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Water fern
Invasive Species Information

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What Is Water fern - (Azolla filiculoides)?

Habitat: Still and slow flowing water bodies (e.g. ponds, drainage channels, ditches, canals).
Distribution in Ireland: Sparse distribution but locally abundant in some places.

Status: Established
Family name: Salviniaceae

Reproduction: Spreads mainly vegetatively though can produce minute spores.

Native to North and Central America and introduced for ornamental use in ponds and aquaria in Ireland. First recorded in 1883 and has spread in Ireland during the last 50 years. 

Water Fern - Azolla filiculoides leaves

Water Fern Leaves

Very small free-floating water plant that forms dense mats and complete coverage of the water in only a few months. Unmistakeable when in its red form and relatively easy to distinguish from duckweeds in its green form. Can be seen most months of the year. 

How To Identify Water fern?

Leaf: Green / Red

A sure method of distinguishing this species is to examine the trichomes on the upper surfaces of the leaves.


Trichomes are small protruberances that create water resistance. They are unicellular in Water ferns

Water fern - Azolla filiculoides leaves
Water fern - Azolla filiculoides infestation
Water fern - Azolla filiculoides ID Guide

Water Fern - Azolla filiculoides ID Guide

Water Fern Leaves

Water Fern Stand

Why Is Water fern A Problem?

Out competes native species by forming a dense covering on the surface of the water, blocking out light, causing deoxygenation, preventing air-breathing insects from reaching the surface and reducing water temperatures.


Dense and continuous stands can be a health hazard as the water surface appears solid. 

Water fern - Azolla filiculoides infestation

Water Fern Stand (Red)

European Communities (Birds and Natural Habitats) Regulations 2011 non-native invasive plant species A-Z (Updated 2017)

There are currently 35 invasive plant species listed in the European Communities (Birds and Natural Habitats) Regulations (annex 2, Part 1)...


Click on a species from the following list to find out more regarding non-native species subject to restrictions under Regulations 49 and 50.

  1. American Skunk-CabbageLysichiton americanus

  2. Brazilian Giant-RhubarbGunnera manicata

  3. Broad-Leaved RushJuncus planifolius

  4. Cape PondweedAponogeton distachyos

  5. Cord-GrassesSpartina (all species and hybrids)

  6. Curly Waterweed - Lagarosiphon major

  7. Dwarf Eel-GrassZostera japonica

  8. FanwortCabomba caroliniana

  9. Floating PennywortHydrocotyle ranunculoides

  10. Fringed Water-LilyNymphoides peltata

  11. Giant HogweedHeracleum mantegazzianum

  12. Giant KnotweedFallopia sachalinensis

  13. Giant-RhubarbGunnera tinctoria

  14. Giant SalviniaSalvinia molesta

  15. Himalayan BalsamImpatiens glandulifera

  16. Himalayan KnotweedPersicaria wallichii

  17. Hottentot-FigCarpobrotus edulis

  18. Japanese KnotweedFallopia japonica

  19. Large-Flowered WaterweedEgeria densa

  20. Mile-a-Minute WeedPersicaria perfoliata

  21. New Zealand PigmyweedCrassula helmsii

  22. Parrots FeatherMyriophyllum aquaticum

  23. Red AlgaGrateloupia doryphora

  24. RhododendronRhododendron ponticum

  25. SalmonberryRubus spectabilis

  26. Sea-Buckthorn Hippophae rhamnoides

  27. Spanish Bluebell Hyacinthoides hispanica

  28. Three-Cornered LeekAllium triquetrum

  29. WakameUndaria pinnatifida

  30. Water ChestnutTrapa natans

  31. Water FernAzolla filiculoides

  32. Water LettucePistia stratiotes

  33. Water-PrimroseLudwigia (all species)

  34. WaterweedsElodea (all species)

  35. WireweedSargassum muticum

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