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Nature Notes
by Bob Thomas

The tiniest fern in the world is alive and well in New Orleans! The mosquito fern (Azolla caroliniana) is a small floating plant species that may cover the surface of still waters much as does duckweed. The name mosquito fern derives from the belief that mosquitoes cannot lay their eggs in water covered by these diminutive plants.

Mosquito fern is olive green in spring and early summer. By mid-summer and extending into winter, the leaves add anthocyanins, thus causing the surface of roadside ditches covered by mosquito fern to appear red.

Mosquito fern does not fill a normal fern niche and its 1/32nd of an inch wide overlapping leaflets are not visually impressive, but this tiny plant is important in the food market of many undeveloped countries. Each leaflet has a pouch which contains a symbiotic blue-green alga (Anabaena azollae) which converts atmospheric nitrogen to a form that can be used by other plants. The fern uses a portion of the nitrogen as it becomes available and when the plant dies and settles to the bottom, it releases the remaining nitrogen for use by other organisms. A good example of human application of this phenomenon is that rice paddies in southeast Asia may be covered with mosquito fern during the early growing season, producing as much as 40 lb of nitrogen per acre. As the rice grows, the ferns are shaded, die and sink to the bottom, thus recycling the vital nutrient to the food crop. Commercial fertilizers are produced at great expense and are virtually unavailable to undeveloped countries.

When present, mosquito ferns give our farmers the same advantages.

The mosquito fern is one of Mother Nature’s natural sources of cost-free fertilizer.

Also published in Delta Journal, The Times Picayune, August 10, 2008 and September 14, 2008

Also published in Nature Profile, The Times Picayune, September 15, 1982.


Red swaths of mosquito fern, Azolla                                  A closer view of reddish mosquito fern
, spread across the pond at                              growing, as it typically does, with other
the Jean Lafitte Nature Trail in Lafitte, La.                         floating plants such as common duckweed
Photo by Bob Thomas.                                                  (Lemna minor) and common salvinia
                                                                                       (Salvinia minima).
                                                                                       Photo by Bob Thomas.


Macro photo of the red form of mosquito                          Macro photo of the green form of mosquito
fern showing the leaflets.                                                fern showing the leaflets.
Photo by Shannon Fortenberry.                                   Photo by Larry Allain, USGS.