Quick answer: Apply crabgrass pre-emergent in early spring, before soil temperatures hold in the mid 50s. On Long Island that window typically falls in April and moves with the weather each year. Forsythia in full bloom is the traditional signal that it is time.
Why does timing matter so much?
A pre-emergent does not kill existing plants. It stops seedlings as they sprout. Once crabgrass has germinated, the window has closed and control becomes harder and more expensive.
What triggers crabgrass to germinate?
Soil warmth. Crabgrass seed begins sprouting after soil temperatures stay around 55 degrees for several days. South facing lawns, thin turf and edges along driveways and sidewalks warm first, so that is where crabgrass shows up earliest.
Does one application last all season?
Not always. Protection fades over weeks, and crabgrass keeps germinating into summer. Many programs split the treatment into two applications to extend coverage.
What are the common mistakes?
- Applying after crabgrass is already visible
- Seeding at the same time, since most pre-emergents also stop grass seed
- Skipping edges along pavement
- Mowing too short, which opens the turf to light and heat
- Using a fertilizer combination product before New York's April 1 start date for lawn fertilizer
What if I missed the window?
Young crabgrass can still be treated with a post-emergent product while it is small. Then plan to thicken the lawn with overseeding in early fall, because dense turf is the best long term defense.
Related: Lawn and ornamental weed control | Green industry best management practices
This article is general information. Pesticides and repellents must always be applied according to the product label and state regulations.