Invasives 2006 Abstracts

Persistence of Polygonum cuspidatum: Lessons Learned From 6 Years of Field Trials.

Jason Dumont, Outreach and Education Coordinator, The Nature Conservancy
821 SE 14th AV. Portland, OR 97214 jdumont@tnc.org 503.802.8100

Doug Kreuzer, Ecological Monitoring Coordinator, The Nature Conservancy
Jonathan Soll, Willamette Basin Conservation Director, The Nature Conservancy
Kyle Strauss, Field and River Access Coordinator, The Nature Conservancy

Since 1999 The Nature Conservancy’s Sandy River Riparian Habitat Protection Project (SRRHPP) has been working to eradicate Polygonum cuspidatum (henceforth knotweed) from the Sandy River Watershed. During 5 years of intensive efforts we have tested a range of treatment options including cutting, spraying, direct stem injection and numerous combinations thereof using mainly glyphosate based products. Year-to-year monitoring of over 650 knotweed sites has demonstrated 60- 80% annual reduction in stem numbers, with more dramatic reductions in above ground bio-mass. However, only 299 sites exhibited no knotweed regrowth in 2005. Some sites that had exhibited no regrowth in past years, were again growing healthy knotweed stems. At other sites, small deformed plants with little to no leaf surface area and with stems too small to inject, have persisted for several years despite repeated retreatment. At these sites, excavated knotweed crowns and rhizomes have, more often than not, contained large amounts of living tissue. These results imply that treatment as we have done it with glyphosate based herbicides, while eliminating much of the above ground plant material, often fails to fully kill the underground crown and rhizome structures. These tissues appear to be able to persist for years, even without growing above the surface. These results demonstrate a need for both alternative treatment methods including the use of other herbicides and excavation, and careful long-term monitoring of treatment areas.