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Water fern spreads like a sponge across the water and can double in area within a few days.
Scientific name: Azolla filiculoides
Family: Salviniaceae
Native to: North and Central America
Type: Perennial
Predators/herbivores: North American weevils
Location: A free-floating aquatic plant most commonly found in still or slow-moving canals, as well as open water spaces like ponds and lakes. You may also find it in swamps and wet woodland areas.
First recorded in the UK in 1886, water fern (also known as 'fairy fern' or 'azolla') was originally used for ornamental purposes in ponds and botanical gardens.
It took a further 20 years to establish itself in the wild. It's unclear how it came to do so, but was most likely accidentally transported on machinery and boats, or on the clothes of staff working in gardens.
Due to its negative impact on native plant and animal species, water fern is listed under Schedule 9 of the Wildlife and Countryside Act 1981, meaning it is illegal to cause it to grow in the wild.
Able to withstand British winters and invade a region rapidly, this innocent-sounding plant has infiltrated many waterways in its time.
Its ability to quickly dominate large areas reduces light beneath the surface and shades out other native plants, as well as causing de-oxygenation, leading to the death of fish.
Air-breathing insects and amphibians including frogs, newts and toads, are unable to reach the surface when trapped below azolla. Meanwhile, diving ducks like pochards and goldeneyes struggle to dip below the water in search of food.
The dense, carpet-like appearance of water fern also poses a safety hazard for humans by appearing solid.
Water fern has distinctive 'rosettes' of leaves, which can grown up to 2.5cm in length. They have a rough, granular appearance, and repel water - meaning that they always look dry.
Hanging just below its leaves are clusters of black-brown roots, which are brittle and break easily.
Usually present all year round, water fern can range from green to red in colour, and is sometimes a blend of the two. It tends to get its red hue in colder weathers, or if established in shady spaces.
The native common duckweed (lemna minor) often grows amongst water fern in a battle for space. Common duck weed is a lighter green in colour, and its round leaves grow singularly from individual white roots, rather than in clusters. Its leaves are also much smaller - usually measuring between 1.5 and 4mm in diameter.
Water fern in its green form
Typically, water fern can be removed by dredging, or by using herbicides.
However, in pioneering science, an eco-friendly biocontrol method has now been discovered.
North American weevils are small beetles which feed off azolla. Their attack is so powerful that they have been known to completely eradicate large sections in just a few weeks, preventing the need for chemical treatment or other control measures. But, whilst they can cause considerable damage to sizeable quantities of water fern, they won't harm other plants.
After many years of research alongside CABI, we successfully used weevils as part of our award-winning Canal & River Invasive Species Eradication Project to tackle areas of canal affected by water fern, as well as floating pennywort - another destructive invasive plant species harming our canals.
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, including water fern, 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: 07 May 2025
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