Knocking out knotweed: research pins down a rogue invasive

Bohemian knotweed spreads aggressively along rivers. This invasive weed chokes waterways, displaces native plants, erodes riverbanks, and keeps tree seedlings from growing. Communities in the Pacific Northwest spend millions of dollars to eradicate it on the assumption that it harms fish habitats.

But knotweed is difficult to kill. It takes years of herbicide applications to destroy the weed, and a single fragment can sprout and start new infestations. Also, control programs typically don’t evaluate whether native plants reestablish themselves after knotweed is cleared. Forest Service researchers wanted to understand whether eradication programs are achieving their goals and how knotweed affects aquatic life and fish habitat.

An analysis of leaf packs submerged in Washington’s Chehalis River revealed that fallen knotweed leaves are low in nitrogen and phosphorus, and high in cellulose, fiber, and lignin. This low nutrient and hard-to-digest leaf litter limits the productivity of aquatic fungi and macroinvertebrates—primary prey for juvenile salmon, trout, and other fish species. Researchers also found that although herbicide application cleared river banks of knotweed and allowed colonization by native plants, it also promoted secondary exotic invaders. Successful reestablishment of native plants following knotweed removal may require active restoration, such as post-treatment plant surveys, controlling secondary invasions, and replanting native species.

Citation

Vizcarra, Natasha; Claeson, Shannon. 2015. Knocking out knotweed: research pins down a rogue invasive. Science Findings 169. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6 p.

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