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Washington State Department of Ecology > Water & shorelines > Freshwater studies > Lake water quality > Aquatic Plant Guide home > Floating Leaved Rooted Plants > Trapa natans

Aquatic Plant Identification Manual for Washington's Freshwater Plants

Floating Leaved Rooted Plants

Species: Trapa natans L., water chestnut
Family: Trapaceae

Water chestnut has broad, toothed, diamond-shaped upper leaves which form tight floating rosettes. The leaves connect to the stem by swollen stalks just below the leaf blades. The thin limp stems give rise to long, narrow or feather-shaped underwater leaves and are rooted in the sediment. The fruit is a hard, woody nut with two to four sharp spines. Water chestnut is an annual plant which was introduced from Asia and has become abundant in the eastern U.S. where it creates a nuisance. It outcompetes native plants, and the mature nuts drift to shore where the sharp spines hurt bare feet.

Leaf: Upper leaves are diamond-shaped with toothed edges and are shiny on the upper side and dull with fine hairs underneath. They are alternately arranged on inflated, spongy stalks and occur in clusters up to 50 cm across. Opposite submersed leaves are long and narrow. Green feather-like structures often replace the linear underwater leaves.

Stem: The submersed, long and limp stem is up to 5 m long.

Flower: Small, solitary flowers have 4 white or light-purple petals on short, thick stalks that float among the upper leaves. The 4 sepals turn into the spines of the fruit.

Fruit: Large (2.5 cm), variously-shaped nuts are swollen at the middle and have 2-4 sharp spines. Each nut contains a single, fleshy seed.

Root: Develop on shoots. Lower roots are unbranched and thread-like, while upper roots are sparsely branched and fibrous. Some botanists consider the feather shaped underwater structures to be roots instead of leaves.

Propagation: Seeds.

Importance of plant: People eat the seeds in Asia, however the water chestnut commonly eaten in North America is actually the tuber from a different plant. Water chestnut plants are a nuisance in the eastern U.S.

Distribution: Native to Asia. Introduced into eastern North America. This non-native plant is currently not found in Washington, but if introduced, could readily become established.

Habitat: Lakes, ponds, canals, and slow water.

May be confused with: Invasive Nonnative Icon Easily distinguished by clusters of toothed, diamond-shaped floating leaves.


Photographs: Trapa natans plant (herbarium specimen - this plant is normally bright green) 

Line Drawings: Trapa natans

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