Knotweed Control

Why control Knotweed?

Knotweed's aggressive growth displaces native plants, reduces food sources for wildlife, and degrades habitat along salmon-bearing streams. Knotweed can completely clog small waterways, contribute to bank erosion, and cause damage to structures and paved surfaces due to its extensive root system. Knotweed spreads quickly -- often during flooding or other disturbance events. A root fragment as small as half an inch can quickly establish in a new area after being washed downstream. 

 

Knotweed Identification:

• Usually found growing along streams, ditches and floodplains

• Grows in dense stands 6-12 ft. tall

• Bright green leaves with smooth edges, often heart shaped

• ​Leaves have an alternate (zig-zag) arrangement on the stem

• Flowers are small, white and clustered

• Hollow, bamboo-like greenish/red canes 

• Emerges in April, reaching peak height in July

• Goes dormant and "dies back" in the winter ​

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What can you do to help?

• Control knotweed on your property

• Reach out to your local conservation district or Noxious Weed Board

• Properly dispose of Knotweed by sealing it in sturdy plastic bags to ensure                                      fragments don't re-sprout. DO NOT put Knotweed in yard waste, compost                                              piles, or along waterways.

• Re-plant areas with native species 

• Help educate your neighbors and work together!​

 

What is the most effective way to control Knotweed?

​*There are several treatment options to control Knotweed, but due to its fast growth and spread, successful eradication usually requires herbicide.

• Organic or manual control

  • Cutting and covering plants with tarps has been shown to be marginally effective​

  • Repeated cutting of shoots may tax the plant's energy and lead to less stems meerging over time. Plan to cut down the plant at least 10 times per season. 

• Chemical control 

  • A special permit or license may be required to use herbicides near waterways. Always read and follow the label, wear proper personal protection equipment, and follow state and local regulations.

  • Most effective during the summer or early fall - timing matters!​

  • A stem injector gun can be used to focus herbicide application. This method is more labor intensive, but more targeted.​

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Need Assistance?

Lewis Conservation District: (360) 996-4560

Lewis County Noxious Weed Board: (360) 740-1215

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Learn more:

• Weed Report: Japanese, Sakhalin, Bohemian, and Himalayan Knotweed (Document)

• Do You Have Knotweed on Your Property? (Document)​​