Waterfern Salvinia molesta

Giant salvinia is an invasive aquatic fern in the water-fern family (Salviniaceae). It is native to Argentina and Brazil but is now a global weed. It is a Noxious Aquatic Weed according to the United States Department of Agriculture and the North Carolina Department of Agriculture and Consumer Services. The species epithet is Latin for “bothersome” and refers to the troublesome habit of this plant.

The plant grows only in the freshwater of lakes and rivers and can tolerate cold but not frozen water. The mats can be several feet thick and block sunlight and oxygen from getting to the water, to the detriment of fish and native plants. Mats impede boating, fishing, and swimming and clog water intakes for irrigation and electrical generation.

Giant salvinia is a free-floating fern with horizontal stems that float just below the surface. A whorl of 3 green leaves (fronds) is produced at each node with a bristly surface caused by the hair-like bristles that join at the end to form eggbeater or whisk shapes. Lower leaves are brown, deeply cut and act as roots. Below those are larger leaves, up to 9 inches, that act as a drag for stabilizing the plant. Sporocarps (spore-producing bodies) develop in elongated chains among the submerged leaves. However, very few spores are formed; this plant reproduces primarily vegetatively.

Due to its invasiveness and the harm caused by this plant to natural ecosystems, this fern is not recommended for any garden, aquarium, pond or waterscape. Do not grow this plant. Seek alternatives.

Insects, Diseases, and Other Plant Problems: Salvinia weevil, a South American beetle that feeds on the plant, has been introduced into the US as a biological control.