Japanese knotweed – Reynoutria japonica

Japanese knotweed is native of eastern Asia: Japan, the island of Sakhalin, Korea, China, Taiwan and Vietnam. In its native range, it grows on riversides, peatland forests, roadsides, forest edges and oak forests. 

In the Netherlands, Japanese knotweed was introduced in 1823 as an ornamental. It soon became a common garden plant in many European countries. In Germany, Japanese knotweed was also used as a fodder plant and for controlling erosion. In the mid-1800s, the species spread into the wild and in the late 1800s, it became established in many European countries, including England, France, Germany, the Netherlands, Poland, Scotland and Wales.

Today, it is widely established in Europe, North America, Asia, Australia and New Zealand. In recent decades, Japanese knotweed has spread rapidly in Europe. Typically, it spreads as fragments of rhizome in soil which is used as backfill in parks, yards and road construction. It also spreads along riversides with garden waste.

So far, Japanese knotweed and giant knotweed are mainly present in southern Finland, and only locally here and there. The occurences are concentrated in urban and other populated areas. Most commonly, they grow along roadsides and as a “weed” in newly planted sites, which indicate to the contamination of the soil used. It is not known that knotweed has propagated from seeds in Finland. Instead, they are accidentally transfered to new sites as fragments of rhizome in garden waste and transported soil. Both species are sometimes found fugitives as large stands, originating from garden waste.