Planting Native Species to Control Site Reinfestation by Japanese Knotweed (Fallopia japonica)

Japanese knotweed (Fallopia japonica) is a highly invasive species that has become a serious problem in riparian zones and along road and railroad right-of-ways in North America and Europe. Once established, it forms solid colonies choking out other herbaceous vegetation, displacing native species, negatively affecting wildlife, and altering hydrological processes. We evaluated the ability of 6 native species mixtures to prevent recolonization by Japanese knotweed at a site receiving either 1 or 2 yr of glyphosate applications and mowing to suppress existing Japanese knotweed stands. One year of spraying and mowing was not sufficient to adequately suppress Japanese knotweed. By 37 months after sowing, only the multi-species riparian buffer mixture (RBM) had plant cover >20%, whereas cover for all other mixtures was <10%. Japanese knotweed had successfully reinvaded all plots with percent cover ranging from 72–96%. Two years of spraying and mowing reduced Japanese knotweed percent cover to an average of 12% (range 7–18%) during the first 2 yr after sowing and to 28–43% by 37 months. Only 2 species mixtures adequately established when sown following 2 yr of Japanese knotweed suppression, the RBM and a mixture of Virginia wildrye (Elymus virginicus) and prairie cordgrass (Spartina pectinata). Percent cover for both mixtures was >80% at 25 months after sowing and ≥50% after 37 months. Two years of Japanese knotweed suppression was necessary before native species mixtures could successfully compete against invasive recolonization.