Accidental Preen Application on Lawn: What to Do

A homeowner mistakenly applied Preen (a pre-emergent herbicide) instead of fertilizer to their entire lawn in New Hampshire. The outcome depends on which Preen product was used:

• Preen Weed & Grass Killer (glyphosate): Kills all vegetation on contact. Lawn likely dead; reseeding required.
• Preen Vegetable Garden (corn gluten meal): Organic pre-emergent; prevents seed germination for 6–8 weeks but won’t harm established grass. Safe.
• Preen Garden Weed Preventer (trifluralin): Pre-emergent only; stops weed seeds from sprouting but doesn’t affect existing lawn. Safe.

Check the label immediately. If it’s glyphosate, water heavily to dilute, but expect significant damage. For corn gluten or trifluralin, water in well and avoid fertilizing until fall. Going forward, fertilize cool-season lawns in late May, early September, and late October—never in summer heat.