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Japanese Knotweed

(Reynoutria japonica)

An invasion champion
This plant, native to Asia, was introduced to the province of Quebec in the early twentieth century.  It is found near waterways, wetlands, ditches and roads.  Japanese Knotweed has a hollow stalk (like bamboo) and can reach heights of 1 to 3 meters.  In July and August, it produces many small white flowers, arranged in clusters at the base of the leaves.  The rhizomes, for their part, can burrow up to 2 meters into the ground and up to 7 meters around the plant.

When Japanese Knotweed invades an area, it forms large, dense colonies.  It prevents other plants from growing nearby by secreting a phytotoxin into the soil.


















Image source: Sentinelle Network of the MELCC


Control method

According to the World Conservation Union, the Japanese Knotweed is one of the 100 most invasive species on the planet.  It is very difficult, if not impossible, to eradicate.  It is, however, possible, to control its spread.

For small colonies (only a few plants)

  • Small specimens (less than 2 meters in height): Dig one meter deep, inside a two meter radius of each plant, to remove all of the roots.
  • Large specimens (more than 2 meters in height): Dig one meter deep inside a five meter radius of each plant, to remove all of the roots.
  • Fill the hole with new, clean soil and seed or plant indigenous species immediately.

For large colonies (several hundred plants)

  • Stalks that measure more than one meter in height:  Cut the plant down twice a month during growing season (May to October), to weaken it.
  • In extreme cases, you can remove the plants mechanically with the help of machinery;
  • Plant or seed the disturbed soil with indigenous species immediately.

It is very important to dispose of Japanese Knotweed debris in airtight plastic bags and send it to a landfill site only, as only one small piece of the plant will generate a new shoot.

For more information, please check out the following websites:

  1. A Space for Life’s Green Pages : Diseases, pests and undesirable plants
  2. Nature-Action Québec’s leaflet on the Japanese Knotweed