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Azolla filiculoides


Common name: Water fern
Synonyms: Azolla caroliniana
Origin: North America, South America
Plant type: Herbaceous
Life cycle: Perennial
Code of conduct: Annex II
Invasive status (ISEIA protocol): Watch list
Main ornemental function: Plants for ponds


This plant is a fern of small size (0.5 to 10 cm long), forming dense mats. Leaves green blue to red coloured (in autumn). No flowers. No possible confusion with another aquatic plant.


Water fern mainly reproduces vegetatively by stem fragmentation. Dispersal potential is high. Sexual reproduction is irregular or even inexistant in Europe. This plant is often sterile under our climatic conditions.


A. filiculoides settles in ponds, ditches, water reservoirs, wetlands, channels and slow flowing rivers, often together with Lemna minuta. It prefers shaded or half shaded habitats. It is able to grow in nitrogen-deficient waters through its symbiotic association with the cyanobacteria Anabaena azollae but is limited by phosophorus availability; it generally prefers (very) eutrophic and alkaline waters with organic bottoms.


Species classified B2 species in Belgium. This species has an unpredictable behaviour, sometimes appearing abundantly in one place, and then disapearing almoast entirely next year or reapprearing at another place. This behaviour probably depends on climatic and environmental conditions. However, A. filiculoides can form dense floating monospecific mats at the surface of water bodies that reduce light penetration and gas exchanges, causing the predominance of respiratory activities and the reduction in dissolved oxygen in water beneath the mats. These mats often reduce the development of algae, other aquatic plants and animals. But these impacts are limited as dense populations seem to be transient and well localised. Azolla species are known to be readily consumed by Cyprinid fish, which can be considered as good biocontrol agents. For more information, click here


Avoid planting this species near aquatic habitats (ponds, lakes, rivers, etc.), especially in the vicinity of protected areas (natural reserves, Natura 2000 sites, etc.).

Possible native alternative

Main ornemental function

Plants for ponds

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