Most of these sprays take time to work, so spraying and then cutting it off the next few hours or maybe even the next day or two will result in it not working. Most sprays say on the bottle that you should spray the most foliage as possible, lots of contact with the surface of the plant. So mowing the grass/weeds before applying is not as affective. Most of these sprays do only work by foliage aplication, so applying to roots may not be affective. It is absorbed by the leaves or it prevents the plant from getting sun and it 'starves', not a process that happens that quickly. Even if the tops die, the roots may still survive and re-sprout, so more than one application may be needed. That is my understanding anyway. Some of these sprays leave a residual in the ground to prevent anything from growing for the rest of the season, so read carefully before using anything. Some are formulated for sidewalks, driveways ect where you don't want anything to grow at all.
