Scientific name

Trapa L.

Common names

European water chestnut, horn chestnut, saligot, jesuit nut, water caltrope



Could be confused with


Native distribution

Eurasia, northern Africa

Species commonly cultivated

Trapa natans L. (with numerous geographical varieties)

Adventive distribution

northeastern United States, Canada

Weed status

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.


attached stem plant with apical, floating rosette

Brief description

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.

Natural habitat

still waters of lakes, ponds, and upper regions of estuaries

Additional comments

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.