Japanese Knotweed

Invasive Species | Terrestrial Plants

Japanese Knotweed

(*Fallopia japonica*)
French common name: Renouée du Japon
Japanese knotweed is an aggressive semi-woody perennial plant that was introduced as a horticultural plant to act as a “living fence”. It forms dense thickets of bamboo-like vegetation that outcompetes native plants and negatively impacts wetland and riparian areas. It’s aggressive growth habit also makes it a risk to infrastructure like roads and building foundations, as it will grow right through them.
(Photo credit: David J. Moorhead, University of Georgia, Bugwood.org)

Description:

Size/ Structure: A small-to-medium, densely branched perennial shrub with thorn-covered stems, typically 1 – 2m tall and 0.3- 1.5m+ in width. Often one of the first plants to leaf out in spring.

Leaves: Small, smooth-edged, and shaped like a water droplet; clustering in tight bunches close to the branch. They exhibit a range of colors (red, purple, or green) with a dominant dark green shade in the wild.

Flowers: Tiny, pale yellow flowers with 6 petals; hang singularly or in groups of 2-4 beneath leaf clusters along underside of stems in early-to-mind spring.

Berry/Seeds: Bright red berries that hang singularly along the underside of the stems; mature in mid-summer and remain through the winter.

Tom Heutte, USDA Forest Service, Bugwood.org

Key ID Features

Barbara Tokarska-Guzik, University of Silesia, Bugwood.org

Hollow “bamboo-like” stems

Leaves grow in distinct “zig-zag” alternating pattern

James H. Miller, USDA Forest Service, Bugwood.org

Leaves have a flat, square base

Common Look-a-Likes

Rob Routledge, Sault College, Bugwood.org

Dogwoods (Cornus sp.)

Most dogwoods have opposite leaves with a wide base (not flat) and veins that arch towards a narrow leaf tip; stems are green to reddish and can have tiny white flecks (lenticels).