This introduction from North America is reputed to have come from people emptying aquarium contents into rivers and streams. It is spreading northwards and in this area of Cheshire, which is about mid way in the island of mainland Britain, it is now well established on the River Weaver when there was none to be seen only ten years ago.

The BSBI distribution maps show it was unrecorded in the British Isles before 1986 and then established itself mostly around London from 1987 to 2000. It has not yet reached Scotland or Ireland and there are only a few outposts in Wales so it is not an introduced pest in the same league as Fallopia japonica (Japanese Knotweed) or Crassula helmsii (New Zealand Pygmy-weed) but could well follow those plants' rapid encroachment.

River Weaver and nearby small ponds, Hartford, Cheshire 10th September 2009

Added on October 1st 2009, updated 7th April 2010