A tiny floating fern with branched stems 1-5cm long. Leaves in two overlapping rows, greenish but often turning red in summer. Sori are produced in groups of 2-4 on the lower leaf of each branch.
Introduced as an aquatic for aquariums and ponds, now increasingly recorded as a garden escape and becoming more widespread.
Designated as a significant invasive plant in Ireland:
Target 10 - Managing Invasive Alien species
Water Fern, Azolla filiculoides. March 2009