Why then
Crabgrass germinates when soil at 1–2″ depth holds around 55°F for several days — typically the weeks right around the average last frost, when forsythia is blooming. Pre-emergent must be down and watered in *before* germination; a week late and the first flush is already up. Frost dates are a serviceable proxy, but a $15 soil thermometer (or your state extension’s soil temperature map) is the honest trigger.
Frequently asked questions
When should I apply crabgrass pre-emergent in Iowa?
Across Iowa's featured cities the window runs Mar 28 – Apr 29. Your own town can sit at either end of that range — use the city table on this page or the ZIP lookup for your exact dates.
Why do the dates vary within Iowa?
Because climate does. The dates here are calculated separately for each city from its own NOAA 1991–2020 frost records, and elevation, latitude, and terrain spread them across Iowa — the same task can be a week or two apart between towns. That local spread is exactly what generic national advice misses.
What if I miss the window?
Pre-emergent after crabgrass has germinated is wasted product — switch plans to post-emergent control on young crabgrass and put the pre-emergent reminder on next year’s calendar instead.