When to apply crabgrass preventer
Apply crabgrass preventer in spring, before crabgrass sprouts. Crabgrass germinates when soil temperatures stay above 55 to 60°F for 7 to 10 days in a row, so apply just before that. University of Illinois Extension suggests late April to early May in northern Illinois, mid to late April in central Illinois and early to mid April in southern Illinois. In a cold spring, wait until the end of your window.
When to apply crabgrass preventer
Checklist
- Find your local window: ask your state Extension office or a local garden centerEarly spring
- Check the soil temperature with a soil thermometerBefore applying
- Hold off if you plan to seed bare spots or overseed (most preventers stop grass seed too)Before applying
- Read the product label for rate, timing and watering-inApplication day
- Apply the preventer evenly before crabgrass sproutsSoil near 55°F
- Mow higher than 3 inches and water lightly but less oftenAll season
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Why timing matters
Preventers stop crabgrass seeds from sprouting. They do not kill plants that are already up. University of Illinois Extension says postemergence herbicides work only on very small crabgrass plants, and crabgrass you notice in mid to late summer is usually too large for them to be effective.
Use soil temperature, not the calendar
Illinois Extension says crabgrass germinates when soil temperatures are greater than 55 to 60°F for 7 to 10 consecutive days, and keeps germinating until soils reach 95°F. Dates differ by region, so use the numbers above as a guide and follow your product label.
If spring is cold, Illinois Extension advises applying at the end of the recommended window.
Seeding and fertilizer
Preemergence herbicides also damage germinating desirable grass seed. Siduron is the exception, according to Illinois Extension. If you plan to seed or overseed, check the label first.
Some products combine a preventer with fertilizer. Illinois Extension does not recommend early spring fertilization, and suggests applying fertilizer separately, later in spring.
Prevent it without chemicals
A dense lawn crowds out crabgrass. Illinois Extension recommends mowing higher than 3 inches, watering lightly but less often, and avoiding late-spring seeding on bare soil. Core aeration, topdressing and overseeding in the right season also help.
Always follow the label on any herbicide. Keep children and pets off treated areas as it directs.
What you'll need
- Soil thermometerShows when soil reaches 55°F
- Broadcast spreaderSpreads granules evenly
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Questions
When should I put down crabgrass preventer?
Before crabgrass sprouts, which is when soil temperatures stay above 55 to 60°F for 7 to 10 days. For Illinois that is early April to early May depending on region, per University of Illinois Extension.
Can I apply crabgrass preventer and grass seed together?
Usually not. Preemergence herbicides also damage germinating grass seed. Siduron is the exception. Read the label.
What if I applied it too late?
Preventers will not stop crabgrass that has already sprouted. Illinois Extension says postemergence products work only on very small plants, so prevent it next spring and mow high this year.
Sources
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