Azolla filiculoides

Azolla filiculoides aka water fern or Pacific mosquitofern has its origins on the American continent and is distributed from Canada to the South of Argentine and Chile. This tinly floating fern inhabits diverse stagnant to slow-flowing waters like ditches, swamps, ponds and lakes. However, it has been introduced into many regions of the world by humans and is found in the wild in Central Europe, too. It is not a popular aquarium plant, however, it is quite frequently sold as a plant for garden ponds.

According to more recent studies, there are only two native American Azolla species: A. filiculoides (synonyme: A. caroliniana) and A. cristata, often erroneously called A. caroliniana as well as A. mexicana. A. cristata is also found in the wild in Europe, however, much less frequently. Neither is it as popular, and it is easily mistaken for A. filiculoides.

A. filiculoides is a hardy floating aquarium plant and requires only medium light intensities to grow well. In nature it is also very robust, its spores survive cold water temperatures and even frost. An interesting characteristic of Azolla species is their symbiosis with a cyanobacterium named Anabaena azollae, which lives in a tiny cavity in the leaf underside and makes athmospheric nitrogen available to its host (Azolla). Thus, these little floating plants' nitrate content is quite high, which is the reason why they are often allowed to grow in rice fields as a natural fertiliser which is then ploughed under.

Azolla filiculoides ramifies and breaks down when it grows, thus reproducing. In the aquarium, you can simply net out all the superfluous plants. As it is a potentially invasive species in many parts of the world, please make sure you do not release these plants into the wild.

Some Azolla species like A. filiculoides, A. cristata and A. pinnata are an attractive complement to open tanks and (during the warm half of the year) for water gardens. They are able to fix great amounts of nutrients, and their roots provide young fish with optimal cover. When cultivated in the open, A. filiculoides assumes an attractive reddish brown in autumn. It survives mild winters outside, however, to make sure it does not die off it should be hibernated in frost-free, bright conditions.