Overseeding in fall: the cool-season lawn reset
If your Chicagoland lawn is thin, the single highest-leverage thing you can do all year happens between late August and late September: overseed. Warm soil, cooling air, fewer weeds germinating, and a full fall of growth before winter — spring seeding can't compete.
Prep determines results
Seed needs soil contact, not a thatch trampoline. Mow shorter than usual (around 2 inches) and bag the clippings, then rough up the surface — core aeration right before overseeding is the gold standard, since seed falls into the holes (see our aeration guide for timing).
Pick the right seed
Match what you have and where it grows: bluegrass/perennial rye blends for sun, fine fescue-heavy blends for shade. Cheap "contractor mix" bags are how annual rye ends up dying in your lawn next July — buy a named blend.
Water like it's a nursery
For two to three weeks, keep the top half-inch moist with light, frequent watering — the one season this is correct (context in the watering guide). Once new grass hits 3 inches, mow it and shift back to deep-and-infrequent.
The payoff
Density. A thick fall-seeded lawn shades out next year's crabgrass before it starts and shrugs off summer stress. Pair overseeding with your fall cleanup and the lawn goes into winter set up to win spring.