Egeria is a genus of submerged freshwater aquatic plants in the frog's-bit family (Hydrocharitaceae), order Alismatales. As treated in this database it is monotypic, containing a single species, Egeria densa (the large-flowered or Brazilian waterweed). The genus was established in 1849 by the French botanist Jules Émile Planchon, who described the plant as Egeria densa; in 1857 Robert Caspary transferred it to the related genus Elodea, and most modern taxonomic sources — including GBIF's backbone taxonomy and Wikipedia — now treat Egeria as a synonym of Elodea rather than a distinct genus, though it continues to circulate widely under the name Egeria in aquarium and horticultural contexts.
Egeria densa is a bright-green, perennial submerged herb with trailing stems that can exceed 2 m in length, rooting at intervals along their length and growing in water up to about 4 m deep. Its narrow, pointed leaves are arranged in whorls of four to eight around the stem. The species is dioecious, producing small three-petaled white flowers on separate male and female plants; stems grow toward the surface, where they spread out and form a dense canopy.
Native to warm temperate South America — southeastern Brazil, Argentina, Uruguay, and Chile — Egeria densa has been spread worldwide through the aquarium trade and is now naturalized on every continent except Antarctica, including as far north as geothermally heated waters in Iceland. Outside its native range it is considered a serious invasive species and ecosystem engineer: its dense surface canopy blocks light to native vegetation, it takes up nutrients and traps sediment from the water column, and fragments readily to colonize new areas. Infestations obstruct boat traffic, clog water intakes, and impede fish migration; a well-documented example is its long-term impact on California's Sacramento–San Joaquin Delta. It also has beneficial uses, however: it is grown commercially as a low-maintenance, beginner-friendly aquarium plant (propagated from cuttings, with plants in cultivation typically a single sterile male clone) and is used in constructed wetlands and wastewater treatment for its nutrient-stripping ability.
Distribution
Native to warm temperate South America (southeastern Brazil, Argentina, Uruguay, and Chile). Through the aquarium trade it has become widely naturalized elsewhere, including North America, Europe, Asia, Australia, New Zealand, Africa, and geothermally heated waters as far north as Iceland — one of the first freshwater alien plants to reach the Arctic.
Taxonomy Notes
Egeria was established by Jules Émile Planchon in 1849 for the species he named Egeria densa; Robert Caspary moved the species to the genus Elodea in 1857, and modern authorities including GBIF's backbone taxonomy and Wikipedia now treat Egeria as a synonym of Elodea rather than a separate genus, even though some checklists (e.g. NCBI Taxonomy) still list it as accepted. This database follows the traditional/horticultural usage, keeping Egeria as a valid, monotypic genus containing only Egeria densa.
Ecology
Grows fully submerged in fresh water up to about 4 m deep, with stable growth between 16–28°C and reduced growth above 32°C. Its ability to match photosynthetic output to available light lets it thrive in low-light conditions and form a dense surface canopy, which shades out native plants and reduces local plant diversity. Regarded as an ecosystem engineer: it takes up nutrients and sediment from the water column, disperses readily through stem fragmentation, and provides shelter for zooplankton and invertebrates. Black-necked swans feed on it, and swan population declines have been linked to declines of this plant. Where invasive, dense mats obstruct boat passage, clog water intakes and aqueducts, and impede fish migration.
Cultivation
Widely sold as a low-maintenance, cool-water aquarium plant suitable for beginners, though banned or restricted from sale in some regions due to its invasiveness. Cultivated stock is generally a single sterile male clone reproducing only vegetatively, and is easily propagated from stem cuttings. It performs best in nutrient-rich, high-light conditions and reportedly secretes antibiotic compounds that may suppress blue-green algae. The same nutrient- and sediment-stripping traits that make it invasive are also exploited deliberately in constructed wetlands and wastewater treatment.
Invasive Status & Control
Egeria densa has escaped cultivation and naturalized across many warm-temperate to subtropical regions worldwide, including South Africa, the Azores, Hawaii, New Zealand, New Caledonia, and much of North America; in California's Sacramento–San Joaquin Delta it has infested roughly 2,400 ha (about 12% of the delta's surface area) since its 1960s introduction. Because fragments readily regrow, full removal or herbicide treatment is generally needed for control; the plant is highly sensitive to desiccation and can die within an hour if removed from water, so water drawdowns are an effective control method, and cold weather also suppresses growth.
