Create Your Dream Garden 
Keeping Grass out of Groundcovers
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.)
I would use newspaper vs plastic to smother weeds.
Remember there are long term consequences to the environment to spreading poison around. Water flows downhill to streams and rivers, where fish (food) swim. I don’t use poison stuff when I can avoid it. Exceptions: Morning Glory aka bindweed, and Poison Oak.
I remove poison oak by hand by putting my hand in a used plastic grocery bag, reaching down and pulling out the plant roots and all. Then I turn the bag inside out, tie a knot, and throw away. It works great for me. No poison! This, of course, works best in moist soil.
Be cautious of using plastic to cover soil. Plastic does not allow for air/water movement to the soil. I would suggest a landscape fabric that allows air/water to move in/out of soil. Plastics that don’t allow this movement can actually destroy the soil composition over time.
Agree! I’m shocked at the suggestion of plastic, which degrades into tatters, and doesn’t allow the soil to breathe and absorb water.
Over-the-Top Grass killer is a great help to me, Fertilome has another grass specific herbicide which will not kill perenials and most ground covers.
Use a grass specific herbicide, like ‘Grass-b-gon’. Beware of spray drifting onto the lawn, however. I have great luck with it, even on wiregrass.