The short answer
Crabgrass seed starts to sprout once the soil near the surface stays around 55°F. A pre-emergent herbicide does not kill plants; it forms a barrier in the top layer of soil that stops seedlings as they sprout. So it has to be down, and watered in, before that happens. Extension specialists usually put the target at soil that has held 50 to 55°F at a 2-inch depth for several days (Michigan State University Extension).
Too early is a smaller mistake than too late. Products break down over a set number of weeks, so very early applications run out sooner in summer, but an application after crabgrass has sprouted does almost nothing to the plants already up.
Two ways to time it
Soil temperature
Our city pages estimate the daily average soil temperature at 2 inches and call the window open once the 5-day average reaches 50°F. It closes when the 5-day average reaches 55°F after enough spring warmth has built up. That second condition matters: a warm week in early March can push shallow soil past 55°F before crabgrass is ready, and the soil usually cools again.
Growing degree days
Michigan State's GDDTracker adds up daily warmth above 32°F from January 1 and puts the crabgrass pre-emergent window at 250 to 500 of those degree days (MSU GDDTracker; the University of Wisconsin's crabgrass timer works the same way). Every city page shows the running total for this year next to its normal.
The two methods usually agree within a week or two. Where they disagree, soil temperature tells you about this week; degree days are steadier against single warm or cold spells.
Before you spread it
- Water it in. Most granular products need about half an inch of rain or irrigation within a few days to move into the soil.
- Do not seed for a while. Most pre-emergents also stop grass seed. If you plan to seed in spring, choose a product labeled safe for new seedings or skip the pre-emergent in that area.
- Read the label for how long it lasts. If your soil warms early and your summer is long, a second, split application keeps the barrier going.
- Shade and slope change things. South-facing slopes and soil next to pavement warm a week or more before shaded lawn.
