Fallopia japonica

The shoots of Japanese knotweed are strong enough to grow up through concrete sidewalks and parking lots.
Photo Credit: © Elaine Haug, USDA-NRCS PLANTS Database. Courtesy of Smithsonian Institution, Dept. of Systematic Biology, Botany.
Fallopia japonica
Common Name: Japanese knotweed
Other Common Names: fleeceflower, Mexican bamboo
Other Scientific Names: Polygonum cuspidatum
Plant Functional Group: Forb
Class > Order > Family: Magnoliopsida > Caryophyllales > Polygonaceae
What does the species look like?

Japanese knotweed is an erect, multi-stemmed, perennial, herbaceous forb to subshrub plant growing 3 to 10 feet tall, and can form large patches with creeping underground stems. Its small, greenish-white to white flowers are densely clustered along showy, branched spikes, and have either male or female parts that occur on separate plants. Male flower stalks are mostly erect, and female flower stalks are drooping.

Japanese knotweed is found on disturbed sites in preferably moist habitats. It grows on a wide range of soil types and colonizes bare volcanic soils, including those high in sulfur in its native range. It is tolerant of shade, high temperatures, salinity, and drought. It is found near water sources, along stream and river corridors, low-lying areas, waste places, utility rights-of-way, and travel corridors, fence rows, and old homesites.
Where is the species found?
States & Provinces
AK, AR, BC, CA, CO, CT, DC, DE, GA, IA, ID, IL, IN, KS, KY, LA, MA, MB, MD, ME, MI, MN, MO, MS, MT, NB, NC, NE, NH, NJ, NL, NS, NY, OH, OK, ON, OR, PA, PE, QC, RI, SC, SD, TN, UT, VA, VT, WA, WI, WV
Special Considerations for Observing

If drought seems to be the cause of leaf senescence for a plant, please make a comment about it for that observation.

This species has separate male and female flowers. If you know whether the flowers you are observing are male or female (or both), please make a comment about it for that observation.

Note that individuals of this species with only male flowers will not produce fruit.
Which phenophases should I observe?
Do you see...?
Leaves Initial growth
New growth of the plant is visible after a period of no growth (winter or drought), either from above-ground buds with green tips, or new green or white shoots breaking through the soil surface. Growth is considered "initial" on each bud or shoot until the first leaf has fully unfolded. For seedlings, "initial" growth includes the presence of the one or two small, round or elongated leaves (cotyledons) before the first true leaf has unfolded.
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Leaves
One or more live, fully unfolded leaves are visible on the plant. For seedlings, consider only true leaves and do not count the one or two small, round or elongated leaves (cotyledons) that are found on the stem almost immediately after the seedling germinates. Do not include fully dried or dead leaves.
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Flowers Flowers or flower buds
One or more fresh open or unopened flowers or flower buds are visible on the plant. Include flower buds or inflorescences that are swelling or expanding, but do not include those that are tightly closed and not actively growing (dormant). Also do not include wilted or dried flowers.
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How many flowers and flower buds are present? For species in which individual flowers are clustered in flower heads, spikes or catkins (inflorescences), simply estimate the number of flower heads, spikes or catkins and not the number of individual flowers.

  • Less than 3
  • 3 to 10
  • 11 to 100
  • 101 to 1,000
  • More than 1,000
Open flowers
One or more open, fresh flowers are visible on the plant. Flowers are considered "open" when the reproductive parts (male stamens or female pistils) are visible between or within unfolded or open flower parts (petals, floral tubes or sepals). Do not include wilted or dried flowers.
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What percentage of all fresh flowers (buds plus unopened plus open) on the plant are open? For species in which individual flowers are clustered in flower heads, spikes or catkins (inflorescences), estimate the percentage of all individual flowers that are open.

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
Fruits Fruits
One or more fruits are visible on the plant. For Fallopia japonica, the fruit is tiny and capsule-like, maturing to shiny black-brown, and is enclosed within remnant flower parts that become tan, papery "wings".
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How many fruits are present?

  • Less than 3
  • 3 to 10
  • 11 to 100
  • 101 to 1,000
  • More than 1,000
Ripe fruits
One or more ripe fruits are visible on the plant. For Fallopia japonica, a fruit is considered ripe when its outer covering has turned tan, dry and papery.
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What percentage of all fruits (unripe plus ripe) on the plant are ripe?

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
Recent fruit or seed drop
One or more mature fruits or seeds have dropped or been removed from the plant since your last visit. Do not include obviously immature fruits that have dropped before ripening, such as in a heavy rain or wind, or empty fruits that had long ago dropped all of their seeds but remained on the plant.
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How many mature fruits have dropped seeds or have completely dropped or been removed from the plant since your last visit?

  • Less than 3
  • 3 to 10
  • 11 to 100
  • 101 to 1,000
  • More than 1,000
What do these phenophases look like?

The following Phenophase Photo Guides for this species have been vetted for accuracy by the USA-NPN National Coordinating Office. Most photo guides are developed for a particular local or regional monitoring effort, and some of the content may not apply to your effort or your region. However, we make them available to provide as much help as they may in illustrating phenophases for this species. If you have high quality phenophase photos that you are willing to share with us, please visit the Phenophase Photo Guidelines page.

Be aware there is variation from individual to individual within a species, especially across different regions, so your plant may not look exactly like the one pictured.

Since they do not always include complete definitions for the species, use these photo guides ONLY in conjunction with the official Nature's Notebook phenophase defintions included in the table above, in the phenophase definition sheet that downloads with the datasheet, or in the Observe screen in the mobile app.

USDA PLANTS profile for Japanese knotweed (USDA Symbol: POCU6 )