Azolla or water fern in green and red phase
A small channel carpeted in Azolla, when the same site was revisited 2 weeks later the plant had matured and was now red in colour.

Water fern

Water fern, Azolla filliculoides(Azolla filiculoides)

This is a small free floating fern that grows and proliferates in canals, ponds and sheltered aquatic habitats. It is tolerant of brackish water. In suitable watercourses Azolla can carpet the water surface over considerable areas. By excluding light it eliminates the native submerged flora. It also prevents diffusion of oxygen from air to water, which can produce anoxic conditions. This may cause fish kills. The physical biomass can prevent boat movement or angling.

During the 2001 weed and water quality survey of the River Barrow, a large stand of Water Fern, was observed upstream of the lock gates at Milford. This was the first record of this species on the River Barrow. Following this  recording of what was thought of as an isolated colony of Azolla on the River Barrow at Milford, more extensive beds of this exotic, free-floating plant were identified upstream of Bagnelstown in early November of the same year. A subsequent survey revealed small stands of this plant in most canal cuts from Carlow downstream to St. Mullins

Azolla has the capacity to rapidly over-grow static or slow flowing channels (e.g. canals or cuts on the Barrow), and may cause major problems for navigation, angling and other amenity activities on the waterway, as is evident from the photographs taken of the cut upstream of Bagnelstown. Careful monitoring of this prolific plant will be required for the coming seasons and all positive sitings will have to be looked at closely. As this plant is free-floating it poses a real threat of spreading to other stretches of this system and to other canals. Where Azolla is present it will be required to remove it from the channel to prevent the further spreading.

Distribution of water fern in Ireland

Distribution of Water Fern