After a weed has emerged the pre-emergent herbicide won’t work. So, we have to use a post-emergent herbicide. This type of herbicide kills the weeds that have emerged and are growing. A lot of our common aggressive weeds such as common Bermuda, Virginia buttonweed, and torpedo grass are controlled with post-emergent herbicides. When post-emergent herbicides are to be applied you must NOT mow your yard for four days prior to OR after the initial application. It must stay dry and undisturbed on the foliage for at least 24 hours.