15th September 2020

One of many projects Colne Valley Regional Park is leading on is to educate people about species that are harmful for our ecosystem and some can be harmful for people if they get in contact with them. These species are called non-native invasive species, and one of them is the Floating Pennywort, which we aim to educate you about in this article.

Despite being green and almost resembling a shamrock, there is nothing lucky about finding these floating pennies in your local river or water bodies. Though the leaves of Floating Pennywort are quite small in size – reaching only up to 7cm in diameter – they are a river-lover’s nightmare, forming matts of vegetation so dense that it suffocates flowing rivers to an almost stand-still and completely blocking off navigation routes. And in typical non-native invasive species fashion, they don’t seem to want to stop until all water bodies are swamped in greenery. But where did they come from? How do they do it? And is there anything we can do to stop them?

Lottery Heritage Fund

Just one Floating Pennywort can turn a river green.


Floating Pennywort is a native species of North and South America, brought to Britain in the 1980s to decorate tropical aquaria and garden ponds – of all the exotic aquatic plants available, we chose the bland, green invasive one. Unfortunately, but not surpisingly, they escaped into the wild and naturalised in the 1990s, spreading far and wide from South-East England to North-West England and Wales, congesting rivers and any natural water source as far as they can reach.


As well as impacting our lives, it comes with no surprise that our aquatic wildlife is heavily affected by their presence. Growing up to 20cm a day(in prime conditions), the dense rafts not only take up a large amount of space but they shade out the most vital energy source – the Sun – leading to:

  • Reduction in the chance of photosynthesis by plants in and around water
  • Reduction of oxygen in water, negatively affecting fish, plants and invertebrates such as dragonflies that need a good quality source of water to breath as well as survive.

It gets worse for the fish as the excessive growth blocks the waterway and smothers the water in silt, leaving the fish in river gridlock and the latter destroying their spawning habitats – truly a fish’ and fisherman’s nightmare!

It doesn’t stop there. Once winter arrives, the pennyworts die and decompose en masse, adding excessive nutrients, or eutrophication, into the river, snuffing out oxygen to create an inhospitable swamp – except for the floating pennyworts (and algae) that is.

The dense matt can often be mistaken for ground – take care with children and pets!


Is there anything we can do to stop these Pennyworts before its too late? With your help, it’s possible. Whether you’re an avid river rafter or just enjoy the scenic view, you can keep an eye out for them and report any sightings in Colne Valley via our online recording form on: cvfc.org.uk/nnis. Fortunately, their dense matts are hard to miss.

If you happen to come across a riverbank with a bad case of Floating Pennywort, it is crucial that you carry out the Biosecurity measures as human footfalls and boats can carry them upstream and into other water courses, spreading the invasive species even further:

  1. Check your clothes and equipment
  2. Clean them thoroughly
  3. Dry to stop the spread

Our beloved Mute Swan on patrol for Floating Pennyworts.


If you would like to help release our rivers from their green shackles and flow freely again, then why not volunteer with our River Rangers to join us on our Floating Pennywort removal days? Though it may look like a fearsome foe when bundled together, they’re not anchored to anything so its surprising how many you can pull out. Or take part in scouting them out by monitoring your local stretch of river. You’ll learn more about our plants, get a good workout session with like-minded people and have a feeling of satisfaction knowing you’re doing your part to protect our rivers from becoming green corridors. To find out more, contact us or check out our website for to see upcoming volunteer events.

Our Floating Pennywort removal events are a fun day out for kids and adults alike!

Team of River Rangers after a successful Floating Pennywort removal