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Azolla filiculoides

Small aquatic annual fern, greenish, bluish or reddish in colour, growing over the water surface.

Scientific nameAzolla filiculoides Lam.

Common name: water fern


Status in Portugal: invasive species (listed in the annex I of Decreto-Lei n° 565/99, of 21 December)

Risk Assessment score: (in development)

SynonymyAzolla caroliniana Willd., non auct., Azolla rubra R.Br.

Last update: 07/07/2014

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How to recognise it

Aquatic annual fern, floating, from 7-10 cm, green, sub-glaucous or reddish. Slender stems, ramified, horizontal and covered with leaves.

Leaves: with 1-2mm, imbricated, arranged in 2 rows, deeply two-lobed, with an herbaceous upper lobe, thick, aerial, with 2,5 x 0,9-1,4 mm, obtuse, with a broad hyaline margin; and a slender lower lobe, transparent and submerse.

Sporocarps: frequently 2, spherical or oval, yellowish-brown; undivided glochidium with 1-2 dividers near the apex.

Sporulation: April to June.


Similar species

Azolla caroliniana Willd. is similar, but the leaves have an almost acute superior lobe and the hyaline margin is much narrower.


Characteristics that aid invasion

Azolla filiculoides has very high growth rates, being able to multiply the invaded area in 7 to 10 days, when the temperature is between 15 to 20°C.

The invasive potential of this specie is also determined by the presence of available phosphorous in the water. When this element is available at very high concentrations, it may double or triple the invaded area.

Azolla filiculoides propagates vegetatively through fragments of stems that root easily, when the water surface isn’t completely covered. When this happens, sexual propagation starts – dissection-resistant spores are released.

Native distribution area

Tropical America.


Distribution in Portugal

Mainland Portugal (Trás-os-Montes, Beira Litoral, Beira Alta, Estremadura, Ribatejo, Alto Alentejo, Baixo Alentejo).


Geographic areas where there are records of Azolla filiculoides

Other places where the species is invasive

Several countries in Europe, North America, South Africa, tropical Asia, Australia, New Zeland.


Introduction reasons

Accidental with rice cultivation.


Preferential invasion environments

Still waters with low flow: lagoons, ditches and rice fields. It also occurs, though less frequently, in rivers with a slightly higher flow, when there is organic contamination.

Impacts on ecossystems

It grows into very wide and dense mats (up to 30 cm thick), causing eutrophication of riverine waters and reducing aquatic biodiversity.


Economic impacts

It reduces water flow and interferes in activities such as sailing, fishing, irrigation systems and hydroelectric complexes.

The application of control measures is quite expensive.


Natura 2000 network habitats more subject to impacts

– Hard oligo-mesotrophic waters with benthic vegetation of Chara spp. (3140);
– Natural euthrophic lakes with Magnopotamion or Hydrocharition-type vegetation (3150).

Controlling an invasive species demands a well-planned management, which includes the determination of the invaded area, identifying the causes of invasion, assessing the impacts, defining the intervention priorities, selecting the adequate control methodologies and their application. Afterwards it is fundamental to monitor the efficiency of the methodologies and recuperation of the intervened area as to perform, whenever necessary, the follow-up control.

The control methodologies used for Azolla filiculoides include:


Physical control

Manual removal by using thin-meshed nets. It is the preferred methodology when the invaded areas are relatively small. However, even assuming that all the specimens were initially removed, further removals may be necessary since the species propagates again from leftover spores.


Chemical control

Foliar application of herbicide. Spray with herbicide (active principle: glyphosate in a formula adapted to aquatic environments). It should be applied in monospecific populations. Since it produces effects on non-target species and its efficiency is very much dependent on the temperature, success might be difficult to attain.


Biological control

The weevil Stenopelmus rufinasus Gyllenhal (Coleoptera: Erirhinidae) has been successfully used to control the species in South Africa and United Kingdom.

This agent, although existent in Portugal, has not yet been tested in our country as to ascertain its harmlessness to native species, so it is not yet an option in our country.


Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.

Agricultural Research Council – Plant Protection Research Institute – weed">Weed Research Division (2014) Management of invasive alien plants: A list of biocontrol agents released against invasive alien plants in South Africa. Available: http://www.arc.agric.za/arc-ppri/Documents/WebAgentsreleased.pdf  [Retrieved 03/03/2014].

CABI (2012) Azolla control. CAB International, Wallingford, UK. Available: http://www.azollacontrol.com/default.aspx [Retrieved 12/11/2012].

CABI (2012) Azolla filiculoides. In: invasive">Invasive species">Species Compendium. CAB International, Wallingford, UK. Available: http://www.cabi.org/isc/ [Retrieved 10/11/2012].

Carrapiço S (2010) Azolla as a superorganism. Its implication in symbiotic studies. In: Seckback J, Grube M (eds) Symbiosis and stress: Joint ventures in biology, cellular origin, life in extreme habitats and astrobiology. Springer, 17: 225-241.

Carrapiço F, Santos R, Serrano A (2011) First occurrence of Stenopelmus rufinasus Gyllenhal, 1835 (Coleoptera: Erirhinidae) in Portugal. The Coleopterists Bulletin 65(4): 436-437.

Dana ED, Sanz-Elorza M, Vivas S, Sobrino E (2005) Especies vegetales invasoras en Andalucía. Consejería de Medio Ambiente, Junta de Andalucía, Sevilla, 233pp.

Dufour-Dror J-M (2012) Alien invasive plants in Israel. The Middle East Nature Conservation Promotion Association, Ahva, Jerusalem, 213pp.

Hill MP (1999) Biological control of red water fern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) in South Africa. African Entomology Memoir n°1: 119-124.

Marchante E, Freitas H, Marchante H (2008) Guia prático para a identificação de plantas invasoras de Portugal Continental. Imprensa da Universidade de Coimbra, Coimbra, 183pp.