Reynoutria japonica (weed)

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Reynoutria japonica (click on image to enlarge it)
Source: Wikimedia Commons

Reynoutria japonica (weed) Houtt. - (Japanese knotweed)

The plant is native to eastern Asia where it is sometimes collected as a wild edible vegetable. It has been introduced into Europe (first record in 1849) and North America (first record in 1877) as an ornamental and has become a serious invasive weed. It forms dense monocultures, especially in flood plains and parks. Planting the weed is no longer permitted in some countries.

Apart from human activity, the natural spread involves the dispersal of small rhizome fragments and stems by rivers during floods. Once it has established its extensive rhizome system, it is nearly impossible to manage and requires intensive efforts over several seasons to eliminate it. It grows in many habitats and the rhizomes can survive extreme temperatures. Repeated herbicide applications is the main approach to control the weed. Digging up the rhizomes with hand tools and heavy machinery is also possible, but usually needs to be repeated over several years and rhizome need to be disposed off carefully, not discarded.

Vernacular names
• Deutsch: Japanischer Staudenknöterich
Japanischer Windenknöterich
• English: Japanese knotweed
• Français: renouée du Japon

Plants grow to a height of up to 3 m and the rhizomes can extend over a horizontal distance of up to 7 m. The rhizomes become thick and woody. The stems are hollow and the leaves are oval, 5-10 cm long. The flowers are small and whitish and plants have either hermaphrodite flowers (male fertile) or female flowers (male sterile). In its introduced range, male fertile plants are rare and viable seeds are usually not produced.

Synonyms:
Fallopia japonica
Polygonum cuspidatum

For details see Gillies et al. (2016) and the respective page in Wikipedia.