Hello. I'm becoming less able to manage arduous weeding and am considering resorting to Preen or other pre-emgergent prevention for a bed on a steep hillside with newly-planted creeping phlox and some established periwinkle groundcovers. There are a lot of bare spots between that I know will fill with weeds. The steepness of the hill makes weeding very difficult and mulching a problem but not impossible.

I know that Preen prevents seed germination and not actively growing weeds, but will Preen, or some other pre-emergent, also prevent the periwinkle and phlox from spreading and filling in the bare spots, as I believe this involves some sort of "germination" from the runners?

Thanks for your help!