Mosquito fern can cover the pond surface. Photo: Adam McClain, Adams County Conservation District
Native to eastern North America, Mosquito Fern, Azolla caroliniana, is a freshwater fern. It is a small, mat-forming plant that has been used in wastewater treatment and water pollution mitigation because of its ability to absorb heavy metals and radioactivity. Azolla is also used as a biofertilizer in rice paddies because it is able to fix nitrogen due to its symbiotic relationship with a blue-green alga.
Worldwide there are six species of Azolla that grow best in slow moving or still water in lakes, ponds, rivers, streams or wetlands. In North America, Azolla is a small, free-floating fern, only one-half inch in size. The leaves are lacy looking and overlapping. At first, the young plants are bright green or grey-green, but later they are likely to turn red in color. Azolla reproduces rapidly and mats can easily cover the surface of a pond, doubling in 4-5 days.
Azolla can be managed mechanically by raking or seining plants off the surface of the pond and composting far enough from the pond so the plants and their nutrients are not washed back in. Chemical control can also be used. The active ingredients that have been successful in treating mosquito fern include diquat and fluridone, which offer good control and penoxsulam, which is considered excellent. Grass carp, which are more effective on submerged vegetation, do not control Azolla well.
With any chemical control method there is a chance of oxygen depletion after the treatment caused by the decomposition of the dead plant material. Oxygen depletion can kill fish in the pond. If the pond is heavily infested with weeds, treat one-half to one-third of the pond and wait for about two weeks before treating another section. Aeration, especially at night, following treatment can help control the oxygen depletion.
A joint Department of Environmental Protection-Pennsylvania Fish & Boat permit is required for application of aquatic herbicides in Pennsylvania. When using herbicides, be sure to follow all safety instructions, label directions and water use restrictions carefully. As with most plant problems, nutrient management is the best control.