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Japanese Knotweed


Polygonum cuspidatum - Polygonaceae Family - Perennial

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  • Flowers: Flowers are tiny, greenish-white in color with 5 petals. They grow in large bundles of 8-1- that resemble stems.
  • Seeds: Fruits are winged on three sides; seeds are triangular, dark brown, shiny and about 0.1 inches long.
  • Leaves: Leaves are variable; about 6 in. long by 3-4 in. wide, broadly oval to somewhat triangular with a truncated base and a tapered tip.
  • Flowering Time:  Flowers in late summer into the fall.
  • Life cycle:  Japanese Knotweed is a perennial that completes the majority of its growth from early summer through late fall. It flowers in late summer, producing seed shortly after. It dies back at the first winter frost, only re sprouting the following spring.  


  • Japanese Knotweed infestations in riparian areas increase flood risk by impeding water flow and creating an excess of debris in stream and river channels.  
  • Knotweed grows extremely quickly and easily shades out many native species, reducing floral biodiversity.
  • Many native aquatic insects and amphibians only reluctantly feed on Japanese Knotweed, resulting in a decrease in their populations in infested areas.


Most effective control methods

  • Local infestations can be controlled through manual pulling, although care must be taken to remove the entire rhizomatous root system. Continual mowing can also be effective in controlling population size.
  • There is currently no biological control agent approved for use against Japanese Knotweed in the US, although several insect species are being researched.
  • Chemical control can be effective, but proper treatment depends on location and stage of growth. Check with your local weeds supervisor for questions about your infestation.

Large Images


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Jan Samanek, Phytosanitary Administration,

Japanese knotweed: flowers and foliage



japanese knotweed_2
Randy Westbrooks, Invasive Plant Control, Inc.,

Japanese knotweed: infestation



japanese knotweed_3
Jan Samanek, Phytosanitary Administration,

Japanese knotweed: foliage



japanese knotweed_4
Leslie J. Mehrhoff, University of Connecticut,

Japanese knotweed: seeds



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Ansel Oommen,

Japanese knotweed: stem



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Chris Evans, University of Illinois,

Japanese knotweed: infestation



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Jan Samanek, Phytosanitary Administration,

Japanese knotweed: flowers



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Leslie J. Mehrhoff, University of Connecticut,

Japanese knotweed: seedlings


  • Japanese Knotweed Fact Sheet

  • References

    Beaulieu, D. (18, July 24). How to get rid of Japanese knotweed. Retrieved from

    Duncan, C. (2017, November). Identification and management of invasive knotweeds. Retrieved from

    Gover, A., Johnson, J., & Sellmer, J. (2007). Managing Japanese Knotweed [PDF file]. Retrieved from View PDF

    Invasive Species Compendium. (2018, July 15). Fallopia japonica (Japanese knotweed). Retrieved from

    Natural Resources Conservation Service. (2007). Pest management – Invasive plant control: Japanese knotweed [PDF file]. Retrieved from View PDF

    Soll, J. (2004, January 16). Controlling knotweed in the Pacific Northwest [PDF file]. Retrieved from View PDF

    Swansea University. (2018, April 25). Scientists lead the way in tackling Japanese knotweed. Retrieved from