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.
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.
Japanese knotweed has rhizomes that can extend more than 2 m deep and 7 m horizontally! These underground stems release toxins that interfere with other plants.
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 up to 10 years. Lastly, its lack of natural enemies also facilitates the spread of this highly invasive species.
Japanese knotweed is found near water sources, in wetlands, ditches, irrigation canals, roadsides and disturbed areas. It is also frequently found in urban areas, where its rhizomes can sometimes even be see pushing up through asphalt!
It forms dense colonies that crowd out native species and reduce ecosystem biodiversity. It also interferes with access to bodies of water and the dead stems that float on the surface in spring increase the risk of flooding.
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.
Don’t try to rip it out, because its rhizomes are very deep and you risk leaving some fragments in the soil. Do not compost any of the clippings and never toss them away!
Carefully collect all the clippings, place them in garbage bags and put them out with the trash. Studies are currently underway in England to come up with biological control methods.