Giant salvinia, Salvinia molesta, is an aquatic fern prohibited in the United States by Federal law. Native to South America, its introduction by humans has caused severe economic and ecological problems in many counties including New Zealand, Australia and South Africa. It forms floating mats that shade and crowd out important native plants. Thick mats reduce oxygen content and degrade water quality for fish and other aquatic organisms. Mats impede boating, fishing, and swimming and clog water intakes for irrigation and electrical generation.
Young plants have smaller leaves that lie flat on the water surface.
As plants mature and aggregate into mats, leaves are folded and compressed into upright chains. These hairs give a velvety appearance and repel water. Distinguish from common salvinia, Salvinia minima, which has leaf hairs with branches always free at the tips. Spore cases are not found on young plants. Plants can be carried overland on anything entering infested waters.