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Pesticide Safety Education Program

Ohio State University Extension

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Japanese knotweed (Polygonum cuspidatum)

Description: Knotweed is an upright, shrub-like, herbaceous plant that can grow to over 10 feet high.

Life cycle: Perennial

Habitat: Grows best in full sun and can grow semi-shade. It is found along roadsides, stream and river banks, wetlands, wet depressions and woodland edges. It can tolerate a wide array of soil and moisture conditions.

Flower: Many white flowers arranged in spikes. Bloom in August and September.

Fruit: Small glossy winged fruits the produce dark shiny seeds. Can be spread by wind, water, birds and insects. Not all seeds are fertile.

Leaves: Alternating and broad. They typically grow to 4- 6 inches in length and 3-4 inches in width with an abrupt pointed tip.

Stems: Upright, round, and hollow. Resemble bamboo.

Root: Rhizomes

The problem is…an aggressive grower. Widespread rhizomes form dense undergrowth. Knotweed is a threat to riparian areas because small pieces of rhizome washed downstream can start new colonies. Knotweed can contribute to both stream and bank erosion and flooding. Its rhizomes and shoots can penetrate asphalt and cracks in concrete.

Japanese knotweed thicket

Japanese knotweed young stems, Anna Rose.

Japanese knotweed stems

Japanese knotweed zigzag stems

Japanese knotweed flowers