Boundary Invasive Species Society
Japanese knotweed (Fallopia japonica)
Perennial semi-woody shrub with a creeping root system growing to 3 m tall. It is an escaped ornamental from eastern Asia forming dense
• Bamboo like stems are green to reddish brown.
• Zig zag leaf attachment on stem.
• Heart to triangle shaped leaves 10-15 cm long.
• Cluster of white flowers in late summer.
Extensive creeping roots can grow through concrete and has been reported growing through concrete foundation walls.
Certain herbicides can be effective. Digging and mowing have not been found effective. Excavating root mass can be effective if entire root system is removed.
Small, white/green flowers grow in showy, plume-like, branched clusters along the stem and leaf axils (joints).
Leaves and Stems:
Leaves are heart to triangle shaped and 10-15 cm long. Stems, or canes, are hollow with varying thicknesses, upright, and bamboo-like with reddish-brown/red speckles and thin, papery sheaths. Stems are generally 1-3 m in height and grow in large, dense thickets. Stems may persist through the winter as bare, reddish-brown stalks.
Reproduction and Dispersal:
Primarily from root (rhizomes) and stem fragments that are dispersed by human activities or by water
to downstream areas. It is often spread by movement of soil from infested sites or by gardeners sharing plants.
Does not produced viable seed unless able to cross pollinate with Bohemian or Giant knotweed.
Extensive creeping root system that can go down 3 m and spread laterally 20m. Root and stem fragments as small as 1 cm can start new colonies.
Japanese knotweed is an escaped ornamental originating from eastern Asia. It forms aggressive, dense thickets maintained by an extensive and deep, creeping rhizome root system. New shoots are known to penetrate thick asphalt and can grow through foundations.
Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. Also found in riparian areas, along stream banks, and in other areas with high soil moisture. Able to grow in partial shade or full sun.