Japanese Knotweed was originally imported from Japan to North America as an ornamental garden plant. Since its arrival it has spread throughout Canada. In PEI it can be spotted along trails, in gardens and along roads. Urban areas are especially prone to invasion by Japanese Knotweed.
Japanese Knotweed is a perennial semi-woody plant that can grow to 2.5 meters (8ft). Here are some key features that may help to positively identify Japanese Knotweed:
Stems are stout, hollow and green but often mottled with a purplish-red. They are swollen at the joints where the leaves or branches extend from.
Leaves are alternate and grow along a red stem in a zig-zag form that makes identification easier. Plants are multi-branching and appear shrub-like.
Leaves are roundish to triangular and can be heart-shaped or shield-like, have smooth edges and a distinctive narrow tip. They measure approximately 10-17 cm long and 7-10 cm wide. Spring foliage is red-tinted.
Blooms August – September.
Clusters of small, creamy-white flowers form branching panicles near the end of the stem.
There are three types of Knotweed species present on PEI, Japanese Knotweed may be confused with Giant Knotweed (Fallopia sachalinensis) or Bohemian Knotweed (Fallopia x bohemica). Giant Knotweed stems are light green and typically lack the mottled stems, leaves are much larger (approximately 15-40 cm long, 10-28 cm wide) and have long thin hairs on the underside of the leaves which is a key identifying feature. Bohemian Knotweed stems are reddish-brown, they have leaves of varied sizes (up to 25cm long, up to 20 cm wide) and have short broad hairs on the underside of the leaves which is a key identifying feature.
What it does in the ecosystem
Japanese Knotweed is one of the Global Invasive Species Database’s 100 worst invaders. It grows in a wide variety of habitats and tolerates a wide range of adverse conditions such as deep shade, high temperatures, salinity, and drought. It grows in dense thickets that shade out neighbouring species and spreads readily via underground rhizomes. Once established, it generally takes great persistence and several years to eradicate. Japanese Knotweed is widespread on PEI.