Japanese Knotweed

Japanese knotweed is a woody-stemmed herbaceous perennial rhizomatous plant (BMP).

Stems: Japanese knotweed stems are hollow, smooth, purple to green coloured, and up to 2.5 cm in diameter. The hollow-jointed stems have reddish-brown solid nodes surrounded by a papery sheath. The stems die back each fall and the dead stalks remain standing over the winter. Numerous new stems emerge in the spring (usually late March to early April in southern Ontario) from the over-wintering root system. Juvenile stems resemble asparagus (Asparagus officinalis) spears and are purplish in colour, fading to green as they mature. Japanese knotweed can grow rapidly; stems can grow up to 8 cm per day. The plant can grow 1 m in height in three weeks, with the mature plant reaching full height by the end of July. This plant grows in large bamboo-like clumps, reaching heights of 1-3 m (3-10 ft).

Leaves: Alternate leaves are oval to triangular with a pointed tip and flat base with a long stalk (petiole) arising from the stem. They are 10-17 cm long and 7-10 cm wide and alternate along the stem in a distinctive zigzag pattern.

Flowers: Small, white-green flowers bloom in sprays near the end of the stem and in the leaf axils in late July or August. The flowers are produced in branching panicles (clusters).  The seeds are winged, triangular, shiny, and very small. The fruit is winged, which assists them in wind or water dispersal. Japanese knotweed is a dioecious plant, meaning there are male and female plants which require pollination to produce viable seeds (BMP).

Rooting system: Japanese knotweed has a horizontal root system that can grow very quickly and spread up to 10 m away from the parent plant. 

In Ontario, Japanese knotweed distribution is not well-documented. The majority of sites have been reported in the last 10-20 years. There have been populations confirmed as far north as Thunder Bay and Sault Ste Marie. There are established populations across southern, central, and eastern Ontario (in over 20 different municipalities). Canada-wide, the primary distribution is in Ontario, Quebec, and the Atlantic Provinces with isolated populations in Winnipeg, Manitoba and southern British Columbia.

Japanese knotweed is somewhat intolerant of persistent freezing conditions, and as a result, its spread may be confined to more southern parts of Canada. However, as the climate warms, it may be able to spread further north. Ontario should be prepared for Japanese knotweed to spread within the province as the average temperatures rise. Road and railroad rights of way and waterways are the main geographic pathways for spread within Ontario. Road maintenance, forestry operations, and construction activities may spread these plants further (Ontario, 2018).

Distribution Map provided by EDDMapS

Impacts on biodiversity

Japanese knotweed can severely degrade the quality of wetland and riparian habitats where it becomes established. Dense thickets can reduce sunlight penetration by more than 90% and its thick mats of dead and decaying vegetation in fall/spring prevent other plant species from growing by shading them out.  

As a result of the reduced native plant biodiversity and lowered invertebrate densities, established knotweed stands do not support the same levels of native amphibian, reptile, bird and mammal populations. For example, it has been shown that native green frog (Rana clamitans) presence is dramatically reduced in knotweed stands in riparian/wetland areas. Although further study is needed, it is believed that Japanese knotweed may have allelopathic properties. The roots contain unique compounds, which may alter soil chemistry or prohibit the growth of nearby native species (Japanese Knotweed BMP).

Impacts on infrastructure

This plant can significantly damage infrastructure. It is able to grow through concrete/asphalt up to 8 cm thick and building foundations. It is of particular concern in new housing developments. In the United Kingdom, developers must dispose of soil containing knotweed fragments at hazardous waste facilities. Japanese knotweed root systems, while strong, are not as dense as those of native plants, and do not hold soil as well. When Japanese knotweed establishes along stream banks, the bank can become unstable and more vulnerable to erosion and flooding. Reductions in available soil (because of erosion) and space (because of the larger root/rhizome biomass) affect the ability of the stream bank to hold water during heavy rains (Japanese Knotweed BMP).

Impacts on recreation

Japanese knotweed can block or interfere with access to water for activities such as canoeing, boating, angling, and swimming (Japanese Knotweed BMP).