When Should You Apply Pre-Emergent in Illinois?

If you live in the Chicago suburbs and you’re wondering when to apply pre-emergent, the answer isn’t a date on the calendar - It’s soil temperature.

Crabgrass begins germinating when soil temperatures approach 55 degrees. In most years around here, that happens in early April. Some years it’s late March. Some years it’s mid-April. But early April is a safe rule of thumb for northern Illinois.

If your pre-emergent isn’t down before soil temperatures consistently reach 55°, you’re gambling.

Once crabgrass germinates, prevention is off the table. Now you’re trying to control it after the fact. That’s more expensive, less predictable, and usually takes multiple applications. It also competes with your turf all summer, and if we get dry conditions like we did last year, those crabgrass patches will stick out even more.

Prevention is always cheaper than correction.

One question we get a lot is whether liquid or granular pre-emergent is better. The honest answer is that timing matters more than the form. Both can work. What matters is getting the product down evenly and before germination. We apply a liquid pre-emergent because it gives us consistent coverage and reliable results, but a properly applied granular product can also be effective.

Another common question: can you seed your lawn and apply pre-emergent at the same time?

Generally, no.

Pre-emergent doesn’t know the difference between crabgrass seed and the grass seed you just put down. If you apply pre-emergent, you’re creating a barrier that prevents new seeds from establishing. That’s great for stopping weeds. Not great if you’re trying to grow new grass.

If your lawn is thin, spring seeding can make sense. If your lawn is full of weeds, prevention usually comes first and seeding is better saved for fall. The right answer depends on what problem you’re actually trying to solve.

The biggest mistake we see every year is homeowners waiting until they see weeds and then reacting. By the time you notice crabgrass, it’s already established. Now you’re managing damage instead of preventing it.

In Illinois, the window isn’t huge. Once soil temperatures climb in early April, things move quickly. That’s why having a plan in place before then matters.

Our Fertilization & Weed Control program is built around proper timing for Chicago conditions. We monitor soil temperatures, apply pre-emergent before germination, and manage the season from there. It’s for homeowners who don’t want to guess, don’t want to waste money on the wrong products, and don’t want to fight crabgrass all summer.

If you’re wondering whether you’re early, late, or right on time, now is the time to ask. Early April comes fast.