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Japanese knotweed

Quick facts

Japanese knotweed is an invasive species. Japanese knotweed is on the  Control noxious weed list meaning you must prevent the spread of this plant.

  • Japanese knotweed tolerates full sun, high temperatures, high salinity and drought.
  • It can pose a significant threat to riparian areas, such as low-lying stream sides, lakeshores and other low-lying areas.
  • It spreads vegetatively, forming dense thickets that suppress native vegetation.
  • Crosses between Japanese, Bohemian and giant knotweed are being found in Minnesota.
  • In Minnesota, roots have grown through both old and new building foundations, producing vine-like roots inside these structures.

Japanese knotweed should be reported. The Minnesota Department of Natural Resources provides detailed recommendations for reporting invasive species.

How to identify Japanese knotweed

Blossoming bush of Japanese knotweed
  • Japanese knotwood (Polygonum cuspidatum) resembles bamboo.
  • Perennial plant, ten feet tall.
  • Often forms dense, leafy thickets.

Stem

  • Young stems emerging from the ground are purplish and green.
  • Mature stems are green during the summer, reddish brown in the fall.
  • Stems are smooth, hollow and swollen at the joint where the leaf meets the stem.
  • Can reproduce from stem fragments.

Leaves

Close up of Japanese knotweed leaf
Japanese knotweed leaf
  • Alternate, broadly oval, straight at the base and pointed at the tip; dark green on the upper surface and light green on the lower surface, about four to six inches long and three to four inches wide.
  • Knotweeds hybridize, making identification tricky.

Flowers

  • Male and female flowers.
  • Greenish-white, branched clusters grow from leaf axils near the end of stems.
  • Blooms in late summer.
Pink and white Japanese knotweed flowers.
Japanese knotweed flower

Seeds

  • Seed production is rare.
  • Small, winged pods carry very small, shiny, triangular seeds.

Roots

  • Long (65 feet or more), stout rhizomes form fibrous roots, allowing vegetative spread.
  • Root fragments can produce new plants.

Angela Gupta, Amy Rager and Megan M. Weber, Extension educators

Reviewed in 2019

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