Japanese knotweed

August 27, 2015

Polygonum cuspidatum Sieb. & Zucc

Life cycle

Aggressive, clump-forming perennial.

Leaves

Alternate, broadly egg-shaped, sometimes heart-shaped with a squared leaf base.

Stems

Stout, semi-woody, red to brown, hollow and bamboo-like, often exceeding 6 to 8 feet in height. Stems are also jointed, forming a zigzag growth pattern. A membranous sheath (ocrea) surrounds the stem at the base of each petiole. Reddish shoots arise from thick, sturdy and abundant rhizomes.

Japanese knotweed new ocrea
Japanese knotweed ocrea.

Flowers and fruit

Flowers are small, white to pink and borne in elongated clusters. The seed is enclosed in a single-seeded, shiny, dark brown to black, three-sided fruit.

Japanese knotweed foliage & flowers Japanese knotweed flower
Japanese knotweed foliage and flowers.

Reproduction

Seeds and aggressive rhizomes.

Similar weeds

Giant knotweed (P. sachalinense F. W. Schmidt) Differs by having a greater height and heart-shaped leaf base.

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MSU Extension Integrated Pest Management