It is an offence to plant or otherwise
cause knotweed to grow in the wild.

The Environmental Protection Act 1990, Duty of Care regulations 1991 cut japanese knotweed material and soil containing rhizomes must be disposed of as a controlled waste If removed from their site of origin.

Third Party Litigation

A land owner may be sued for costs and damages if found to have allowed japanese knotweed to spread from your property to that of an adjacent land owner. More information about knotweed and the control of japanese knotweed

More information about japanese knotweed
and the control of japanese knotweed

http://www.environment-agency.gov.uk/business/sectors/31364.aspx

http://www.cornwall.gov.uk/default.aspx?page=13789