Pre-emergent herbicides act as an invisible weed killing barrier within the soil. As the weed seed begins the germination process, the pre-emergent herbicide stops the weeds in their tracks. Not all weeds are controlled by pre-emergents, but the best way to take care of crabgrass, annual bluegrass and goosegrass is with a good pre-emergent herbicide routine. The times to apply pre-emergent herbicides are February, April/May and then in September/October. All pre-emergents must be watered into the soil for at least 1 hour per section of turfgrass within 48 hours of the initial application.