Salvinia molesta

Lore & Background

Salvinia molesta belongs to a complex of closely-related water fern species that are only visually distinguishable by their unique sporocarps. It was originally grown as an ornamental plant but has since escaped its native habitat. The fern exhibits multiple growth forms; its primary invasive form has small, flat leaves, while its tertiary form has larger leaves that are crowded and folded. Under favorable conditions, the plants can form a dense mat up to two feet in thickness, often large enough to interfere with recreational activities on lakes and waterways. S. molesta reproduces by asexual reproduction, starting from small fragments and doubling in dry weight every 2.2–2.5 days. Each node can produce up to five buds, allowing rapid spread. The plant also generates genetically defective spores that do not produce viable offspring. It prefers nutrient-rich, slow-moving waters such as lakes, ponds, oxbow lakes, streams, ditches, marshes, and rivers, and grows best at a pH of 6–7.7 and water temperature of 20–30 °C. It can tolerate dewatering and exposure to low or very high temperatures, but cannot grow in water with high salt concentrations.

Reader's Guide

Salvinia molesta is significant primarily as an invasive species that has been accidentally introduced to several lakes worldwide. Since 2019, it has been included in the European Union's list of invasive alien species of Union concern, making it illegal to import, cultivate, transport, commercialize, plant, or intentionally release it in the EU. In the United States, it has been introduced into Caddo Lake in Texas, causing significant eutrophication. The fern's explosive growth clogs waterways, blocks sunlight needed by other aquatic plants, and causes rapid deoxygenation of affected water bodies. As it dies and decays, decomposition further uses up oxygen, and the mat prevents natural gas exchange between air and water, leading to stagnation. This can kill plants, insects, or fish trapped underneath. Large infestations may also pose problems for migratory birds and provide ideal breeding conditions for disease-carrying mosquitoes. The fern has been classified as an invasive weed in Australia, the United Kingdom, New Zealand, and parts of the United States. Biological control using the weevil Cyrtobagous salviniae has been successfully employed in several locations, including Lake Moondarra in Australia, where it reduced the fern population to a few small patches by mid-1981. The salvinia effect, a physical phenomenon describing the stabilization of a thin layer of air on a submerged hydrophobic surface, was discovered on this fern. Research has also shown its potential in bioremediation of blackwater effluent and in cancer research, where extracts demonstrated promising signs of inhibiting growth of human cancer cells without destroying nearby healthy ones.

Did You Know?

• The weevil Cyrtobagous salviniae was first used as a biological control for S. molesta in Australia at Lake Moondarra in 1980.
• Extracts of giant salvinia have shown promise in inhibiting growth of human cancer cells without destroying healthy ones, according to 2011 research at Stephen F. Austin State University.
• S. molesta can double its dry weight every 2.2–2.5 days under favorable conditions.