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Yes, knotweed is an entirely introduced species.
Japanese knotweed is a pernicious weed and should be avoided at all costs.
Japanese knotweed is a herbaceous perennial plant. It can be found in Japan or you can buy it on the internet or at various gardening stores.
Tom Heutte has written:
'Japanese knotweed' -- subject(s): Control, Invasine plants, Japanese knotweed
knotweed