Most Unwanted: Water fern
Azolla filiculoides
Habitat: Freshwater
Threat: Threatens standing water quality and dependant species
Status: Established
Azolla is a floating aquatic fern that is almost moss-like in appearance. It is bright green in colour with a noticeable fine red border round each leaf. Often acquires a rusty red colouration in winter, or when stressed. Found in freshwater systems such as lakes, rivers, and canals. Slow moving, or low energy sites are particularly vulnerable to infestation by this species. It is not tolerant of turbulence or waves, and is usually flushed out of fast-flowing rivers and streams.
What is it
It 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 are larger but not overlapping so that the stem is visible. 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.
Impact
Dense rafts of Azolla can accumulate, especially under the influence of winds and currents. These will reduce light levels penetrating beneath the surface of the water to waterweeds and algae, which could die off, seriously reducing oxygenation in the water.
Where is it from
Western North America from Washington State to Mexico. Now found in continental Europe and Britain.
Where is it in Ireland
Established at a number of sites across the island.
How did it get here
Garden and aquaria trade
How are they spreading in Ireland
Discarded garden or pond waste and flood events.
What you can do
Do not buy invasive aquatic plants.
Dispose of garden waste in a responsible manner.
Report all sightings.
Additional reading
Habitas Invasive Alien Species in Northern Ireland
