Japanese knotweed (Polygonum cuspidatum)

japanese knotweed

Description:

Appearance: Shrub-like, arching perennial herbaceous plant, over 10' high, reddish-brown stems, smooth, stout hollow and swollen at the joint where the leaf meets the stem.

Leaves: Alternate broadly oval and pointed at the tip, about 6" long, 3 - 4" wide.

Flowers: Greenish-white, branched clusters which grow from leaf axils, near the end of stems, blooming in late summer.

Fruit: Small winged, carry triangular, shiny and very small seeds.

Roots: Long, stout horizontal stems growing below the soil surface, called rhizomes, forming roots and producing new plants allow the plant to spread vegetatively as well.

Ecological Threat:

  • Japanese knotweed spreads primarily vegetatively to form dense thickets that suppress native vegetation.
  • It can pose a significant threat to riparian areas, such as disturbed stream sides, lakeshores and other low lying areas, where it can rapidly colonize. It tolerates full shade, high temperatures, high salinity and drought.
  • It is currently occurring from Maine to Minnesota and south to Louisiana and scattered in midwestern and western states. It was introduced to the U.S. in the late 1800s for ornamental purposes and erosion control.
  • Japanese knotweed and the related giant knotweed are MDA Prohibited Noxious Weeds on the Control List in Minnesota. Anyone selling Japanese or giant knotweed must include a label advising that the plants are not to be planted within 100 feet of a waterbody or a floodplain.

 

Control Methods:

Japanese knotweed can be challenging to control.  See the "Additional Resources" section for more detailed information.

Mechanical

  • Digging plants is effective for small infestations and in sensitive areas
  • Pulling of juvenile plants

Plants can resprout from small fragments.  Mowing may spread plants to new areas.  See the "Additional Resources" section below for more information on disposal.

Chemical

  • Cut stem treatment with glyphosate or triclopyr
  • Foliar spray in large single species populations

 

Native Substitutes:

Additional Resources