Japanese knotweed (Polygonum cuspidatum) belongs to the buckwheat family (Polygonaceae). It was first introduced into the U.S. from Asia as an ornamental plant and to control soil erosion, according to the U.S. Department of Agriculture. The plant has a very aggressive rhizome root system. Each rhizome spreads up to 30 feet. The plant itself reaches a height of 10 feet and looks similar to bamboo with long, hollow, jointed stems. Leaves measure 6 inches in length. White sprays of flowers occur in the early summer months. The top foliage dies after the first hard frost but the rhizomes continue to flourish even in cold weather.
Mow the knotweed as low as possible once a week. Mowing will not eradicate the plant but it will weaken it over time. Mowing must be performed weekly because the underground rhizomes will be stimulated to spread when the top growth is mowed; however, by removing the top growth repeatedly the plant will become less vigorous in growth and easier to kill. Use a lawnmower or a weed eater.
Pull or dig up knotweed plants in areas of soft, moist soil. Take great care to remove all the rhizomes to prevent them from growing. Promptly discard the plant.
Apply glyphosate in the late spring and fall. Apply to the foliage of the plant using a backpack sprayer or handheld sprayer. Mix according to the directions on the herbicide's label. Take care not to spray other plants using glyphosate because it will kill virtually anything it touches.
Cover the mowed and sprayed knotweed with black plastic. Lay the plastic flat over the knotweed and weigh the plastic down using bricks or rocks. The black plastic will prevent light from reaching the knotweed, which will aid in killing it. Look under the plastic in a week to see if the knotweed is dying. Leave the plastic over the plant until all top growth is dead. Complete death could take up to a month.
Remove the plastic. Discard the dead knotweed top growth. Dig the rhizomes up and discard.