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Japanese knotweed is a dense growing herbaceous perennial, reaching heights of 10 feet, often occurring in large monocultures. The semi-woody stem is hollow with enlarged nodes, resembling bamboo. Its large, alternate, dark-green leaves are about six inches long, three to four inches wide, somewhat triangular and pointed at the tip. Minute, greenish-white flowers occur in attractive, branched sprays in summer and are followed soon after by small, winged fruits.

Introduced to the U.S. from East Asia in the late 1800s, Japanese knotweed was used as an ornamental, for erosion control and for landscape screening. Currently it is found along streams and rivers, in low-lying areas, waste places, utility rights-of-way and disturbed areas.

Japanese knotweed spreads primarily through its stout rhizomes. It often is transported to new sites as a contaminant in fill dirt or on construction equipment, and can also be distributed by water or the wind. The plant spreads quickly to form dense thickets that exclude native vegetation. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands.

The most widely used method to control large established stands of Japanese knotweed is through a combination of cutting and chemical control. Manual control alone is not an effective long-term control method and may actually exacerbate the problem by encouraging new growth from rhizome segments.
More Resources:

Element Stewardship Abstract
Forest Pests
U.S. Forest Service
Biocontrol of Invasive Plants
Morris Arboretum

Southeast Exotic Pest Plant Manual
Plant Conservation Alliance
Plant Invaders of Mid-Atlantic Natural Areas
Pennsylvania State University

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