Elodea is a genus of approximately eight species of submerged freshwater aquatic plants belonging to the family Hydrocharitaceae, within the order Alismatales. The genus was described by André Michaux in 1803 in his Flora Boreali-Americana and is native to the Americas, where species range from Canada and the United States south through South America.
Plants in this genus are slender, fully submerged herbs with whorled leaves, growing rooted in freshwater sediments across lakes, ponds, rivers, and streams. They are commonly known as waterweeds or, under the older synonym Anacharis, by that name in North America. Elodea species are well known for their use as aquarium vegetation and as model organisms in biology education, where they are frequently employed in demonstrations of photosynthesis, cellular respiration, and cytoplasmic streaming.
Several species, most notably Elodea canadensis and Elodea nuttallii, have been introduced beyond their native range into waterways across Europe, Australia, Africa, Asia, and New Zealand. In these regions they are regarded as serious invasive weeds: newly colonized habitats experience rapid growth for five to six years as nutrients are exploited, and fragmentation through mechanical removal or boating activity spreads the plant further. Chemical control using aquatic herbicides has proven largely ineffective against Elodea.
Notable species include Elodea canadensis (common waterweed), widespread across the United States and Canada; Elodea nuttallii (Nuttall's waterweed), similarly widespread in North America; Elodea granatensis, native to much of South America; and Elodea densa, sometimes placed in the closely related genus Egeria.
Etymology
The name Elodea derives from the Greek helos (ἕλος), meaning "marsh" or "swamp," reflecting the genus's aquatic habitat. An older and still-used common name, Anacharis, is a synonym recognized by GBIF; it remains in informal use across North America.
Distribution
Elodea is native to the Americas, with species distributed from western and central Canada and the United States (E. canadensis, E. nuttallii, E. bifoliata) through South America (E. granatensis, E. callitrichoides, E. potamogeton). Multiple species have been widely introduced outside this native range and are now naturalized — often invasively — across much of Europe, Australia, Africa, Asia, and New Zealand.
Ecology
Elodea grows fully submerged in freshwater lakes, ponds, rivers, and slow streams, rooting in soft sediments. When introduced to new ecosystems, populations typically experience rapid growth for five to six years until soil nutrients are exhausted. The genus reproduces vegetatively via fragments, which can disperse downstream or attach to boats, anchor chains, and fishing equipment, facilitating long-distance spread. Dense mats reduce light penetration, alter water chemistry, and compete with native aquatic vegetation. Chemical control via aquatic herbicides is largely ineffective; mechanical removal by raking or mowing boats can inadvertently accelerate spread through fragmentation.
Taxonomy Notes
The genus Elodea Michx. (1803) carries a long list of synonyms in the GBIF backbone, including Anacharis Rich., Apalanthe Planch., Diplandra Bertero, Egeria Planch., Helodea, Hydora Besser, Philotria Raf., Serpicula Pursh, and Udora Nutt. The name Anacharis remains widely used colloquially in North America. The closely related genus Egeria (containing Egeria densa, the aquarium plant sold as "Brazilian waterweed" or "Anacharis") was formerly treated within Elodea and is still sometimes conflated with it in the aquarium trade.
