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Floating Pennywort

Floating Pennywort - River Colne - Credit Martin Lydon

Floating pennywort can completely smother a river. Image credit: Martin Lydon.

Background information

Floating pennywort (Hydrocotyle ranunculoides) is a highly damaging non-native invasive aquatic plant that originated from North America. It is not to be confused with a similar native species - marsh pennywort.

Floating pennywort was first naturalised in 1990 as a result of discarded plants from garden ponds and has now spread to hundreds of water bodies throughout England and Wales, although it was banned from sale in 2014. Pennywort roots in the shallow margins of lakes and lowland rivers, encouraged by sluggish flow, and it can grow up to 20cm per day, spreading rapidly to quickly dominate a waterbody by forming thick mats and impeding water flow and amenity use. It is also able to regrow from small fragments, making transmission to new water bodies very easy and the plant itself very difficult to eradicate completely.

Marsh Pennywort - Credit Trevor Renals (EA)

There is a native species of pennywort - Marsh Pennywort (left) - which doesn't show the vigorous growth associated with Floating Pennywort. Image credit: Trevor Renals (EA).

Impacts of Floating Pennywort

Given its rapid rate of growth, floating pennywort can quickly grow out from each bank of a small water course, creating dense mats of vegetation. These mats can meet in the middle of the water course and block off open water entirely. Floating pennywort can severely limit access for angling, while rafts of pennywort floating downstream can make keeping a bait in the water very difficult, but there are also a number of other damaging impacts, including:

  • Out-competing native species by blocking out light;
  • Reduced flow and consequently increased silt deposition - this has a positive feedback effect by creating more favourable conditions for pennywort growth;
  • Reduced levels of dissolved oxygen;
  • Choked drainage systems and therefore increases risk of flooding;
  • Can make navigation difficult on canals and navigable rivers.

Control of Floating Pennywort

  1. When it comes to the control of floating pennywort, there are broadly four main options:Mechanical control - removal of the large mats of vegetation using long-arm machinery from the bank, or specialist aquatic vegetation removal machinery.
  2. Manual control - despite the initially daunting task, man-power can be very effective at removing pennywort. In shallow rivers/lakes, mats of pennywort can be removed by getting into the river in waders and using rakes to drag the vegetation to the bank. Manual removal of small remaining fragments from a small boat is also very effective following mechanical control.
  3. Chemical control - emergent and floating vegetation can be sprayed using glyphosate (the active ingredient in Round-Up). Chemical control can be highly effective under the right circumstances but the dead and dying plant matter should be manually removed.
  4. Biological control - in recent years, there have been trials carried out to test the effectiveness of using a particular species of weevil as a biocontrol. The weevil doesn't actually eat the pennywort directly, but it lays its eggs in the stems of the plant and when the larvae hatch it causes the pennywort to break down and die. The trials have been very promising but there are many risks associated with utilising a bio-control and therefore Defra must sign the use of the weevil off before it can be deployed. This could be several years away.

You can find out more about the various techniques for controlling pennywort here.

Problems with the Government's current approach to pennywort

Although the Environment Agency spends over £500,000 annually controlling pennywort, the Angling Trust believes that there are a number of issues with the current approach:

  • The EA doesn't respond quickly enough to pennywort invading new water bodies;
  • The EA doesn't control pennywort for long enough to ensure total eradication;
  • The EA's approach varies across the country.

Consequently, floating pennywort is still spreading exponentially.

What is the Angling Trust doing?

Given its ever-increasing impact on angling, the Angling Trust will be applying pressure to the Environment Agency to dedicate specific budget for tackling pennywort and also to more effectively coordinate its approach across different areas of the country.

There have already been indications that the EA wish to:

  • Agree a common position on pennywort across the business
  • Work better with partners and other waterbody managers
  • Be clear on our management objectives and react quickly to achieve them.

Floating pennywort is a species notified on the EU Invasive Alien Species Regulation (1143/2014) list of invasive species, which came into effect in August 2017. The government is legally obliged to bring in an effective management plan to contain and reduce the impact of floating pennywort by the end of 2018. The Angling Trust would urge the government to bring in this management plan as soon as possible with the Environment Agency as a regulator, to ensure that there is coordinated and effective action across government departments.

Biosecurity - Check, Clean, Dry!

Anglers have the potential to easily spread floating pennywort (and other invasive species) from one waterbody to another on nets, unhooking mats and waders. This is why the Angling Trust promotes the Check, Clean, Dry initiative to encourage anglers to:

  • Check your clothing and equipment for living plants and animals. Pay particular attention to areas that are damp or difficult to inspect such as the tread of boots or rims of nets. If you do come across any plant or animals, leave them at the water body where they were found.
  • Clean and wash all clothing, footware and equipment thoroughly.
  • Dry all equipment and clothing after each trip. Where possible, leave to dry in direct sunlight. Some species can live for many days in moist conditions.

Find out more HERE. Download the Check, Clean, Dry poster to display at your fishery HERE.

Learning lessons from the River Kennet

In February 2018, work commenced to remove pennywort from the Foundry Brook in Reading, a tributary of the River Kennet. It is thought that the Foundry Brook is the main source of pennywort entering the River Kennet, and consequently the River Thames.

The £140,000 of funding for this work came from an Enforcement Undertaking of Thames Water, and includes provision to fund a Masters research project at Reading University looking into how best to eradicate pennywort.

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