://gddtracker.msu.edu/. With the warmer weather we’ve had, we are ahead of last year at this time.
Crabgrass herbicides available to homeowners are primarily “preventer” products, meaning that they need to be in place before crabgrass germinates. The way they work is that as soon as new seeds sprout, they come in contact with the product, die, and fail to emerge. Many of these products are combined with fertilizer, however, early spring fertilization of turf should be minimized. Look for combination products with mostly “slow-release” forms of nitrogen.
Do not use most pre-emergence crabgrass preventers on newly seeded lawns, as they may inhibit desirable lawn species from growth and establishment.
Post-emergence herbicide products are available and are most effective on small crabgrass plants, but the products are very difficult to use effectively by most homeowners. Do not attempt to control crabgrass with herbicides after mid-July because crabgrass plants are usually too large to control effectively.
When using any herbicide, always read and follow all label instructions.
To learn more about what crabgrass looks like, and about management strategies, access an article of Patton’s on crabgrass at https://turf.purdue.edu/crabgrass/.
For additional information, access Purdue Extension’s publication, “Control of Crabgrass in Home Lawns,” number AY-10-W at The Education Store, www.edustore.purdue.edu. Find additional turf management tips at Purdue’s turfgrass science website: https://turf.purdue.edu/.
Crabgrass seedhead, photo by Aaron J. Patton, Purdue University