Invasive Knotweed Management

Knotweed is an invasive species in the Pacific Northwest that is quickly infesting and disrupting our waterways. It forms dense thickets and outcompetes native plants, but provides poor forage for native animals. It is difficult to control and grows quickly, making it one of the most threatening invasive plants in the basin if left uncontrolled. The Watershed Council has conducted knotweed surveys and control efforts since 2010 to remove the plant, stop its spread, and reduce its impact on native species. Within the Marys River Watershed we have identified Giant Knotweed ( Polygonum sachalinense), Japanese Knotweed (Fallopia japonica), as well as some hybrid variations. 

To complete these knotweed removal projects Marys River Watershed Council receives support from the Forest Service, Oregon Department of Agriculture, Marys Peak Stewardship Group, and Pacific Power Blue Sky Habitat Fund. We also rely heavily on the landowners who allow us to do this work on their properties.  

Knotweed is detrimental to the ecosystem because it grows rapidly and aggressively shades out native plants. Knotweed plants can also directly harm other plants by releasing chemicals into the ground. Large knotweed thickets can contribute to soil erosion by weakening tough soils and materials. Knotweeds uproot easily during flooding, which weakens soil structure. Animals in infested areas can starve or become malnourished when they try to eat less nutritious knotweed leaves.

Alongside risks to the environment, knotweed plants can also damage built structures. The stems also break off and uproot easily during floods, which can increase flood damage. Knotweed infestations speed the decay of roads, sidewalks, and bridges by colonizing cracks in materials like concrete and asphalt. Weakened infrastructure can be more expensive to monitor and repair, and increase the risk of catastrophic failure.

Knotweed plants are difficult to permanently remove because they can regrow even from small shards of cut stems and rebound quickly from roots left in the ground. Knotweed plants also send underground runners that can travel as far as seven feet from the original plant. These properties make mechanical knotweed removal difficult and labor-intensive, requiring careful management of plant fragments and multi-stage treatments. Knotweed is resistant to most herbicides. The Watershed Council has identified only a few herbicides that can adequately address infestations. 

Despite the difficulty of knotweed control, the Watershed Council has developed a plan to eradicate it throughout the basin. Knotweed tends to grow only in riparian areas, and typically colonizes new areas when root or stem fragments wash downstream and re-root. Knotweed infestations can be addressed by walking downstream and using a backpack herbicide sprayer to kill each stand spotted. This strategy has led to sustained decreases in knotweed populations in Woods, Greasy, Oak, and Dixon Creeks, and the mainstem Marys that cover over 25 stream miles. MRWC has found that due to our treatments we address around 4 fewer knotweed stands each year.