How to Remove Knotweed in Seattle


A member of the buckwheat family, Japanese knotweed (Polygonum cuspidatum) resembles small stalks of bamboo with segmented stems and plumes of yellowish-white flowers. A fast-growing, shrubby plant, knotweed's rhizomes can spread underground 25 feet in all directions. According to a Seattle Times article, pulling the weed will break the rhizome and encourage it to spread more. Even small pieces of stem can regenerate under the right conditions. Japanese knotweed is tough to eradicate, and it may take a long time. In fact, experts with the King County Noxious Weed Control Program say that even a small patch of Japanese knotweed may take "several years and multiple treatments" to eradicate. Begin your efforts in May or June.

Manual Control

Step 1

Cut back any small patches of knotweed with a machete, or similar cutting tool. Experts with the King County Noxious Weed Control Program say that this should be done with any patches that contain 50 stems or less. Cut them back to the ground, twice a month, between April and August. Cut once a month thereafter, until the first frost.

Step 2

Rake the cut plant material into a pile in an area that will allow the weeds to dry out. Once dry, burn or bag them and dispose.

Step 3

Repeat this procedure, beginning every April, for three years.

Chemical Control

Step 1

Cut each knotweed stem so that only one joint (the line on the stems) remains. This should be done in late summer or early fall.

Step 2

Pour 3ml of undiluted herbicide into the hollow cavity of each stem. A child's medicine syringe is useful for both measuring the herbicide and shooting it into the stem. Be careful not to splash or drip the herbicide on desired plants. Use an herbicide with glyphosphate, such as Roundup or Gallup, for terrestrial sites. Use an herbicide with triclopyr, such as AquaMaster or Renovate, for use around water.

Step 3

Dispose of the cut plant material by either burning it or bagging it. Do not allow any of it to be left behind on the soil.

Tips and Warnings

  • Wear gloves, long sleeves, pants, closed-toe shoes and eye protection when applying herbicide. Follow all label directions.

Things You'll Need

  • Machete
  • Rake
  • Herbicide with glyphosphate
  • Herbicide with triclopyr
  • Pruning shears


  • Seattle Times: Knotweed Battle Keeps Growing
  • The Forestry and Agriculture Guide: Stopping the Invasion of Japanese Knotweed
  • State of Washington: Knotweed Management Plan
  • King County: Noxious Weed Control Program
Keywords: eradicate knotweed Seattle, rid of knotweed, kill knotweed Seattle

About this Author

Victoria Hunter, a former broadcaster and real estate agent, has provided audio and written services to both small businesses and large corporations, worldwide. Hunter is a freelance writer specializing in the real estate industry. She devotes her spare time to her other passions: gardening and cooking. Hunter holds a Bachelor of Arts in English/creative writing.