In theory, a barrier of preemergene herbicide is created in the soil and limites emergence of crabgrass. Any gap in this barrier could allow crabgrass to germinate. This is why a uniform application of preemergence herbicide is required and practices like power raking or aerification are usually not recommended after application. However, research consistently shows that aerification does not reduce effectiveness of the herbicide. When aerification plugs are returned to soil, the herbicide may be simply redistributed in soil and not actually removed. The size of an aerification hole is only 0.5 to 0.75 inche in diameter and even though a crabgrass plant may germinate in a hole, its roots will explore well outside that hole and come into contact with the herbicide. Our recommendation is to not tempt fate and limit aerification following preemergence herbicide application if possible. But if turf areas will benefit long-term from aerification, aerify whenever you can regardless if a preemergence herbicide has been applied.