European water chestnut, horn chestnut, saligot, jesuit nut, water caltrope
Could be confused with
Eurasia, northern Africa
Species commonly cultivated
Trapa natans L. (with numerous geographical varieties)
northeastern United States, Canada
The high weed potential of this plant is widely accepted. Trapa is a noxious weed in many U.S. states and a prohibited import in many countries including Australia, Canada, and New Zealand.
Annual. Stem attached to substrate, submersed, elongate, relatively unbranched. Leaves dimorphic: submersed leaves opposite, sessile, linear; floating and emergent leaves alternate in apical rosette, petiole often swollen with spongy aerenchyma, leaf blade rhombic, venation pinnate, margin serrate. Submersed linear leaves, once abscised, are replaced with green pinnately-branched adventitious roots that develop from leaf scar. Inflorescence a solitary, bisexual flower, borne on short axillary pedicel. Sepals 4, these developing into 2 to 4 distinctive, but highly polymorphic spines on fruit; petals 4, white to purple. Dispersal by seed.
still waters of lakes, ponds, and upper regions of estuaries
The common name water chestnut is also applied to Eleocharis dulcus (Burm.) Hensch. and can lead to some confusion. Both species have edible seeds or tubers. Trapa natans is considered threatened in some regions of its native distribution (e.g. Germany, Belgium, Holland, and Sweden) but is a troublesome weed in much of its adventive distribution.