Invasive Plants in PNW Ecosystems
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Landscape-Scale Knotweed Control in the Upper Skagit River Basin.Melisa L. Holman. The Nature Conservancy of Washington. 410 N 4th St., Mount Vernon, WA 98273. (360) 419-0556. mholman@tnc.org. The Skagit Knotweed Working Group was formed in 2000 to begin a landscape-scale effort to control Polygonum cuspidatum Sieb. & Zucc. and related congeners (knotweed) to help prevent degradation of riparian function and biological integrity in the Upper Skagit River Basin. Extensive surveys and outreach have taken place alongside a chemical-based treatment strategy. Considering all the new patches that have been found and those that have died or washed away since 2000, the upper Skagit project area has 55% fewer knotweed patches. A review of four years of treatment data suggests the following: a) Average percent mortality is 54% and average initial patch size has been reduced by 45% (for patches chemically treated at least one year), b) Integrating manual and chemical control increases mortality by 16%, c) Mortality is 35% lower for larger patches (>50 stems), d) Mortality is highest for patches that have only been treated one year, e) Regardless of patch size, mortality rates decrease as years of treatment increase, f) In 2005, 11% of patches that appeared dead for one or more previous years experienced re-growth, g) These “resurrections” are more likely for large patches (>100 stems). These data indicate that it may be important to apply new treatment strategies to larger patches and to patches that survive beyond one year of treatment. While many components contribute to the overall success of a landscape-scale invasive species program, the exploration of alternate treatment methods will be an important step in maximizing control efficacy. A multi-year commitment will be critical.
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