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.
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.
Symbiosis
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.
Green Manure
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.
Companion Plant
Another 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.
Feed Supplement
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.
Pest
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.
However, 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.
History
Azolla once changed the history of the world. This is described in The Azolla Event.
Most of the time, we tend to just refer to Azolla as if it were only one species. In fact 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.
Azolla carolinian. (A native of the Americas.)
Photo by Kurt Stueber.
Permission granted to use under GFDLby Kurt Stueber
Azolla pinnata
Azolla pinata is native to much of the "old world" including tropical Africa, South Africa, India, China,
Japan, Malaysia, The Philippines, Vietnam, New Guinea, and Australia
Photo By Tpa2067 [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (www.creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
Azolla africana in Mare aux hippopotames, SW Burkina Faso
Photo By Marco Schmidt [1] [CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
Azolla filiculoides growing with Duckweed (Lemna minor)
Azolla filiculoides is native to warm temperate and tropical regions of much of the world.
