Japanese knotweed (Reynoutria japonica var. japonica, syn. Fallopia japonica) is native to Asia and was introduced to North America as an ornamental plant in the 19th century. This fast-growing perennial reaches a height of 2 to 3 m in summer. Its hollow stems with raised nodes resemble bamboo, hence its other common names of Japanese bamboo or Mexican bamboo. In late summer, it produces panicles of creamy white flowers.
Giant knotweed (Reynoutria sachalinensis) and Bohemian knotweed (Reynoutria x bohemica) closely resemble it and are also highly invasive.
A highly invasive plant
Although these features might make it appealing to gardeners, don’t be fooled! This attractive plant is listed by the World Conservation Union (UICN) as one of the world’s 100 most invasive species. Cultivars with mottled foliage are also invasive. Cultivars with mottled foliage are also invasive. In addition, other very similar species are equally invasive, the giant knotweed (Reynoutria sachalinensis) and the Bohemian knotweed (Reynoutria x bohemica).
Japanese knotweed has rhizomes that can extend more than 2 m deep and 7 m horizontally!
It forms dense colonies that crowd out native species and reduce ecosystem biodiversity.
Reproduction
In North America, this plant spreads mainly vegetatively. This is a highly effective means, however, as a new plant can grow from just a tiny fragment of a stem or rhizome.
Moreover, rhizome fragments can remain dormant in the soil for many years. Lastly, its lack of natural enemies also facilitates the spread of this highly invasive species.
Dissemination
Japanese knotweed stem and rhizome fragments can be dispersed during floods but mainly, human activities contribute to their spread (landscaping, excavation work with displacement of soil).
Where can you find Japanese knotweed?
Japanese knotweed is found near water sources, in wetlands, ditches, irrigation canals, roadsides and disturbed areas. It limits access to waterways, promotes bank erosion and could increase the risk of flooding due to dead stems that float on the water surface in the spring, obstructing the water flow.
It is also frequently found in urban areas, where its rhizomes can sometimes even be see pushing up through asphalt!
How to fight against this plant?
Because Japanese knotweed is so terribly difficult to eradicate, it should never be grown. If you do have any of these plants in your garden, cut the stems off at ground level several times over the growing season, to exhaust it. Do this for a number of years. Note that cutting a colony only once may stimulate its growth.
You can try to eliminate young plants or a small colony by completely removing the root system from the soil. Beware, its rhizomes are very deep and there is a risk of leaving some fragments in the soil. When the colony is too large or established, this method is not appropriate. Close monitoring is also necessary.
Do not compost any of the clippings and never toss them in a natural environment.
Carefully collect all the clippings, place them in heavy-duty and waterproof garbage bags and put them out with the trash. Then, store these bags in the sun for a few weeks before throwing them away during waste collection.
After ripping it out, it may be a good idea to install a dark tarp (such as heavy geotextile, geomembrane, or dark polyethylene sheeting) to prevent light from reaching the ground. The membrane should extend a few meters beyond the perimeter of the colony to prevent new stems from growing on the periphery. In addition, it must be left in place for many years.