Invasive Knotweed Complex

Polygonaceae Family | Class B Noxious Weed

The invasive knotweed complex consists of four species in the family Polygonaceae. Knotweeds are large, rhizomatous, herbaceous perennial plants that vary in height from 5 to 20 feet (Duncan, 2013; Parkinson & Mangold, 2017). The round, hollow, and thick stems with swollen nodes resemble bamboo. The stems are green to reddish in color, often red-speckled. The multiple high erect stems are often arched near the top (Parkinson & Mangold, 2017; King County BMP, 2015; WA NWCB). The large sized leaves are alternately arranged, bright green with smooth edges. In general leaves are ovate or broadly elliptic with a pointed tip, however leaf shape varies between species (Bobis et al., 2019; King County BMP, 2015). The rhizomatous root system of knotweed forms a deep, dense mat. Underground rhizomes are thick and extensive, storing large quantities of carbohydrates and spread aggressively (WA NWCB; King County BMP, 2015).

Rhizomes can grow 23 feet from parent plant, 50 to 65 feet laterally and penetrate 7 feet deep into the soil (King County BMP, 2015; Duncan, 2013). Rhizomes are root-like subterranean stems commonly horizontal in position that usually produce roots below and send shoots up towards the surface. Flowers are small and white to green in color with the exception of Himalayan flowers which are pink to white in color. Flowers grow in showy plume-like clusters and emerge from where leaves meet the stem; flowers have 5 petals (technically tepals), rarely 4, all of which are upright (Parkinson & Mangold, 2017; King County BMP, 2015). In the Pacific Northwest the invasive knotweed complex consists of four species including Bohemian, Giant, Japanese and Himalayan. All four species are Class B state listed noxious weeds in Washington State.

Integrated Weed Management

The Four Species of Knotweed

Works Cited

  • Bobis, O., Dezmirean, D. S., Bonta, V., Moise, A., Pasca, C., Domokos, T. E., & Urcan, A. C. (2019). JAPANESE KNOTWEED (FALLOPIA JAPONICA): LANDSCAPE INVASIVE PLANT VERSUS HIGH QUALITY HONEY SOURCE. Scientific Papers, LXII(1).
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  • Byers, J. E., Reichard, S., Hayes, D., Chornesky, E., Williamson, M., Seastedt, T., . . . Randall, J. (2001). Directing Research to Reduce the Impacts of Nonindigenous Species. Conservation Biology, 16(3), 630-640.
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  • Claeson, S. M., & Bisson, P. A. (2013). Passive Reestablishment of Riparian Vegetation Following Removal of Invasive Knotweed (Polygonum). Weed Science Society of America, 6, 208-218. Retrieved November 12, 2020.
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  • Davenport R. 2006. Control of knotweed and other invasive species and experiences restoring native species in the Pacific Northwest US. Native Plants Journal 7(1):20-26.
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  • Duncan, C. (2013). Identification and Management of Invasive Knotweed. Retrieved November 17, 2020, from https://www.techlinenews.com/articles/2013/identification-and-management-of-invasive-knotweeds
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  • King County Noxious Weed Control Program Best Management Practices: Invasive Knotweeds. (2015, July). Retrieved November 12, 2020, from https://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-control-practices/bmp.aspx
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  • Parkinson, H., & Mangold, J. (2017). Biology, Ecology and Management of the Knotweed Complex. Montana State University Extension. Retrieved November 12, 2020, from https://www.montana.edu/plantinvasions/publications/pdfs/EB0196_knotweed.pdf
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  • Reardon, R. Japanese Knotweed Biological Control. Biological Control & Biopesticides. USDA Forest Service. Retrieved November 12, 2020, from https://www.fs.fed.us/foresthealth/technology/pdfs/FS_jaknotweed.pdf
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  • Urgenson, L. S., Reichard, S. H., & Halpern, C. B. (2009). Community and ecosystem consequences of giant knotweed (Polygonum sachalinense) invasion into riparian forests of western Washington, USA. Elsevier, Biological Conservation.
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  • Written Findings of the Washington State Noxious Weed Control Board. (2004). Retrieved November 12, 2020, from https://www.nwcb.wa.gov/images/weeds/Polygonum-bohimicum-2004.pdf