Japanese knotweed is an aggressive perennial weed. It often invades empty properties like development sites and empty lots. Before you begin developing a site, ensure the knotweed on the property is completely eradicated. If not managed properly, it may spring back up at an inopportune moment. The best way to ensure that Japanese knotweed is gone for good is to treat it chemically when it is most vulnerable.
Cut the Japanese knotweed back to within a few inches of the ground with lopping shears in spring or summer when it is actively growing. The best time to do this is when the plant is flowering.
Spray the knotweed with a glyphosate herbicide one to two weeks after you cut it back. The new growth is more susceptible to herbicidal treatment. Use a hose-end sprayer to treat large stands of knotweed. Treat individual plants with a hand sprayer. Coat all of the foliage. Stop just before the herbicide runs off of the plant. Follow the manufacturer's instructions.
Respray the knotweed as necessary, at the rate recommended by the herbicide's manufacturer.
Monitor the area for regrowth for the rest of the season. Spray any plants that resprout as soon as they reach a few inches in height.