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What is Azolla?

The fairy fern or floating water fern (Azolla filiculoides) is an attractive aquatic plant with delicate fern-like foliage that originates in the Americas. It was first recorded in the UK in around 1840 and has been continually arriving ever since as a popular garden aquatic.

Why is it a problem?

Azolla has a remarkable ability to multiply; fronds grow rapidly and elongate until fragments finally break off to form new plants. The mats that form on the water's surface can be 30cm thick, and during hot weather, can double in size in just 4 or 5 days. These mats:

  • - block out light, killing our rich and diverse aquatic flora
  • - reduce the oxygen availability which can lead to the death of fish and invertebrates
  • - can impede flood defenses and water-based recreation, and block irrigation pumps
  • - can cause livestock deaths when stock mistake water for land


clearing boat 5 

Azolla has escaped from gardens into the wider environment, becoming a problem on ponds, lakes, rivers and canals throughout the UK, including areas of conservation importance such as the New Forest.

Until very recently, despite warnings about Azolla's 'weedy' tendencies, and a Royal Horticultural Society ban at its flower shows, it was imported and sold by garden and aquatic centres around the country. However, in April 2014, legislation was introduced that bans the sale of this and four other invasive aquatic plants in the UK.

There are no organisms native to the UK that feed on Azolla and the imported plant has not brought all the native American natural enemies with it that keep it in check.

Why are current control methods unsuitable?

Fragmentation of the fronds makes control by mechanical means virtually impossible. This is compounded by the annual production of millions of tiny spores, which are released in autumn and grow into new plants the following year.

There is now only one herbicide that is licensed for use on water in the UK and there is public pressure to find more environment-friendly alternatives.