Bluegrass germinates throughout the winter and can reseed even after being mowed over, making it a nuisance. To prevent it from growing, use a pre-emergent herbicide.
This means applying the weed killer preemptively, timing the applications carefully and deliberately at certain points in the year before seed growth occurs.
Apply the pre-emergent herbicides between August and November. The treatments will last approximately three to five months, after which you can apply post-emergents.
When temperatures don't rise above 75 degrees F for at least four days in a row, apply the first of two fall applications. Wait eight to 10 weeks for the second one in late fall.