Azolla mexicana

Azolla mexicana is a small floating fern from the Americas that the Flora of North America treats as a separate species: closely related to A. caroliniana, but less cold-tolerant.¹ In other treatments the name belongs to A. cristata or A. filiculoides subsp. cristata, whose native range extends from western Canada to Argentina and which has been introduced in Europe, Asia and Africa.²³

In early summer the plants can build up layered mats as much as 4 cm deep; otherwise they float freely, their colour ranging from green through blue-green to dark red.¹ Because the cyanobacterium Anabaena azollae lives in the upper lobe of the minute, two-lobed leaves and fixes atmospheric nitrogen, Azolla is used as a green manure in rice cultivation.²⁴ It is one of the cultivated aquarium and pond plants.⁴

1. Azolla mexicana – Flora of North America, eFloras.org (accessed 5 October 2026)
2. Kasselmann, C. (2019): Aquarienpflanzen. 4th edition (German). - Ulmer Verlag, Stuttgart: p. 163.
3. Azolla filiculoides subsp. cristata – Plants of the World Online, Royal Botanic Gardens Kew (WCVP data as of 4 June 2026, accessed 5 October 2026)
4. Azolla – Aquarium and Pond Plants of the World, idtools.org (accessed 5 October 2026)