Loading pre-emergent timing
Crabgrass preventer and fall poa annua timing based on current soil temperature readings from your nearest monitoring station.
Live verdict
Soil has fallen below 65°F, so the poa annua window is closing and germination is already underway. Apply immediately if you have not yet.
Measured at the Boulder 14 W monitoring station (32.4 mi away), reading from July 28.
Based on long-term station averages, the 2-inch soil near Thornton typically crosses the crabgrass germination threshold (55°F) around June 11. 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 July 9, 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.
The Thornton 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.
Full lawn care schedule for Thornton →
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.