Introduced into the United States in the 1800s as an ornamental plant for its attractive foliage, Japanese knotweed (Polygonum cuspidatum) is classified as invasive throughout much of the United States. This plant can quickly crowd out garden plants and grow up to 10 feet tall. Underground rhizomes make it challenging for gardeners to remove Japanese knotweed, yet they must do so before it takes over garden beds. Remove knotweed at any time of year using either a chemical or mechanical control method.
Dig up small outbreaks of Japanese knotweed with a shovel. Digging does not make sense for large stands but will successfully remove small amounts of knotweed. Using a shovel, lift up the earth under the plant. Grasp the knotweed by its stem and tug the plant and rhizomes free from the ground. Remove all knotweed in this manner.
Cut plants down to the ground using shears or mow over them with a lawnmower. Repeat this every two to four weeks, mowing or cutting any knotweed stems back to ground level. Continue this throughout the growing season to discourage the knotweed from growing back.
Cut tall knotweed plants down to a height of 4 feet in preparation for herbicide treatment if you prefer to remove them chemically. Dispose of all clippings. If your knotweed plants are shorter than 4 feet, skip this step.
Spray knotweed plants with an herbicide containing either a 2 percent glyphosate or a 3 to 4 percent solution of triclopyr, recommends the University of Rhode Island. Work on a still day so wind will not carry the herbicide to other plants. Spray the plant's leaves and stems until the leaves become wet but not dripping. Repeat this process throughout the season, waiting the recommended time between doses, which will be on the container of herbicide and varies by type and concentration.
Work the earth with a shovel to aerate it and remove any final rhizomes. Plant something else in the area as soon as possible after removing the knotweed to discourage it from growing back.