I have a customer that has a mulch bed that was improperly made and the grass has migrated into the mulch. He wants me to get rid of the grass and properly prepare the bed and remulch. There is a stand of arbor vitaes that is the focus of the bed. What product can I use to kill the grass in the bed without damaging the arbor vitaes? I don't want to dig it all out. Yes I am certified and liscensed but I don't do much with a selective, nonselective herbicide. What do you recommend?