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Home > Weeds > Japanese Knotweed

Japanese Knotweed—Polygonum cuspidatum (Fallopia japonica)

Japanese knotweed is an invasive that grows quickly and aggressively, forming dense thickets. It thrives especially in riverbanks, roadsides and moist areas.

japanese knotweed along road japanese knotweed plant japanese knotweed flowers
Dense thickets can clog small waterways and lower the quality of wetland habitats. Leaves are oval or heart–shaped with pointed tips, 6 inches long, 4 inches wide. Flowers are small and white and grow in showy lace-like branched clusters. Flowers appear in July and August.
japanese knotweed stem japanese knotweed stem japanese knotweed flower
Spreads aggressively, sprouting from rhizomes which can be 30 feet long. Bamboo–like, hollow, reddish–brown stem that can grow 3 to 9 feet tall. Each female flower can produce 1000 wing-like seeds.

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[Photos, left to right: Leslie J. Mehrhoff, University of Connecticut,; Richard Old, XID Services, Inc.,; James H. Miller, USDA Forest Service,; Leslie J. Mehrhoff, University of Connecticut,; Philip Rusted, Thurlow Countryside Management (r&d),; Leslie J. Mehrhoff, University of Connecticut,]

It is the policy of the State of Maine to minimize reliance on pesticides. The Maine Department of Agriculture and the Maine IPM Council encourage everyone to practice integrated pest management and to use pesticides only as a last resort. The mention of pesticides in the fact sheets linked to these pages does not imply an endorsement of any product. Be sure that any product used is currently registered and follow all label directions.