I have a customer with a southern exposure foundation bed. It is planted with azaleas and variegated boxwood. Every year the bed is taken over by very vigorous and widespread grass like weeds. They are think, and tough, and grown like crazy. I pull them out by hand, I reduce them with the string trimmer I have treated the bed with Preen and yet they keep coming back. They grow right through the mulch and make the bed look like hell. Does anyone have any suggestions for taming a problem bed like this? It is an ongoing problem.