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Japanese knotweed (Polygonum cuspidatum)

Posted in Species Profiles

Description: Japanese knotweed is a herbaceous perennial plant, growing in dense patches between 3 and 15 feet in height. Japanese knotweed forms stout, zig-zag stalks swollen at the node, which are hollow and smooth like bamboo. The stalks persist through the winter, but new plants arise from the extensive root system. .

Picture of Japanese knotweed leaves and flowers.
Leaves are oval to triangular and pointed at the tips. Flowers appear in small, creamy-white clusters in the leaf axils in late summer.

Habitat: Japanese knotweed is tolerant of saline environments, heat, shade, drought, and scouring conditions in riparian zones. It also colonizes disturbed areas such as fill dirt and construction areas.

Invasive Traits: Once established, his plant grows vigorously, quickly building a hearty root system. Roots are known to spread through the substrate sometimes penetrating foundations, and breaking apart even the smallest fissures in rock or cement, causing foundational damage to property. When growing in riparian zones, plant tissue can be carried by water and spread downstream where it can root and establish new populations.

Control: Mechanical control is extremely difficult as manually removing the entire root system of Japanese Knotweed is a daunting task. Applying glyphosate products with a foliar spray technique is more effective, because the systemic herbicide will kill the root system. Care must be taken not to displace any living stalks into waterways, because that is the method by which the plant most often spreads. With persistence, the plant can be controlled using a combination of mowing in the spring, followed by foliar spray in late summer (Gover, Johnson & Selmer 2007)

Further Reading

Gover, Johnson, & Sellmer 2007

Devon Knotweed Forum 2008



[polygonum_2] stem


[polygonum_4] flower


Photo Credits