Floating Pennywort: Part 2 – Spread and control in Cambridgeshire


Cam Valley Forum Volunteer clearing Floating Pennywort, photo courtesy of Cam Valley Forum


Floating Pennywort can be found on the Cam, Gt .Ouse, Lark, Bourne Brook and the Old West. Fragments of plant detach from existing rafts of the weed and float downstream to catch on overhanging trees and vegetation and where they will start to growth. Boats definitely play a role in spreading plant fragments but the case against water birds is less certain. Interestingly it is not as widespread on the Cam as it flows though the centre of Cambridge. This is thought to be because the plant fragments appear to prefer “sticking to” natural vegetation or obstructions – the banks of the Cam in the city centre are hard piled and smooth and don’t seem to be very attractive to the plant.  This may have serious implications about how banks and overhanging trees are to be managed – some tough choices ahead perhaps?

Control measures


The riparian  owner is ultimately responsible for the control of the plant if it is not on a main river. Environment Agency permissive powers allow the Agency to gain access for removal to other watercourses if it poses a risk to flooding or navigation on main rivers.

Control methods in Cambridgeshire so far…

  1. Rakes : Cam Valley Forum Volunteers have been holding regular working parties to clear the weed from the Cam. They have found the type of rake used to be important – the tines needing to be curved and close together. Booms are put in place to prevent small fragments floating downstream. The main problem with this approach has been accessing the plant where there are overgrown and steep banks. Disposal has also proved tricky due to its bulk and weight. It can be composted under a tarpaulin but can live up to a month like this before it dies. There have been varying reports about whether cattle will eat it or not, some do, some don’t!
  2. Mechanical weed cutting: used by the Environment Agency and Cam Conservators. The latter have been clearing Floating Pennywort for about 8 years when it first became a menace. Again, the use of a boom to prevent fragment dispersal is important.



  3. Herbicide: varying levels of success have been experienced using the herbicide glyphosate as the chemical appears to have little/no effect on submerged plant parts. Best results have been achieved by spraying when the plant is young in early spring and adding Topfilm  –  a naturally based aquatic adjuvant approved for the use in the control of aquatic weeds. This improves the way the herbicide “sticks” to the leaves. It has been found that the plant needs to be sprayed every 3 weeks when it is growing strongly and when temperatures are above 6⁰C with control taking up to 8 weeks. Problems arise when the weed has got into the natural bank vegetation – spraying to kill the weed will also kill the bank vegetation, winter spraying when the native vegetation is dormant but the Pennywort is still growing is an option.  Cam Conservators have found an integrated regime of mechanical removal, followed by hand picking and then herbicide use has given  the best control so far.

The future – harnessing the power of weevil…

CABI  (Centre for Agriculture and Biosciences International) have identified a South American weevil (Listronotus elongatus) which may have potential for biological control of the weed. The weevil larvae mine into the stems and the adults eat the leaves of the Pennywort. At present, indications are that it is specific to Floating Pennywort and can survive colder temperatures. Extensive tests have been carried out to ascertain its effectiveness and safety which have been submitted to the UK regulators who are now considering its suitability for release.


Listronotus weevil , photo courtesy of CABI