Invasive Alien Species in Northern Ireland

Azolla filiculoides, Water fern

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Azolla filiculoides
© Mark Wright
Azolla filiculoides
© Mark Wright
Azolla filiculoides Lam.
Water fern is a floating aquatic fern, native of western North America, introduced as a decorative water plant for garden ponds and aquaria. It has entered lakes and ponds and has become well established in some areas in Northern Ireland.
It is called mosquito fern in America as it is said to cover some lakes so thickly that mosquitoes are unable to hatch. It is also called fairy moss.
Free-floating aquatic fern, appearing almost moss-like. Consists of a branched stem with attached leaves divided into two lobes; the upper lobes floating on the water surface, c.1-1.5mm across, overlapping and concealing the stem; the lower, submerged lobes larger but not overlapping so that the stem is visible; bright green with a noticeable fine red border round each leaf. Individual roots originate from the lower surface of the stem at the junctions with the side branches; sori are in one or two pairs on the lower lobe of the first leaf of a lateral branch. Often acquires a rusty red colouration in winter, or when stressed.
Country of origin
Western North America from Washington State to Mexico.0
Location in Ireland
Established in Glastry clay pits (Ards), Lough Neagh and some inter-drumlin lakes.
Life cycle
Reproduces vegetatively as the fronds grow, and sexually by producing spores which are the main means of overwintering.
Human impacts
None known.
Key vectors
Discarded material from garden centres and houses can be flushed into drainage systems and from there into rivers and lakes; propagules can be carried in mud on boots and shoes.
What you can do as an individual
Dispose of unwanted material and rinsing water from ponds carefully; filter discarded water, bin material.
Management measures
Large rafts can be scooped up with buckets, but this will not be effective except in small lakes or ponds, nor provide anything other than a temporary or cosmetic respite in larger areas. This would have to be repeated in order to catch germinated sporelings.
The plant can be effectively controlled by treatment with herbicides suitable for use in water: glyphosate, which will kill off almost all emergent and floating plants; diquat (as Reglone) will control but not kill the plant. It is best to apply herbicides before the surface of the water is completely covered. Several repeat applications will probably be needed.
Biological control: a weevil, Stenopelmus rufinasus, about 2mm long, with a yellow-striped back, is effective against Azolla in South Africa, and has been observed several times in the UK; grass carp feed on small infestations.
Further information
The Centre for Aquatic Plant Management (CAPM) gives details on their web site of methods of control:
Centre for Aquatic Plant Management: Information Sheet 22
See the Invasive Species Ireland web site for further information -
Text written by:
Catherine Tyrie, Curator, Botany, Ulster Museum