Japanese knotweed is a stubborn perennial. Its spreading and staying power are the result of its underground rhizome system, but it can be removed and controlled. These underground stems extend deep into the soil, spread quickly and are capable of producing new plants from even small pieces left behind after digging. The key to effectively treating a Japanese knotweed infestation is to kill the rhizomes or remove them completely.
Dig up individual knotweed plants. Remove the bulk of the roots, using a shovel. Expand the hole and look for any overlooked root pieces. Remove them. Remove and dispose of all pulled weeds and rhizomes from the area immediately.
Cut the Japanese knotweed back to the ground repeatedly with a lawn mower or lopping shears. Remove the foliage when it is 3 to 5 inches tall to prevent the plant from photosynthesizing and to starve its rhizomes. Repeat this method until the Japanese knotweed is eradicated.
Spray the Japanese knotweed with a glyphosate herbicide. Follow the manufacturer's instructions. Coat all of the plant's tissue until just before the point of runoff. Take precautionary measures. Cover any desirable plants with plastic. Spray on a day with little to no wind. Dip a paintbrush into the herbicide and brush it onto the knotweed's foliage, as an alternative to spraying. Re-treat the plants as necessary at the intervals dictated by the manufacturer.
Monitor the affected area for growth. Retreat as soon as the plants show evidence of growth.