How to remove Water fern
By Jennifer Holmes

How to remove Water fern

water fern (Azolla)

what is it?

Water fern (or Fairy fern) is most commonly known as Azolla. An invasive weed found in southern England, coastal Wales and is scattered predominantly across coastal areas in the north of the UK. Characteristically it is known for its striking colour change in Autumn as seen below, where it turns from bright green to a deep red. How to control or remove it is covered further on in this article, because it’s important to know what type of species Azolla is in order to effectively eradicate it.


Azolla is predominantly found in ponds, lakes, canals, ditches and slow flowing rivers, as well as in seepages and the backwaters of rivers and streams. In its native habitat, Azolla is originated from the tropical regions but it is thought to have evolved a cold-tolerant strain since its introduction to the UK at the end of the nineteenth century.

The problem

Azolla is listed in Schedule 9 of the Wildlife and Countryside Act, 1981 (as amended), which makes it illegal to distribute or allow the release of water fern into the wild. Azolla spreads rapidly, plant fragments and spores being easily dispersed by waterfowl and amphibians and through agricultural practices. It can cover 100% of the water’s surface creating a dense mat or carpet that can be up to 30cm deep. This carpet stops light penetration to the water underneath which creates an anaerobic environment that many native species cannot survive in. It also prevents aquatic plants from photosynthesizing. The thickness of its mass stops apmphibians and invertebrates from reaching the water surface, and inhibits movement of wildlife traversing the water.

economic impact

Azolla can significantly impact the biodiversity of aquatic ecosystems, but mostly its impact can be seen on the fish farming industry, but by covering the surface of reservoirs and slow-moving waterbodies it can also cause economic loss to water suppliers and users. Drinking water quality can be reduced, which has resulted in farmers losing their livestock due to contaminated waterbodies. So how can we control this invasive species?

Options for control

  1. PREVENTION: Really the best option is simply to not allow it to spread, because once it has taken over a water body removal is difficult and biosecurity measures must be particularly vigilant as Azolla can easily be spread by fragmentation during the removal process, allowing small particles to travel downstream and re-establish further along the water.
  2. CHEMICAL: Herbicide treatments (spraying) can be effective but they can also kill other native species in the ecosystem, and are mostly carried out via specially converted boats. There is now only one group of herbicides licensed for use on UK waters, and the active ingredient is glyphosate. Only companies who hold an Aqua Herb licence are allowed to carry out chemical treatments.
  3. MECHANICAL: Dredging or hand pulling Azolla from the water by boat has been tried however does not guarantee 100% success as any disturbance can lead to fragmentation.
  4. BIOLOGICAL: Using natural predators such as weevils is a safe option, the only downside is the longer timescale it takes to be effective. Stenopelmus rufinasus (below) is one such weevil that is prevalent in the UK (first recorded in 1921), and as it hosts on Azolla, and has been used with some success – so much so that it is known as the ‘water fern weevil’. A study carried out by CABI showed “The absence of biocontrol, the expected average annual costs of A. filiculoides management were estimated to range from £8.4 to 16.9 million.” It concluded; “The unintentional introduction of the weevil S. rufinasus to GB is estimated to have resulted in millions of pounds of savings annually in management costs for A. filiculoides.” So it’s clear that using weevils to destroy an Azolla outbreak saves a considerable amount of money – if you’re prepared to wait for them to do their job!







Azolla has a high environmental impact, and is difficult to control. Biological methods appear to provide the optimum and most cost-effective solution but takes a little longer than other methods. Treatment must be carried out by a professional company, holding the required licenses and by fully trained operatives qualified to carry out treatments near or on water (PA6).

If you’d like our help to remove an Azolla outbreak please get in touch.