How to kill knotweed

Updated August 08, 2017

Knotgrass (often referred to as Japanese Knotweed) is a shrublike perennial weed that's native to Eastern Asia and has spread to the Western world. In fact, it grows so quickly that knotgrass is difficult to contain and spreads easily from one site to another. As it spreads, Japanese knotgrass alters and threatens ecosystems. For that reason, the Plant Conservation Alliance, a United States government-run group, advocates killing the plant. Once you've killed the weed it's important to dispose of it properly. The best way to do so is to place it in a garbage bag and throw it in a can or dumpster to prevent future re-establishment.

Cut the knotgrass stems just below the bottom joint with gardening shears at any time during the growing season. Put the tops of the plants in a pile where they can dry out and die.

Fill a small glass with liquid Glyphosate herbicide. Do not dilute the herbicide with water. Doing so will only make it weaker.

Pour a tablespoon of the herbicide into the hollow stem of each knotgrass plant. Be cautious about dripping it on any surrounding plants or grass as it will likely kill them.

Make a mixture of diluted herbicide in a garden sprayer. All herbicides are different, so follow the directions on the package carefully.

Spray the plants with the herbicide once every three weeks until the ground freezes over. If the ground does not get cold enough, continue spraying the herbicide every three weeks until the plant dries out completely and dies.

Repeat the same process the following year. Knotgrass has roots that can grow up to 10 feet deep, so it can take up to three years of work to completely kill it off. Once completely dead, it's best to remove the entire knotgrass system, including the roots. This prevents any future growth.

Things You'll Need

  • Gardening shears
  • Glyphosate herbicide
  • Small glass
  • Garden sprayer
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About the Author

Wendy Rose Gould is a professional journalist who has contributed to "Glamour" magazine and the Huffington Post, among other publications. After internships at the "Indianapolis Business Journal," "Kiwanis International" and "NUVO Newsweekly," she earned BA degrees in journalism and philosophy from Franklin College in 2008. Gould specializes in lifestyle topics.