Japanese knotweed (Polygonum cuspidatum) is a perennial plant that grows on a thick single stem and blooms white flowers during the summer months. It reproduces and spreads very quickly and therefore is an extremely difficult plant to control. According to Columbia University, it has been declared a noxious weed in many states because it forces out native and other wanted plants. When cut or dug out, it should be discarded in your local yard waste facility or burned to prevent it from growing back.
Dig out you Japanese knotweed in the late fall or early winter when it has dried out. How deep and how wide depends on the plant. Try not to leave any roots. If you do, it will grow back. Before digging, check with your utility company for any underground wires and pipes.
Cut it down close to the ground with a pair of lopping shears. Then, spray it with an herbicide that contains glysophate, such as Roundup. Follow dosing and application instructions on the label. (Color the herbicide with food coloring so you know where and how much you have sprayed.) When it grows back in the fall, cut it back again and spray it with the herbicide again. This method can take several years until the plant completely dies.
Cut down the plant to the ground and cover it with a piece of piece of black plastic. Secure the plastic with stakes or heavy objects, such as bricks. Leave it there for up to two years to starve all the roots so the plant never grows back. If you notice holes, patch or replace the plastic. If you notice the plant growing along and out of the perimeter of the plastic, add more plastic to widen the area, or replace the plastic with a larger piece.