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
- Japanese knotwood (Polygonum cuspidatum) resembles bamboo.
- Perennial plant, ten feet tall.
- Often forms dense, leafy thickets.
- 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.
- 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.
- Male and female flowers.
- Greenish-white, branched clusters grow from leaf axils near the end of stems.
- Blooms in late summer.
- Seed production is rare.
- Small, winged pods carry very small, shiny, triangular seeds.
- Long (65 feet or more), stout rhizomes form fibrous roots, allowing vegetative spread.
- Root fragments can produce new plants.
Reviewed in 2019