Findings from the invasive, non-native species survey of the River Irwell released

Invasive, non-native species (INNS) of plant are a major threat to the health of our rivers and are also damaging to the wider environment and economy.

Between June and October 2022, we conducted the largest and most comprehensive survey of INNS plants of the Irwell catchment.

Greater Manchester Combined Authority (GMCA) commissioned the River Stewardship Company to survey the main rivers of the Irwell Catchment, whilst some of the smaller tributaries were surveyed by a group of volunteers who were trained by GMCA and GroundWork GM.  The survey was managed in liaison with the Irwell Catchment Partnership, and this collaboration led to the creation of a bespoke recording app to capture the data.

The survey focussed on the widest spread and most harmful plant species: Giant Hogweed, Japanese Knotweed and Himalayan Balsam.

The results revealed:

  • There are over 2000 instances of recorded Japanese Knotweed and over 700 instances of Giant Hogweed across the whole Irwell Catchment.
  • Japanese Knotweed is present on over 109 km of riverbank and a further 45 km contains Giant Hogweed.
  • Within the Irwell catchment there is approximately 436 km of Statutory Main Rivers length, comprising a left and right bank. This means that out of a possible linear riverbank of 872 km approximately 1/8 or 12% of riverbank has Japanese Knotweed present.
  • This also means that in isolation that Giant Hogweed also covers 5% of all the river corridor in the Irwell catchment.

Mike Beard, Natural Course Project Officer and Greater Manchester Environment Team said…

“This is a particularly impactful survey because the catchment wide results can be used to attract more resources to tackle the infestations and to improve the targeting of their removal.”

You can read the full report here (opens in new tab)