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Japanese Knotweed
  • Knotweeds are usually introduced as ornamentals

  • An escaped ornamental from eastern Asia forming dense thickets

Japanese Knotweed Stalks

Knotweeds - Japanese, bohemian, giant
(Fallopia japonica, Fallopia x bohemica, & Polygonum sachalinense)

Quick Identification

  • Bamboo like stems are green to reddish brown

  • Zig zag leaf attachment on stem

  • Heart to triangle shaped leaves 10-15 cm long

  • Cluster of white flowers in late summer

Knotweeds are escaped ornamentals originating from eastern Asia.  They form aggressive, dense thickets maintained by an extensive and deep, creeping rhizome root system.  New shoots are known to penetrate thick asphalt and can grow through foundations.

Certain herbicides can be effective. Digging and mowing are not effective. Excavating root mass can be effective if entire root system is removed.

Small, white-green flowers grow in showy, plume-like, branched clusters along the stem and leaf axils (joints).

Leaves & Stems
Leaves are heart to triangle shaped range from 15 cm in length in Japanese knotweed up to 30 cm in giant knotweed. Stems, or canes, are hollow with varying thicknesses, upright, and bamboo-like with reddish-brown speckles and thin, papery sheaths. Stems are generally 1-3 m in height and grow in large, dense thickets. Stems may persist through the winter as bare, reddish-brown stalks.

Japanese Knotweed Grows Through Concrete and Foundations

Extensive creeping root system that can go down 3 m and spread laterally 20m. Root and stem fragments as small as 1 cm can start new colonies.

Reproduction & Dispersal
Primarily from root (rhizomes) and stem fragments that are dispersed by human activities or by water
to downstream areas. It is often spread by movement of soil from infested sites or by gardeners sharing plants.

Japanese knotweed does not produced viable seed unless able to cross pollinate with Bohemian or Giant knotweed.  Knotweed seeds are small and have wings which give them a heart shape.

Preferred Habitat
Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. Also found in riparian areas, along stream banks, and in other areas with high soil moisture. Able to grow in partial shade or full sun.

Interesting Facts
Extensive creeping roots can grow through concrete and has been reported growing through foundation walls.

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