When to put down pre-emergent (hint: not by the calendar)
Soil temperature is the trigger, not the date. Here’s the number to watch and the easy backup cue if you don’t own a thermometer.
Pre-emergent herbicide works by stopping seeds from sprouting. Put it down too early and it wears off before the weeds are done germinating. Too late and they’ve already sprouted.
Spring: 55°F soil
Crabgrass starts germinating when the soil holds around 55°F for several days in a row. That’s your window. In much of the South that’s February; farther north it’s March or April. No thermometer? When the forsythia blooms, you’re close.
Fall: soil dropping through 70°F
Winter weeds like poa annua and henbit germinate as the soil cools in fall. A second pre-emergent as soil drops toward 70°F stops them.
Make it work
- Water it in with about half an inch within a few days
- Don’t core aerate after applying — it breaks the barrier
- A second application 6–8 weeks later extends protection through summer
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