Extension Logo
Extension Logo
University of Minnesota Extension
https://extension.umn.edu

Giant knotweed

Quick facts

Giant knotweed is an invasive species. 

  • Giant knotweed grows in disturbed areas and can tolerate varying sun availability.
  • It is the largest of the knotweeds.
  • Its growth shades out and displaces native vegetation.

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

tall giant knotweed plants along a sidewalk

How to identify giant knotweed

  • Perennial shrub that can grow nine to twenty feet in height, typically with no branches.
  • Resembles bamboo.

Stem

  • Large, light green and hollow with reddish joints.

Leaves

  • Simple, alternate, toothless and heart shaped.
  • They are large, reaching twelve inches across and six to fourteen inches wide.
  • Tips of leaves are blunt.

Flowers

  • Small, off-white or greenish flowers are compact, usually shorter than the leaves.
  • They form in clusters and bloom up from the leaf axil from July to October.
  • Giant knotweed has perfect flowers, including both female and male fertile flowers.

Seeds

  • Female flowers produce a winged papery fruit.
  • Each fruit contains a small, shiny, brown-black three-sided viable seed.

Roots

  • The root system extends deep into the soil and has thick mats of rhizomes.
  • Reproduction primarily occurs through these rhizomes.
close up of thin, redish green giant knotweed stem
Giant knotweed stems
Comparing 2 giant knotweed leaves to a pen
Giant knotweed leaves

Angela Gupta, Extension educator; Amy Rager, Extension educator; Megan M. Weber, Extension educator

Reviewed in 2019

Share this page:

© 2019 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.