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Preventing Crabgrass

Banner photo by Aaron Patton

You might be thinking that a better title for this article, designed for the average homeowner, might be “Controlling Crabgrass.” However, I’ll explain why I believe the word ‘preventing’ is more appropriate when it comes to crabgrass in the home lawn.

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

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, with one exception. Patton said, “Siduron allows cool-season grasses to germinate but prevents crabgrass from emerging.” He said it is good for seeding but doesn't last as long as other preemergence herbicides, so it is not a good season-long preemergence herbicide.

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. Post-emergence treatment is probably best left to professionals. 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

For additional information, access Purdue Extension’s publication, “Control of Crabgrass in Home Lawns,” number AY-10-W at The Education Store, Find additional turf management tips at Purdue’s turfgrass science website:

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