When to Apply Crabgrass Preventer in Iowa

Why timing beats product

Pre-emergent herbicide does not kill crabgrass plants; it stops seed from establishing. Once the seed has germinated, the preventer is too late and only post-emergent treatment (harder, uglier, less effective) is left. Crabgrass germinates as soil hits 55 to 60 F and keeps germinating for weeks, so the product needs to be down and watered in before that threshold arrives.

The Iowa windows

Source: Iowa State University Extension and Outreach crabgrass control guidance. A common local calibration: the window generally aligns with when lilacs begin blooming.

Three mistakes that break the program

• Applying too early (March): the product degrades before the late germination waves and the lawn breaks through in July.
• Seeding at the same time: standard pre-emergent blocks grass seed too. If you plan to seed, do it in the fall window instead; by late summer a spring preventer has broken down enough to allow seeding.
• Relying on chemistry alone: a lawn mowed tall (3 to 3.5 inches in summer) shades the soil and stops much of the germination for free.

Our lawn application programs put the pre-emergent down inside the window as the first round of the season, then handle broadleaf weeds when fall makes them vulnerable.

Done reading, want it done? See our Lawn Applications (Fertilizer & Weed Control) service.