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Crabgrass preventer and fall poa annua timing based on current soil temperature readings from the nearest station.
Live verdict
Soil is in the 65–70°F falling range, the window for stopping poa annua and other winter annual weeds. Apply now if you have not already.
Measured in the ground · Elsberry PMC (USDA SCAN #2220), 23 mi N · read yesterday (Oct 2, 2026) · updates nightly with about a 1-day sensor lag
Based on long-term station averages, the 2-inch soil near O Fallon typically crosses the crabgrass germination threshold (55°F) around April 12. Apply pre-emergent 1–2 weeks before that date for best control. The spring window effectively closes once soil settles above 60°F, typically around April 27, by which point crabgrass has germinated and a post-emergent is the better tool.
Crabgrass and most summer annual weeds begin germinating when the 2-inch soil temperature sustains 55°F for 3–5 consecutive days. Pre-emergent herbicides, the products sold at retail as crabgrass preventer, create a chemical barrier at the soil surface that prevents weed seeds from sprouting. The barrier must be in place before germination begins.
Pre-emergent has a second season. Poa annua (annual bluegrass) and other winter annual weeds germinate in fall as the 2-inch soil temperature drops back through 70°F after the summer peak, with most germination in the 65–70°F falling range. A fall application ahead of that crossing stops poa annua the same way a spring application stops crabgrass.
Based on long-term station averages, the 2-inch soil near O Fallon typically falls back below 70°F around September 7 and drops below 65°F around October 6. Plan to have the fall application down before that first crossing, and check the annual maximum on your product label if you also applied in spring.
The O Fallon lawn care schedule tracks this window live against daily station readings. For products, rates, and the germination research behind the thresholds, see the fall pre-emergent timing guide.
Look for products containing prodiamine, dithiopyr, or pendimethalin. Dithiopyr has mild post-emergent properties and can suppress very young crabgrass (1–2 leaf stage) even if applied slightly late. Weed-and-feed products that pair fertilizer with a pre-emergent follow the same soil-temperature window: the preventer half only works if it is down before the 55°F crossing.
Common products
Full lawn care schedule for O Fallon →
Spring pre-emergent herbicide guide →
Enter your city or ZIP code to see current 2-inch soil temperatures and find out whether the crabgrass prevention window is still open.