I have a fairly large area with Vinca groundcover (aka myrtle or periwinkle) that was planted almost 2 years ago. Problem is there are numerous weeds that have taken root in the same area. I am unable to keep up with the weeding, which is quite difficult because the weeds are tightly mixed in with the groundcover. My question is...is there a chemical (herbicide) that will kill the weeds, but not harm the groundcover. Thanks.