Apply pre-emergent when soil hits 55°F for 3 consecutive days

What to Do

Monitor soil temperature at 4-inch depth. When it reaches 55°F for 3 consecutive days, apply a pre-emergent herbicide. This is the threshold at which crabgrass and other annual grassy weeds begin germinating.

Why It Works

Pre-emergent herbicides create a chemical barrier in the top layer of soil that kills weed seedlings as they germinate. They must be in place before germination occurs — once you see crabgrass, it is too late for pre-emergent. Wisconsin Extension notes this usually happens in early May for northern lawns, not March or April.

Tips

• A cheap soil thermometer ($10-15) or your local extension's soil temperature map can guide timing.
• Water in the product lightly after application to activate the barrier.
• The old rule of "apply when forsythia blooms" is a rough but useful proxy.