The Continuing Spread of Japanese Knotweed

Japanese knotweed was initially indigenous to Japan, China and Korea – it has now spread worldwide, and it is listed by the World Conservation Union as one of the world’s worst invasive species. Read on below for how this weed left Japan and spread worldwide!

Europe


Japanese knotweed was bought to Europe by the physician and botanist, Phillipp Franz Balthasar von Siebold. von Siebold travelled across Japan for 8 years before returning to Europe, bringing hundreds of samples with him, including Japanese knotweed. A specimen was donated to the Botanic Gardens in Kew in 1850 and from there, the plant has spread. Japanese knotweed is now found UK wide, with the weed spreading thanks to the rail and water networks. Japanese knotweed is also found in Ireland, with the first record of the plant found in the country in 1872.

Japanese knotweed is still most prevalent in the UK, due to the plant’s popularity in the late Victorian era in themed gardens.

United States

Japanese knotweed has been introduced (through various means) to 42 of the 50 states. It is currently not located in North Dakota, Wyoming, Nevada, Arizona, New Mexico, Texas, Alabama and Florida. The plant is currently documented by the National Resources Conservation Service to ensure that it does not spread more widely.

Canada

The weed is found in all but two provinces (not Manitoba or Saskatchewan). The infestation rate is lower than in the UK, as Canada will use strong herbicides closer to waterways to help aggressively treat knotweed.

Other countries


There have been cases of knotweed in other countries, including New Zealand, Germany and a number of countries in Scandinavia. Many countries will stop knotweed entering the country at the border to ensure it is not planted and will not grow in an uncontrolled way.

 

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