When would be the optimum time of year to go after Quackgrass in a KBG lawn here in the Upper Midwest? Using Certainty. I plan to treat 1k sq ft or so (front lawn) for a customer who's lawn has been infiltrated with the stuff. Too much to dig out. I have made this customer fully aware of the ramifications of using Certainty on his lawn (namely loss of fescue). He says fine.