Conservation Evidence strives to be as useful to conservationists as possible. Please take our survey to help the team improve our resource.

Providing evidence to improve practice

Action: Floating pennywort: Physical removal Control of Freshwater Invasive Species

Key messages

Read our guidance on Key messages before continuing

  • A study in Western Australia found that following a two-week program of physical removal of floating pennywort, the rate of growth exceeded the rate of removal.
  • A study in the UK, found that removal using a mechanical digger and monthly picking by hand greatly reduced the cover of floating pennywort but did not completely eradicate it.

Supporting evidence from individual studies


A before-and-after study from 1991 to 1992 in a river in Western Australia (Ruiz-Avila & Klemm 1996) found that a two-week program of physical removal did not reduce floating pennywort Hydrocotyle ranunculoides biomass.  Following the removal program in late 1991, by September 1992 the estimated biomass of floating pennywort in the river had increased from an initial 175 tonnes to 420 tonnes.  Control attempts in 1991 involved a two-week programme of physical removal by cutting the floating mats of floating pennywort with sickles from small boats.  The mats were then pushed by small boats to an aquatic harvester, floated to the bank and removed by a backhoe.   Follow up maintenance control was continued until January 1992, when growth rates exceeded the rate of removal.


A study in 2005-2006 by the Broads Authority at Gillingham Marshes, Suffolk, UK (Kelly 2006) found that removal of floating pennywort Hydrocotyle ranunculoides using a mechanical digger and extensive hand picking, along with monitoring, greatly reduced the cover of floating pennywort but did not completely eradicate it.  Hand-picking was undertaken at least once a month (usually every fortnight) throughout the growing season (March – September 2005-2006 ongoing). In addition, a mesh grid was added to the upstream end of the water pump at Gillingham Marshes to try to prevent floating fragments from entering and infesting the River Waveney, adjacent to the marshes.  To dispose of the pennywort, it was piled on the site and monitored for regrowth. Monthly monitoring of the pile was undertaken and if signs of growth were observed, they were sprayed with the herbicide glyphosate.

Referenced papers

Please cite as:

Aldridge, D., Ockendon, N., Rocha, R., Smith, R.K. & Sutherland, W.J. (2019) Some aspects of control of freshwater invasive species. Pages 569-602 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.