Mosquito fern is easy to overlook until it covers the surface of a pond completely, at which point it becomes impossible to miss. Native across eastern North America and northern South America, this aquatic annual colonizes still or sluggish water — ponds, ditches, slow streams — in partial to full sun, floating on the surface in a dense mat of overlapping scale-like leaves less than an inch tall. New growth emerges bright green or gray-green, but exposure to strong light triggers a shift to rusty red that can color an entire pond surface by midsummer.

The ecological value of azolla is quietly remarkable: each tiny leaflet shelters a symbiotic blue-green alga called Anabaena azollae that fixes atmospheric nitrogen, making the plant genuinely beneficial to the water chemistry of ponds and slow waterways. Its surface coverage also blocks the still water that mosquitoes require to deposit eggs, though it should be noted that azolla and mosquitoes favor the same environments — this is a plant that fits the problem rather than eliminating it. In a water garden or aquarium it spreads quickly, which is worth accounting for when choosing where to introduce it. Divisions overwinter easily indoors in water. Fish typically leave it alone, and it moves easily into new habitats by transferring a portion of the mat.