New Jersey Pesticide Applicator Training Category 3A Practice Exam

Preemergence herbicides work by forming a chemical barrier in the top layer of soil that prevents weed seeds from germinating and/or emerging. Because they must be present before seeds start to grow, the best time to apply is before germination—generally right as soil temperatures approach the level that triggers germination. In turf, this window is tied to soil temperature cues for the weeds you’re targeting (for example, crabgrass). After weeds have already germinated, the herbicide won’t prevent those seedlings, and applying at times not aligned with the germination cue (like waiting for summer heat or random timing) won’t be effective. Rainfall or irrigation after application helps move and activate the product in the soil.