Common Waterweed (Elodea canadensis)

Common Waterweed (Elodea canadensis)

The Common Waterweed (Elodea canadensis) also called the Canadian Waterweed or American Waterweed is of the order Hydrocharitales and Tape-grass Family (Hydrocharitaceae). The specimen shown in the photograph above was removed from French Hill Pond for study in October 2011. The genus name (Elodea) is from the Greek for "marsh." The species name associates this plant with Canada. The order and family name is from the Latin indicating a plant associated with water.

The leaves are very small and simple. Each leaf is ovate to oblong, ½ inch (1.3 centimeters) or less long and attached directly to the stem. The leaves are usually arranged along the stem in whorls of two or three leaves with the two-leaved whorls being lower on the stem. The Common Waterweeds in French Hill Pond have whorls of four leaves, which is very rare. However, the small size, shape and spacing of these leaves identify the plant as a variety of Common Waterweed. Similar invasive plants have much larger and more tapered leaves in whorls closer together. The leaves and stems of the Common Waterweed remain green all year round. This characteristic of remaining green and its small size makes this plant popular for aquarium use. Propagation is by seed or through the rooting of stem fragments. The latter means of propagation is more common. The root systems are simple. 

Claw on the Margin of a Common Waterweed Leaf

The edges (margins) of the leaves appear entire (toothless) but there may be microscopic claws or teeth on the edges as shown in the microphotograph above. The magnification in this microphotograph is 400X. However, these teeth may be seen under lesser magnification. The photograph below shows the tip of an Elodea canadensis where a bud is beginning to form. Note the whorls of four leaves. These four-leaved whorls appear at the top of the stems. The distortion around the bud occurred because the bud is partially above the water.

Common Waterweed (Elodea canadensis) with Bud

The Common Waterweed flowers from May to October producing small white flowers that float on the water. Each flower has three petals less than 1/20 inch (5 millimeters) long  and may be male or female. The male flower has nine stamens. The female flower has three carpels (containers for the female organs).  Male and female flowers occur on different plants. Such plants are said to be "dioecious." The fruit is an ovoid capsule.

 

Dense Mat of Common Waterweed (Elodea canadensis) 

These plants grow in dense mats as shown in the photograph above. The Common Waterweed grows rapidly and can clog shallower water. This plant is considered invasive in Europe. The German name for this plant is "Wasserpest," literally, "Water Pest." However, home, aquatic gardeners will plant Common Waterweed for the dense, green mats they produce, not usually for the tiny, inconspicuous flowers.

The Slender Waterweed (Elodea nuttallii) with narrower leaves is very similar to the Common Waterweed (Elodea canadensis).

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