DO NOT seed for at least 90 days after a pre-emergent has been applied. Pre-emergent weed killers are designed to prevent seeds from germinating and can remain in the soil for up to 90 days. Check pre-emergent label for exact time period to wait before seeding.
Do not apply a weed control herbicide until the newly seeded area has fully germinated and become established (usually after the 3rd or 4th mowing).
Why Weeds Grow Around Planted Grass SeedSummer annual grassy weeds, such as crabgrass, germinate and thrive during the warmest spring and summer months. These hot months also are the months in which the cool season grasses slow down growth in order to protect themselves against the extreme high temperatures and intense sunlight.
If the area to be seeded was prepared even a day or two before seeding, the crabgrass could have a head start on germination and could quickly take over the desired lawn site. To prevent this from occurring, it is recommended that a pre-emergent crabgrass control using Tupersan be used at the time the site is prepared, to prevent the germination of unwanted grassy weeds. If a seed such as Kentucky bluegrass is used (which takes a longer time to germinate than other cool season grasses), a second application of Tupersan is recommended to control the crabgrass until the lawn has had a chance to establish.
It is also critical to provide the most optimum growing conditions for the new seed (adequate water and fertilizer) and not allow the site to dry out. Tupersan must be used for new seedings! Any other crabgrass control will kill young grass seedlings and may only be used on established lawns.