Japanese knotweed (Polygonum cuspidatum)

Japanese knotweed patch

Native to Japan, China and parts of Korea and Taiwan, Japanese knotweed was introduced to North America in the late 19th century as an ornamental plant. This invasive herbaceous perennial grows in a variety of moist soil types and is highly adaptable to extreme temperatures, salinity, droughts and floods. It grows very quickly and often forms dense thickets, crowding and shading out native plants and grasses.

Japanese knotweed has hollow bamboo-like stems which grow up to 4 m tall and are ridged, jointed and light green to tan or reddish in colour. It has tear-drop shaped leaves that are sharp-pointed and flattened at the base. They alternate up the stem and are dark green with reddish veins. Japanese knotweed has small light green to white flowers that bloom from August to September.

Japanese knotweed is an aggressive, semi-shade tolerant plant that prefers moist to wet soils and grows well in or near wetlands, watercourses and roadside ditches. Its root system or rhizomes, can spread 15-18 feet and are so strong that they can even grow though pavement.

Young Japanese knotweed shoots are edible and can be harvested and prepared like asparagus. To remove Japanese knotweed mowing or cutting the large stalks at least 3 times per season (every 2 weeks is recommended) can help drain the energy stores in the rhizomes. For younger plants hand pulling can work, but not for older plants because they are likely to re-sprout from broken rhizomes. Foliar herbicide application is also effective, but requires a licensed pesticide applicator.