Small aquatic annual fern, greenish, bluish or reddish in colour, growing over the water surface.
Scientific name: Azolla filiculoides Lam.
Common name: water fern
Risk Assessment score: (in development)
Synonymy: Azolla caroliniana Willd., non auct., Azolla rubra R.Br.
Last update: 07/07/2014
How to recognise it
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.
Sporulation: April to June.
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
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
Several countries in Europe, North America, South Africa, tropical Asia, Australia, New Zeland.
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.
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:
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.
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.
The weevil Stenopelmus rufinasus Gyllenhal (Coleoptera: Erirhinidae) has been successfully used to control the species in South Africa and United Kingdom.
Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.
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CABI (2012) Azolla control. CAB International, Wallingford, UK. Available: http://www.azollacontrol.com/default.aspx [Retrieved 12/11/2012].
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