Remember, pre-emergent herbicide can affect desirable plants. That includes turf. Caution must be taken if you're applying pre-emergent and seeding the turf in the same season. Seed first, then apply pre-emergent at least 6 weeks later to allow for lawn establishment. Or seed at least 3 months after the pre-emergent has been applied. Principle #2: Pre-emergent must be mixed correctly and applied evenly over the target area for best results. Pre-emergent herbicides need to be mixed correctly for the spray solution to be at the appropriate strength.