Asphalt shingles work well too, but they have to be pinned down to the soil with those metal U-shaped pins. You can make your own pins from wire coat hangers. I got my shingles from the Habitat for Humanity Re-Store for $4. Cut them lengthwise into strips and overlap the ends slightly, using one pin at each junction. They lay nice and flat, get really hot in the sun and bake anything that tries to grow under them and are easy to walk, mow and trim around.

Thanks thats a great idea. I want something greay to match the concrete castle pieces I want to put on the garden side.

I used the roofing shingle strips to ring a stone walled bed that always had to be weed whacked, not any more! Looks very tidy and the mower goes right over it. Also used the landscape pins cut from wire hangers. Love to recycle, reuse, save money!
Tina