I have a rock garden along one side of my house, which has various plants in it. It is watered by an inground system. There are 3 patches of variegated phalaris which I like, but it is spreading so much as to be a problem. I took my hedge trimmer and cut it way back thinking I could get in and dig up most of the roots, just leaving a small clump. Not happening! The roots are way too deep. What can be used to kill back the roots? And will I have to wait for the irrigation to be off? Will the other plants be harmed by run off? I appreciate any help.