Wait at least 16-20 weeks after applying pre-emergent herbicide before seeding your lawn.

Pre-emergent herbicides create a chemical barrier in the soil that prevents weed seeds from germinating. This barrier also inhibits grass seed germination, making timing crucial for successful seeding. Most pre-emergent products remain active in soil for 3-5 months, depending on the active ingredient and environmental conditions.

Recommended waiting periods by pre-emergent type

Steps to seed after pre-emergent application

• Check product label for specific waiting period recommendations
• Water lawn thoroughly to help break down herbicide barrier
• Test soil by attempting to germinate a few seeds in a small area
• Choose appropriate grass seed for your region and season
• Aerate soil if compacted before seeding

Alternative approach: Use siduron-based pre-emergent

Siduron (Tupersan) is the only pre-emergent herbicide that selectively controls weeds while allowing certain cool-season grass seeds to germinate. This makes it ideal for spring seeding projects where crabgrass prevention is needed simultaneously.

Signs pre-emergent barrier has broken down

• Weeds begin germinating in treated areas
• Grass seeds sprout when tested in small patches
• Soil tests show reduced herbicide concentration

Factors affecting pre-emergent persistence

Temperature, rainfall, soil type, and microbial activity all influence how long pre-emergent remains active. Warmer temperatures and frequent watering accelerate breakdown, while clay soils may retain herbicide longer than sandy soils.