Wildflowers of the United States

Japanese Knotweed, Crimson Beauty, Mexican bamboo, Japanese Fleece Flower, Reynoutria - Fallopia japonica

Native Status: Introduced

Dicot Perennial Subshrub Herb

Fallopia japonica - Japanese Knotweed, Crimson Beauty, Mexican bamboo, Japanese Fleece Flower, Reynoutria. Japanese Knotweed is a highly invasive species native to Asia, introduced as an ornamental in the United States in the 1800s. It reproduces vegetatively from its rhizomes, so removing it and discarding the remains propagate rather than limit the plant. In England it is classified as Controlled Waste and must be burned or transported to a licensed disposal site; it is illegal to include Japanese Knotweed in household garbage. Reportedly UK mortgage lenders are refusing to make loans on properties where the plant is found until remediation plans are made. In the United States, it is officially considered a noxious weed in Alabama, California, Oregon, Vermont, and Washington State.

Fallopia japonica was formerly included in Polygonum (along with the other 11 members of the genus Fallopia), and is still classified as Polygonum cuspidatum in many publications. But some authorities, based on molecular study as well as erect stems, classify Japanese Knotweed as Reynoutria japonica. ITIS lists it in Fallopia; Weakley lists it in Reynoutria; USDA lists it in Polygonum.

Found in:
AK, AR, CA, CO, CT, DC, DE, GA, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, NE, NH, NJ, NY, OH, OK, OR, PA, RI, SC, SD, TN, UT, VA, VT, WA, WI, WV

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Distribution of Fallopia japonica in the United States and Canada:

Map courtesy of The Biota of North America Program.
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Site: Johnson County, TN Date: 2013-August-22	Photographer: Gerald C. Williamson Nikon D7000
Part of the problem with Japanese Knotweed is that it is an attractive plant, so folks that don’t know about its invasive nature may not attempt to eradicate it as soon as they find it. It has 6-inch white sprays of attractive flowers above a branch with ovate leaves to 6 inches long. The mid-branch leaf bases of Giant Knotweed are deeply cordate; those of Bohemian Knotweed are slightly cordate The bases of Japanese Knotweed leaves are truncate or slightly v-shaped.

Site: Camp Vesper Point, Hamilton County, TN Date: 2013-September-13	Photographer: Gerald C Williamson Nikon D7000
The sepals turn red, giving an attractive change of color - once again leading some folks to ignore the invasive nature of this plant.
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Site: Camp Vesper Point, Hamilton County, TN Date: 2013-September-13	Photographer: Gerald C Williamson Nikon D7000
I’m finding mixed reports on whether this plant is monoecious or dioecious. I think that there may be plants with female flowers, and plants with bisexual flowers, and that these are infertile, with the plant reproducing purely vegetatively - however, I am not sure of this.
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Site: Johnson County, TN Date: 2013-August-22	Photographer: Gerald C Williamson Nikon D7000
Japanese Knotweed forms large, dense colonies up to about 10 feet high, crowding out native plants.
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Site: Camp Vesper Point, Hamilton County, TN Date: 2013-August-22	Photographer: Gerald C Williamson Nikon D7000
Japanese Knotwood stems are bamboo-like.
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References used for identification and information:

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Fallopia japonica initially published on USWildflowers.com 2014-01-05; Updated 2016-01-27

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Code Update 20230302