Recycle that old carpet, and shrink the landfills. Cut the carpet in to any shape, you want your flower bed to be. Lay the carpet face down, and cover with mulch. You use less mulch and it last for years and years. When you are ready to plant, cut a X and plant.This is not organic for your vegetable garden.
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Old carpet and rugs that are still in decent condition make a nice addition to a porch, laundry room, or even in the garage in front of a tool bench. If you can't use it, maybe you know someone who can.