Elodea densa - Brazilian pondweed

Elodea canadensis, the Canadian waterweed or water pest, is a hardy, perennial aquatic plant of prime importance, reserved for ponds. It has long, slender stems completely submerged, except for the small white flowers that bloom on the water's surface, connected to the plant by a thin peduncle. The stems bear small translucent leaves, light green to dark green. The white flowering is insignificant. It is an invasive plant whose development should be monitored.

Canadian waterweed is a species of monocotyledonous aquatic plant in the hydrocharidaceae family. It is native to North America. It is commonly mentioned and used as an aquarium plant. Care should be taken when using it, as it can cause certain imbalances in nature. The introduction of this plant into rivers in Europe and other parts of the world, followed by the introduction of Nuttall's waterweed, sometimes causes fatal alkalosis in fish due to the uncontrolled proliferation of waterweeds. It has gone from intentional dissemination in aquariums to uncontrolled dissemination in natural areas.

Canadian waterweed is a dioecious species, meaning it has separate sexes. In Europe, only female plants exist. The slender, long stems of several meters are equipped with whorled leaves in groups of three. These leaves are small, sessile, thin (2 to 3 mm (1in) wide), light green to dark green, paler on their lower surface. They produce terminal buds that winter at the bottom of the water and produce new stems in spring. The flowers are insignificant.

Canadian waterweed is hardy, at least down to -15°C (5°F). It is necessary to test it to place it in a specific location in a pond. The brittle stems allow this plant to attach to other areas of the pond. It can also be planted in a basket or in the bottom of a natural pond. It should then be placed under 30 to 150 cm (12 to 59in) of water. Canadian waterweed tolerates almost all exposures but produces more oxygen with strong brightness. It is imperative to limit the development of these plants by regularly eliminating the excess. Prune regularly throughout the summer and heavily prune in autumn, to 30 cm (1 to 12in) from the base.

When the size of the pond allows, it is preferable to install two to three species of submerged plants to vary the foliage. Mix Canadian waterweed with crisp waterweed and dense waterweed, which are less invasive than the former. Waterweeds are an important element of lacustrine ecosystems in North America, providing habitat for many aquatic invertebrates and cover for young fish and amphibians. Aquatic fauna, particularly ducks, as well as beavers and muskrats, feed on them. Ceratophyllum, Crassula, Hippuris, Myriophyllum, and Stratiotes aloides can also be used to promote oxygenation and purification of the pond.

Elodea canadensis is used in cooking. The leaves are commonly used in biology to observe the simplicity of cellular organization. The term "elodea" comes from the Greek helôdês, "of the marshes".