Crabgrass pre-emergent timing

How a pre-emergent works

Crabgrass is an annual: last year’s plants died in the frost, and this year’s come from seed in the soil. A pre-emergent herbicide forms a thin barrier in the top layer of soil that stops those seeds from establishing. It does nothing to plants that have already come up, so timing is everything.

When to apply

Crabgrass germinates once the soil at about 5 cm (2 in) deep stays around 13 °C (55 °F) for several days. In a continental climate that is usually between early March and early April. A useful sign is forsythia: apply when it is in full bloom, before its flowers drop. Too early and the barrier weakens before germination ends; too late and the seedlings are already up.

How

• Use a pre-emergent lawn herbicide authorized in your country, at the rate on its label.
• Spread it evenly with a spreader, then water it in as the label says to activate the barrier.
• Avoid raking, scarifying or aerating afterwards; breaking the soil surface breaks the barrier.

The catch: it stops grass seed too

The barrier cannot tell crabgrass from lawn seed. Do not use it the same spring you overseed. A thick, well-fed lawn mown at the right height is the first line of defense; a pre-emergent is optional.

For crabgrass that gets through anyway, see post-emergent treatment.