Water fern is a free-floating aquatic plant native to North and Central America and is found growing in slow flowing or still waters. Water fern can form dense mats on the surfaces of ponds, lakes, ditches. Water fern was introduced as part of the ornamental and aquaria trade. It has now become established in Ireland.
Classified as a medium impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.
- Leaves are small, green, and fern-like with a red border in summer, and leaves take on a red appearance in autumn and winter.
- Leaves have trichomes on the surface of the leaves that make them resistant to water and are non-wettable.
- The roots are brown.
It spreads mostly vegetatively but can also reproduce by spores.
- Forms dense mats on lakes and reservoirs
- Blocks out light and outcompetes native plant species
- Can impede boating and recreational water activities.
- Increase flooding
- Causes deoxygenation in water
- Mechanical: The best option for control of Water fern. Treatment can be carried out using large rafts and seine nets to scoop up the water fern out of the water. This causes the vegetation to float to the surface where it can be collected with nets. The waterweed can then be composted on dry land. This action can be repeated if required.
- Chemical: Chemical herbicide can be used where suitable but can kill off native plants and is often not viable.
- Biological: Stenopelmus rufinasus, a weevil, has been a successful biological control of water fern in South Africa.
(National Biodiversity Data Centre, Ireland, Water Fern (Azolla filiculoides), image, accessed 28 June 2022)