Trapaceae - water-chestnut family
 

WeedsPoisonous PlantsHay Fever
Gerald A. Mulligan (Agriculture Canada, Ottawa, retired)
Trapa natans L., European water-chestnut, châtaigne d'eau
European water-chestnut is an annual aquatic herb that forms mats of triangular, saw-toothed rosettes at the water's surface and feather-like leaves on submerged stems 8 to 15 feet (2.5 to 4.5 meters) long. Flowers with four, tiny, white petals develop in the center of rosettes. A, sharply 4-barbed, hard, heavy, 1.25 inch (3 cm), nut-like fruit matures underwater and soon sinks to the bottom. Germination of this heavy fruit, that can remain viable for up to 12 years, allows the plant to overwinter. During the growing season, spread of the plant occurs mainly by means of plant fragments that are dispersed by water-movement, waterfowl and boats. European water-chestnut was first observed naturalized in North America at Concord Massachusetts in 1879 (not 1859 as is often reported). It is now widely distributed, in water 8 to 15 feet (2.5 to 4.5 meters) deep, in sheltered, nutrient rich bays of lakes and rivers throughout the New England States, and locally in southern Ontario and southern Quebec. In Canada, it was first recorded in 1998 at Rivière du Sud, Quebec, and in 2005 was discovered in the Ottawa River near Hawkesbury, Ontario. European water-chestnut has proven to be a very aggressively invasive weed in North America and seams very much on the move. Its dense floating mats of rosettes severely limit space, light, and oxygen levels for native aquatic ecosystems, and impede fishing, hunting, swimming, boating and commercial navigation. The spiny fruit can also caused injury. It it is sometimes cultivated for its nutritious fruit in Europe, Asia and Africa. It is not related to Chinese water-chestnut, Eleocharis duicis, a sedge that is cultivated for its small, round, corms.

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European water-chestnut, châtaigne d'eau
(from Wikipedia)
European water-chestnut, châtaigne d'eau
(rosette, photo by Mike Naylor)
European water-chestnut, châtaigne d'eau
(4- barbed fruit; photo by Leslie J. Mehrhoff)
European water-chestnut, châtaigne d'eau
(mat of floating rosettes; photo by University of Connecticut)