|Zoomed picture of Azolla filiculoides|
It is a floating aquatic fern, with very fast growth, capable of spreading over lake surfaces to give complete coverage of the water in only a few months. Each individual plant is 1–2 cm across, green tinged pink, orange or red at the edges, branching freely, and breaking into smaller sections as it grows. It is not tolerant of cold temperatures and, in temperate regions it largely dies back in winter, surviving by means of submerged buds. It harbors the diazotrophic organism, Nostoc azollae, in specialized leaf pockets. This ancient symbiosis allows N. azollae to fix nitrogen from the air and contribute to the fern's metabolism.
The only sure method of distinguishing this species from Azolla cristata (long incorrectly known as A. caroliniana) is to examine the trichomes on the upper surfaces of the leaves. Trichomes are small protuberances that create water resistance. They are unicellular in A. filiculoides but septate (two-celled) in A. cristata.
The species has been introduced to many regions of the Old World, grown for its nitrogen-fixing ability that may be used to enhance the growth rate of crops grown in water, such as rice, or by removal from lakes for use as green manure. It has become naturalized, sometimes also an invasive species, in several regions, including western Europe, southern Africa, tropical Asia and New Zealand.
Ireland: Introduced into Clandeboye Lake, Co. Down.
- Lamarck JB (1783). "Name - Azolla Lam". Encyclopédie Méthodique, Botanique. Saint Louis, Missouri: Missouri Botanical Garden. 1 (1): 343. Retrieved February 19, 2010.
Annotation: a sp. nov. reference for Azolla filiculoides
Type Specimens HT: Azolla filiculoides
- Hussner A (2006). "NOBANIS -- Invasive Alien Species Fact Sheet -- Azolla filiculoides" (PDF). Online Database of the North European and Baltic Network on Invasive Alien Species. Heinrich Heine University, Düsseldorf. Retrieved February 19, 2010.
- Brouwer P, Bräutigam A, Buijs VA, Tazelaar AO, van der Werf A, Schlüter U, et al. (2017-03-31). "Azolla Ferns without Nitrogen Fertilizer". Frontiers in Plant Science. 8: 442. doi:10.3389/fpls.2017.00442. PMC 5374210. PMID 28408911.
- Meeks JC (2009). "Physiological Adaptations in Nitrogen-fixing Nostoc–PlantSymbiotic Associations". In Pawlowski K (ed.). Prokaryotic Symbionts in Plants. Microbiology Monographs. 8. Berlin, Heidelberg: Springer. pp. 181–205. doi:10.1007/7171_2007_101. ISBN 978-3-540-75460-2.
- Li FW, Brouwer P, Carretero-Paulet L, Cheng S, de Vries J, Delaux PM, et al. (July 2018). "Fern genomes elucidate land plant evolution and cyanobacterial symbioses". Nature Plants. 4 (7): 460–472. doi:10.1038/s41477-018-0188-8. PMC 6786969. PMID 29967517.
- "Can A Tiny Fern Help Fight Climate Change and Cut Fertilizer Use?". Yale Environment 360 Digest. 11 July 2018.
- Evrard C, Van Hove C (2004). "Taxonomy of the American Azolla Species (Azollaceae): A Critical Review". Systematics and Geography of Plants. 74 (2): 301–318.
- Hackney P, ed. (1992). Stewart & Corry's Flora of the North-east of Ireland (Third ed.). Institute of Irish Studies & The Queen's University of Belfast. ISBN 0-85389-446-9.
- Hyde HA, Wade AE, Harrison SG (1978). Welsh ferns: clubmosses, quillworts, and horsetails (PDF). National Museum of Wales. ISBN 0-7200-0210-9. Archived from the original (PDF) on 2012-10-15.
- Sainty G, Jacobs S (2003). Waterplants in Australia. Potts Point, Sydney, Australia: Sainty and Associates. ISBN 0-9581055-1-0.
- Media related to Azolla filiculoides at Wikimedia Commons
- "Azolla filiculoides". Germplasm Resources Information Network (GRIN). Agricultural Research Service (ARS), United States Department of Agriculture (USDA).
- Flora of North America: Azolla filiculoides
- USDA Plants Profile: Azolla filiculoides
- Plants for a Future: Azolla filiculoides
- NSW Flora Online: Azolla filiculoides