Water chestnut
European water chestnut



Trapa natans L.



Water-nut family



Trapa natans


Trapa natans is an aquatic annual that grows as a rooted, floating plant. Its floating leaves are arranged in a rosette. Individually, the 2-4 cm (0.75-1.5 in.) long upper leaves are slightly rhombic to rhombic-ovate and are sharply dentate along the leaf margins. There are conspicuous veins on the lower surface as well as short, stiff hairs. The submerged lower leaves are alternate and feather-like and can reach up to 15 cm (6 in.) long. The petioles of the floating leaves are 0.6-1.8 m (2-6 ft.) long. The inconspicuous white flowers consist of four 8 mm (0.3 in.) long, white petals and four green sepals, and are located in the center of the rosette. Flowering begins in the northeastern United States in July and continues until the plants are killed by frost. The fruit is a four-horned nut-like structure about 3 cm (1.2 in.) wide that develops underwater. Fruits ripen in about a month and can remain viable for up to about twelve years. Each seed can give rise to ten to fifteen rosettes, and each rosette may produce as many as twenty seeds. 


It has often been reported that waterfowl or water currents can move the seeds of Trapa natans long distances. However, the fruits weigh 6g and have been described as falling to the bottom of lakes "like sinkers," making them unlikely to be carried in the feathers of birds or downstream by moving water. The empty husks of the fruits do tend to float, possibly leading to the belief that they could be moved in these ways. Trapa natans may also disperse by fragmentation. Plant fragments can be carried by water, waterfowl and boats to new locations.



Trapa natans is native to the warm temperate regions of Eurasia. It has been naturalized in Australia and has recently (1998) been reported from Ontario, Canada. In the United States it is present in the northeast in the states of New York, New Jersey, Pennsylvania, Delaware, Maryland and Virginia. In New England, it has been reported from Massachusetts, Vermont, New Hampshire and Connecticut.