Elodea canadensis

Elodea canadensis is a perennial aquatic plant, or submergent macrophyte, native to most of North America. It is closely related to Elodea nuttallii, which generally has narrower leaves under 2 mm broad. It is usually fairly easy to distinguish from its relatives, like the Brazilian Elodea densa and Hydrilla verticillata. These all have leaves in whorls around the stem; however, Elodea canadensis usually has three leaves per whorl, whereas Elodea Densa and Hydrilla usually have four or more leaves per whorl. Elodea densa is also a larger, bushier plant with longer leaves

Common Names
Elodea canadensis is commonly known as Canadian waterweed, Canadian pondweed, American waterweed, or American pondweed.

Growth
Young plants initially start with a seedling stem with roots growing in mud at the bottom of the water; further adventitious roots are produced at intervals along the stem, which may hang free in the water or anchor into the bottom. It grows indefinitely at the stem tips, and single specimens may reach lengths of 3 m or more. The leaves are bright green, 6–17 mm long and 1–4 mm broad, borne in whorls of three (rarely two or four) round the stem. It lives entirely underwater, the only exception being the small white or pale purple flowers which float at the surface and are attached to the plant by delicate stalks. It is dioecious, with male and female flowers on different plants. The flowers have three small white petals; male flowers have 4.5–5 mm petals and nine stamens, female flowers have 2–3 mm petals and three fused carpels. The fruit is an ovoid capsule, about 6 mm long containing several seeds that ripen underwater. The seeds are 4–5 mm long, fusiform, glabrous (round), and narrowly cylindrical. It grows rapidly in favorable conditions and can choke shallow ponds, canals, and the margins of some slow-flowing rivers. It requires summer water temperatures of 10-25 °C and moderate to bright lighting.

Cultivation and uses
It is frequently used as an aquarium plant. Propagation is by cuttings. It is an invasive species in Europe, Asia, Africa, and Australia. It was introduced into County Down, Ireland in about 1836, and appeared in Great Britain in 1841, spreading through both countries in ponds, ditches and streams, which were often choked with its rank growth. Other common names for this plant include Anacharis (an older name for the genus Elodea), water thyme, common elodea, and ditch moss.