Will Kelly explains why you should never spray herbicide and mow on the same day. The chemical needs time to absorb through leaves and roots before the grass is cut, so most labels require at least one full day between the two tasks for the weed killer to work.
You want to try to avoid spraying the same day that they get mowed. On all of the labels it says for best results and the best effectiveness is to actually treat at least one full day before, which would technically be 2 days, or one full day after, which would be the second day after the mow, for best results. Because you want the chemical to uptake the root system, uptake the leaves, and actually start to be effective on the weed control before you come through and cut it. And then ideally what you want to do is schedule your sprays when you're not out there same day mowing for best results.
