Elodea is an aquatic plant in the Hydrocharitaceae family that is commonly used in aquariums and ponds to provide oxygen for fish. Although native to North America, this perennial has spread into local waters in many parts of the world, and is often considered a noxious weed. The common name for the American or Canadian waterweed, Elodea canadensis, is Anacharis — an older term for the genus. Anacharis is often confused with other invasive aquatic plants, such as Brazilian elodea, Egeria, and Hydrilla.
American elodea can be distinguished from Brazilian elodea and other plants by its characteristic whorls of leaves around the stems. The American species has a whorl of three leaves, while the other species generally have a greater number of leaves in each one. Also, the Brazilian species is a much larger plant.
Waterweeds are vigorous plants that can grow at temperatures ranging from 34° to 77°F (1° to 25°C). The plants grow best in nutrient-rich water, but can grow in a variety of environments. They can survive brief stays in ice, and being out of water for a few weeks, if exposed as a dense colony. These plants require water that is fairly still, and will not grow in fast-running water.
Elodea grows to the surface of the water, but will not grow above it. It produces small, white flowers at the surface. Due to a shortage of male plants, seed is rarely produced.
These types of plants can fragment, however. Pieces break off and can live independently while floating loose in the water. This is the primary way in which the plant spreads. Such fragments can be picked up by ships or by migrating birds for their nests. The stems of the plants can grow to be 6.7 to 10 feet (2 to 3 m), but colonies can spread to be hundreds of acres.
Eradication efforts have been difficult, since small pieces of the plant can spread to start new colonies. Mechanical efforts at harvesting have been tried. It has been found to be necessary to set up a filtering system downstream to catch any of the elodea pieces that may have broken off. Efforts to introduce fish to eat the plants have run into various problems. One solution that has worked is to introduce an opaque material that causes so much shade that the plants die.
In North America, elodea is generally a useful plant. It provides shelter for aquatic animals, such as small invertebrates and fish. Some sources say that animals, like waterfowl and beaver, eat the plants. Also, it is of economic importance as an aquarium plant. It is illegal to sell in some parts of the United States, however.