The challenge we turf grass managers face every spring season (in particular) is that in order to prevent the annual unwanted grassy weed called crabgrass from growing in a lawn we either need to maintain great turf grass density to stop undesirable crabgrass (seed) from ever germinating in open soil or we need to apply a preventative herbicide to create a short lasting “barrier” that will stop any/all crabgrass seed from growing or fully maturing. Even when we have decent overall lawn density and have applied a pre-emergent crabgrass herbicide there is still a strong possibility that crabgrass will grow in weakened lawn areas like those found in heavily compacted soil areas, lawn areas cut too short (usually along curbs, roads, walks, base of trees, etc.), areas that are exposed to extreme heat for long periods of time or lawn areas that were simply left bare (not repaired) from growing seasons past.
When a granular pre-emergent crabgrass herbicide is applied it will generally provide between 6-10 weeks of crabgrass protection. The life of this synthetic “crabgrass barrier” is dependent on the amount of rain or irrigation following the application and the amount of possible soil disturbance that takes place (post application). The lower the amount of soil surface disturbance or lessor water applied will extend the life and effectiveness of a crabgrass pre-emergent herbicide application.
No grass seed should ever be planted in a lawn that recently had a pre-emergent crabgrass herbicide applied (+/- 6-10 weeks post application).
The majority of crabgrass pre-emergent herbicides will not allow desirable grass seed to germinate over the course of its effective 6-10 week life span. The crabgrass herbicide has no way to distinguish between unwanted crabgrass seed and desirable grass seed. We highly recommend applying a crabgrass pre-emergent herbicide in the spring and planting grass seed mid-August through September especially in full sun exposed lawn areas where crabgrass is more prevalent.