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American waterweed (Elodea canadensis) is what many people commonly think of as "that aquarium plant"; It is also known by several other common names such as Canadian waterweed, common elodea, or anacharis. The use of these names causes it to be confused with similar-looking nonnative plants like Brazilian elodea or hydrilla. American waterweed is an attractive aquarium plant, and is a good substitute for Brazilian elodea since it is native to Washington's lakes, ponds, and rivers. In fact, due to its availability in the aquarium trade, it has been introduced to several countries where it is not native, and is now considered a noxious weed in those regions (parts of Europe, Australia, Africa, Asia, and New Zealand).
American waterweed is usually fairly easy to distinguish from its more notorious relatives, like Brazilian elodea and hydrilla. All of them have leaves in whorls around the stem. However, American waterweed has three leaves per whorl, whereas hydrilla and Brazilian elodea almost always have more than three leaves per whorl. Brazilian elodea is also a much larger, bushy plant with longer leaves. In the photograph, Brazilian elodea is the plant to the right. The two American waterweeds plants are at the top left - the plant in the middle is bushy because it was growing in higher light than the more spindly waterweed plant to its left. American waterweed also looks very much like another native elodea, Elodea nuttalli, which generally has three narrower leaves per whorl.
American waterweed lives entirely underwater with the exception of small white flowers which bloom at the surface and are attached to the plant by delicate stalks. It produces winter buds from the stem tips which overwinter on the lake bottom. It also often overwinters as an evergreen plant in mild climates. In the fall leafy stalks will detach from the parent plant, float away, root, and start new plants. This is American waterweed's most important method of spreading, with seed production playing a relatively minor role.
Silty sediments and water rich in nutrients favor the growth of American waterweed and in nutrient-rich lakes, it is sometimes perceived as a nuisance. However, it will grow in a wide range of conditions, from very shallow to deep water, and in many sediment types. It can even continue to grow unrooted, as floating fragments. It is found throughout temperate North America, and is one of the most common aquatic plants in Washington.
American waterweed is an important part of lake ecosystems. It provides good habitat for many aquatic invertebrates and cover for young fish and amphibians. Waterfowl, especially ducks, as well as beaver and muskrat eat this plant. Also, it is of economic importance as an attractive and easy to keep aquarium plant.
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