Azolla is a very small floating fern. It will only grow in fresh water and does not tolerate a lot of salt. In good conditions in summer it can grow at an astonishing rate, doubling its size in a few days.
Azolla and Duckweed
Some people confuse Azolla with Duckweed. When plants are very small they may have a slight superficial resemblance to duckweed, but they are not closely related. Duckweed is technically a flowering plant (although its flowers are very rarely seen) while Azolla is a fern. Azolla reproduces by fragmentation. It can also reproduce by spores like most ferns and this may explain why it sometimes appears in water previously free of it. Azolla is very widespread round the world.
Duckweed grows best in high nutrient conditions which have to include nitrogen. Azolla is slightly different in growing even in the absence of fixed Nitrogen, although it does require enough Phosphorous.
Azolla is actually a symbiotic entity made up of a fern and a filamentous blue green alga. The Alga fixes atmospheric nitrogen which is available to both partners in the relationship. The fossil record suggests that Azolla has been around for at least 80 million years in its present form.
In parts of Asia, Azolla is grown as a green manure crop before a rice harvest and greatly improves the yield of the rice. Azolla taken from a garden pond is a useful plant to compost. The growing of rice contributes to the Methane content of the atmosphere. Some of this Methane comes from the decomposition of Azolla under anaerobic conditions.
way that Azolla is used in rice growing is as a companion plant. Once the
rice is tall enough it is above the water surface where the Azolla grows.
Under these conditions the Azolla fixes atmospheric Nitrogen which become
available to the Rice, the thick mat of Azolla makes it difficult for weeds to
grow, and reduces the number of mosquito larvae by making it more difficult for
them to reach the surface to breathe.
Azolla can be fed at low levels to egg laying poultry and can increase their egg production. Like many water plants Azolla has a high water content. Of the dry matter in Azolla, tests suggest a crude protein level of between 18 and 32 percent.
Azolla can grow so well that it can be a problem. A thick mat of Azolla makes it difficult for mosquito larvae to breathe. Fish will eat it and also the tiny animals that grow on and around it. In a pond it can greatly reduce the amount of algae, but will also make it very difficult for plants to grow underwater.
often problems blamed on Azolla on rivers and lakes are really problems of too
much Phosphorous in the water and are an indication that the waterway is
polluted, often by run off from agricultural fields carrying Phosphorous from
the fertilizers used in the fields.
Sources and Picture Credits
The picture of Azolla carolinian is by Ingrid Taylar from San Francisco Bay Area - California, USA (Vegetation Blanket) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons.
The picture of
is by Marco Schmidt  (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.
the Botanical Building at Balboa Park in San Diego, California, USA, Identified
by the sign is by Sickpen who has kindly released his picture into the public
The Picxture of Azolla pinata is by Laxskinn (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons.
The picture of Azola rubra is from New Zealand Ferns.
the time, we tend to just refer to Azolla as if it were only one species; there
are many species in the genus, both ones that exist now and ones that have
become extinct. As well as the different species, Azolla can also look
quite different depending on the conditions. In full sun, Azolla tends to
be red while in shade it is green.
Carolina mosquitofern. Azolla caroliniana.
This species of Azolla is native to parts of North and South America.
Azolla Africana is native to parts of Africa.
Azolla filiculoides is a widely distributes species, native to parts of Australia, Asia, Europe, North America and South America.
mixed with Duckweed. This species of Azolla
is native to parts of Africa, Asia and Australia.
comes from Australia;
south west Western Australia, and parts of the Northern Territory, South
Australia, southern Queensland, Victoria and Tasmania) as well as the North
Island possibly the South islands of New Zealand.