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Introduced to the UK in the 1980s by the aquatic nursery trade, this fleshy-stemmed plant grows into large floating mats of dense foliage and smothers our waterways.
Scientific name: Hydrocotyle ranunculoides
Family: Araliaceae
Native to: North America
Type: Perennial broadleaf
Predators/herbivores: None
Location: Typically seen on the surface of the water, more prominent in the south-east of England, and north-west of England and Wales
First recorded in the UK in 1990, floating pennywort (also known as just 'pennywort') was originally introduced as an ornamental plant for garden ponds. It has the ability to grow from even the smallest fragments, which enabled it to enter canals and rivers after being transported or discarded.
Floating pennywort is so damaging to native flora and fauna that it is listed under Schedule 9 to the Wildlife and Countryside Act 1981, meaning it is illegal to cause it to grow in the wild. Since 2014, it has also been banned from sale in the UK.
Floating pennywort
In the warmer months, floating pennywort grows rapidly - as much as 20cm a day. Its dominating presence steals sunlight, smothers habitats and crowds out native plants. It can cause an increase in water temperatures, and create mosquito breeding areas.
When pennywort dies back in the winter, excessive nutrients can build up in the water and damage ecosystems.
Floating pennywort's excessive coverage across the surface of water can impede flow, causing oxygen depletion for fish and other wildlife, which in turn impacts food webs.
Boaters and other craft users can face major problems from its long, fleshy stems tangling in propellers and oars. Similarly, those who are fishing in pennywort-ridden areas can have issues with their rods and equipment.
It is distinguishable by its distinctive bright green, kidney-shaped leaf, which has crinkled edges. The leaves are also usually quite shiny, and grow to around 7cm wide. Pennywort is often seen in thick, dense mats across long stretches of water. This is in contrast to the native marsh pennywort, which has smaller, rounder shaped leaves of about 1.5cm in size, and is always rooted in the ground.
Between July and August, tiny white flowers can also appear, although this is uncommon.
To ensure every fragment of pennywort is removed from the water, the process can be incredibly difficult and expensive.
We use specialist machinery to rid canals of large quantities, and our volunteers remove smaller pieces from the water by hand.
As part of our Invasive Species Eradication Project 2021-2025, we were thrilled by the success of using weevils to help rid the plant from large stretches of canal.
After years of research alongside our partners at CABI, these south American beetles were released into controlled areas in a first-of-its-kind trial, and carefully monitored as they effectively munched their way through large swathes of the foliage.
Canoeists removing Floating Pennywort
If you see an invasive, non-native plant species on one of our canals or rivers, please contact us online or call us on 0303 040 4040 and we'll check if it is already included in our treatment schedule.
Various invasive plant and animal species can be transported on equipment such as paddleboards and oars, and wet clothing including footwear.
Whether you use our canals and rivers for work or pleasure, please remember to Check, Clean, Dry.
These three things should always be done before you leave a waterway, even if you are moving to another spot on the same stretch just a few miles away.
Last Edited: 24 April 2025
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