Keeping Grass out of Groundcovers

How can I get rid of the grass growing through groundcover?
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Question: How can I get rid of the grass growing through a groundcover? It keeps invading the creeping phlox that borders the walkway to my doorstep.

Answer: There's no easy solution. In October, or in early spring, dig up the phlox, pull any weeds or grass from the clumps, and cut out the weakest phlox shoots (which won't flower anyway). Protect the excavated phlox roots from drying out by temporarily heeling them in or by packing them in containers and storing them in a shady spot. Then scalp the empty bed by cutting under and lifting out the sod and weeds. Work in some compost to lighten the bed and to replace lost soil, then cover the entire bed with black plastic. Cut holes in the plastic, replant the phlox clumps through the holes, and water well. Finally, cover the plastic with two or three inches of bark mulch for appearance's sake, keeping the mulch three inches away from the phlox stems. The following spring, feed the planting with a granular 5-10-10. (If you've done the work in the spring, fertilize with a liquid feed one month later and again about the first of July.)