Puget Sound Shoreline, Rae A. McNally

Knotweeds are classified as noxious weeds. They spread quickly, particularly along rivers and steams where they can out-compete native plants and destroy habitat for spawning fish. These weeds have massive root systems--sometime as deep as 9 feet! They grow in dense stands up to 12-feet tall.

If left unchecked, knotweed will steadily take over river and stream banks and beaches.  

Four genetically related species of non-native knotweeds are classified as Class B noxious weeds on Washington State ’s Noxious Weed List.

  • Japanese (Polygonum cuspidatum)
  • Giant (Polygonum sachalinense)
  • Bohemian (Polygonum bohemicum)
  • Himalayan (Polygonum polystachyum)
A Class B listing means that these weeds are either absent from or limited in distribution in some portions of the state but very abundant in other areas. The management approach for Class B weeds is to contain the plants where they are already widespread and prevent their spread into new areas.

What’s the problem?

Knotweeds displace normal riparian vegetation, disrupting the food chain by altering the timing and quality of leaf litter introduced along riparian corridors. This in turn changes nutrient runoff, soil composition and the source of food available to invertebrates that form the base of the aquatic food web. Knotweed stands apparently accelerate bank erosion, thereby damaging salmon and fish habitat.

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How did invasive knotweeds get to Puget Sound?

Originally, these species were introduced as garden ornamentals. They’re hardy plants, successful colonizers and they spread quickly to less hospitable environments including areas with poor soils, such as gravel beds in rivers and streams.

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Where are knotweeds in this region?

Knotweeds are found through out the Puget Sound basin. The state Department of Agriculture produces annual reports on control efforts that show the current distribution of knotweed in the basin.

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What’s being done?

The Department of Agriculture works with local noxious weed control boards, state agencies, not-for-profit organizations and tribes to survey for and treat infestations.  

>> Learn more

For more information about ballast water management and the Puget Sound Partnership's Aquatic Nuisance Species Program, contact Kevin Anderson, 360.725.5452.

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Japanese Knotweed. | Shutterstock.com/Steven McWilliams
Japanese knotweed. | Shutterstock.com/Steven McWilliams