Crabgrass Prevention in Illinois: Timing Is Everything

Crabgrass is the weed we get asked about most, and it is almost entirely a timing problem. Get the application window right and you barely see it. Miss the window by two weeks and you spend the rest of the summer looking at it.

Watch soil temperature, not the calendar

Crabgrass seed germinates once the top inch or two of soil holds around 55 degrees Fahrenheit for several days running. On the North Shore that typically happens somewhere between mid April and early May, but it swings by a couple of weeks depending on the year. A warm March can pull it forward, and a cold wet April can push it back.

The classic local shortcut still works: when the forsythia finishes blooming and the flowers start dropping, the window is closing. Lilacs leafing out is another decent cue.

Pre emergent only works before germination

A pre emergent does not kill crabgrass. It creates a chemical barrier in the top layer of soil that stops germinating seed from establishing. That is why it is useless once you can see crabgrass in the lawn, and it is why the single most common mistake is applying too late.

Water it in with about half an inch of irrigation or rainfall within a few days of applying, or the barrier never forms properly. And do not aerate after applying, since punching holes through the barrier opens germination lanes.

The seeding conflict

This trips people up every spring. Pre emergent cannot tell the difference between crabgrass seed and Kentucky bluegrass seed. If you have bare patches to repair, you have to pick one: prevent crabgrass this spring, or seed this spring. You cannot do both in the same area.

This is another argument for doing your seeding in fall, which frees spring up for weed prevention.

If it is already up

• Hand pull isolated plants before they set seed in late summer. One plant can produce thousands of seeds.
• Post emergent crabgrass products work best on young plants and get less effective as the plant matures.
• Do not bother treating in September. Crabgrass is an annual and the first hard frost kills it anyway.
• Mow high all summer. Turf cut at three inches or more shades the soil and suppresses germination on its own.

The real long term fix

Crabgrass is opportunistic. It needs sunlight on bare soil to get going, which means it colonizes thin lawns, scalped areas, and the hot strip along a driveway. A thick, tall, well fed lawn crowds it out without any product at all. Every season you spend thickening the turf is a season you need less chemistry.

Timing a pre emergent correctly year after year is exactly the kind of thing a maintenance program handles for you. Call or text 224-634-0673 or 224-200-8363 and we will keep your lawn on schedule.

Ruiz Landscaping serves Vernon Hills, Libertyville, Mundelein, Buffalo Grove, Lincolnshire, Long Grove and the rest of Vernon Hills's surrounding North Shore and Lake County communities.