When should you
apply pre-emergent?
Enter your ZIP code. We'll tell you if it's time — based on real weather station data and Growing Degree Days.
Data details
How Growing Degree Days Work
Growing Degree Days (GDD) measure the accumulated warmth in your soil since January 1. Each day the average temperature exceeds 50°F, the difference is added to your running total. When that total reaches 150–200 GDD, crabgrass and other summer annual weeds are about to germinate. That is the window for pre-emergent herbicide.
Why Pre-Emergent Timing Matters
Pre-emergent herbicides form a chemical barrier in the top layer of soil that stops weed seeds before they sprout. Apply too early and the barrier breaks down before peak germination. Apply too late and the weeds are already growing. GDD tracking removes the guesswork by tying your application to actual soil temperature rather than calendar dates, which vary year to year.
This tracker pulls daily temperature observations from your nearest NOAA weather station and calculates GDD for over 33,000 US ZIP codes.
Pre-Emergent Guides by State
Application timing varies across the country. Southern states may need pre-emergent as early as January, while northern states often wait until April or May. Find your state for local timing and grass type recommendations.