I used to put Preen (actually the landscaper equivalent) the way Julia did- before and after planting. My boss at the time saw that, blew up at me, and sent me to applicator training. They warned about the root-pruning effect. It's the reason the bags warn not to use it more often than every 90 days.
i think established plants could handle it accidentally touching the roots, but anything newly planted this time of year is going to have roots in active growth phase, and that will be retarded. I know it is not supposed to act as a growth retardant,....but it can. Won't kill the plants, but it won't do them any favors, either.
