The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

Trapa natans
Trapa natans
(water chestnut)

Copyright Info
Trapa natans L.

Common name: water chestnut

Synonyms and Other Names: Trapa bispinosa Roxb., Trapa natans var. natans L., Trapa natans var. bispinosa (Roxb.)

Makino, European water chestnut, water nut, horned water chestnut, water caltrop, bull nut

Taxonomy: available through www.itis.govITIS logo

Identification: According to Crow and Hellquist (2000):

Habit: floating, rooted, aquatic annual

Stems/Roots: submerged, flexuous stem with feathery, adventitious roots that anchor into the mud and extend upwards to the surface of the water where they can photosynthesize.

Leaves: heterophyllous; submerged linear leaves (above the feathery, green roots) and emergent rhomboid leaves arranged in a floating rosette, with each emergent leaf having a slightly inflated petiole (leaf stem) and dentate (tooth-like) leaf margins

Flowers: perfect (hermaphroditic), solitary, small, white flowers with four petals in the center axils of the floating rosette; ovary inferior (epigynous)

Fruits/Seeds: large nut-like drupe with four, orthogonal, sharp spines that develop from hardened sepals; single seed

Look-alikes: Ludwigia sedioides (Humb. & Bonpl.) H.Hara and Ludwigia peploides (Kunth) P.H. Raven

Size: up to 5 m (16 ft) in stem length (Muenscher 1944)

Native Range: Europe, Asia, and Africa (Muenscher 1944; Gleason and Cronquist 1991; Crow and Hellquist 2000).

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences: Trapa natans occurs throughout the Northeast U.S. and parts of Canada, from the Ottawa River, Ontario to northern portions of the Chesapeake Bay, Maryland, and from the Erie Canal, New York and the Allegheny River, Pennsylvania to the Nashua River, New Hampshire and the Ten Mile River, Rhode Island. The populations historically established in the Potomac River were eradicated in the 1970s (Scott 2011).

Trapa natans was first introduced to North America in the 1870s, where it is known to have been grown in Asa Gray's botanical garden at Harvard University in 1877. By 1879, the Harvard University gardener along with other enthusiasts had deliberately planted T. natans in ponds near the Sudbury and Concord Rivers and it soon became established in those rivers (Countryman 1970). Another initial introduction of T. natans was at Collins Lake near Scotia, NY (in the Hudson River-Mohawk River drainage) around 1884, possibly as an intentional introduction for waterfowl food or as a water garden escapee (Wibbe 1886).

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Trapa natans are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
CT199820239Farmington River; Housatonic; Lower Hudson; Outlet Connecticut River; Pawcatuck River; Quinebaug River; Quinnipiac; Shetucket River; Thames
DC189519501Middle Potomac-Anacostia-Occoquan
MD192320244Chester-Sassafras; Gunpowder-Patapsco; Lower Potomac; Middle Potomac-Anacostia-Occoquan
MA1874202315Ashuelot River-Connecticut River; Blackstone River; Charles; Chicopee River; Concord River; Deerfield River; Housatonic; Merrimack River; Middle Hudson; Narragansett; Nashua River; New England Region; Outlet Connecticut River; Quinebaug River; Westfield River
NH199520163Black River-Connecticut River; Nashua River; West River-Connecticut River
NJ200220235Hackensack-Passaic; Middle Delaware-Musconetcong; Raritan; Rondout; Sandy Hook-Staten Island
NY1884202330Bronx; Chaumont-Perch; Chenango; Conewango; Hackensack-Passaic; Hudson-Hoosic; Hudson-Wappinger; Irondequoit-Ninemile; Lake Champlain; Lake Ontario; Lower Genesee; Lower Hudson; Mettawee River; Middle Delaware-Mongaup-Brodhead; Middle Hudson; Mohawk; Niagara River; Northern Long Island; Oak Orchard-Twelvemile; Oneida; Oswegatchie; Oswego; Rondout; Salmon-Sandy; Sandy Hook-Staten Island; Schoharie; Seneca; Southern Long Island; Upper Delaware; Upper Susquehanna
PA1977202316Brandywine-Christina; Conewango; Crosswicks-Neshaminy; Lackawaxen; Lehigh; Lower Delaware; Lower Susquehanna-Swatara; Middle Allegheny-Redbank; Middle Delaware-Mongaup-Brodhead; Middle Delaware-Musconetcong; Schuylkill; Shenango; Upper Allegheny; Upper Delaware; Upper Susquehanna; Upper Susquehanna-Lackawanna
RI200720234Blackstone River; Narragansett; Pawcatuck River; Quinebaug River
VT194220226Black River-Connecticut River; Hudson-Hoosic; Lake Champlain; Lamoille River; Mettawee River; Otter Creek
VA193619451Middle Potomac-Anacostia-Occoquan

Table last updated 4/20/2024

† Populations may not be currently present.


Life history: Sexual reproduction occurs annually with a seed bank (dormancy) of at least 2 years and up to 5 years in stable conditions (Kunii 1988, Menegus et al. 1992, Cozza et al. 1994). Flowers bloom for one day and are self-compatible and apomictic (producing asexually via seed) in variety japonica (Kadono and Schneider 1986). Each T. natans plant has 15 - 20 rosettes, and each rosette can generate up to 20 seeds (Maryland Sea Grant 2012). Seeds overwinter in the benthic sediments and germinate the following spring (Swearingen et al. 2002). Fruit production of T. bispinosa may be lower than for T. natans at similar depths (Dodd and Schad 2021). Asexual reproduction occurs from stolons and stem fragments, typically prior to allocation of resources to fruit production (Groth et al. 1996, Les 2018).

Habitat: shallow (less than 3 meters), nutrient-rich lakes, ponds, canals, and slow-moving rivers and streams, especially bays (Les 2018). Trapa bispinosa may produce more fruits at shallow (<1 m) compared to deep (>2 m) waters while T. natans fruit production seems highest between depths of 1-2 m (Dodd and Schad 2021).

Tolerances: pH range of 6.7 to 8.2 and alkalinity of 12 to 128 mg/L of calcium carbonate (Les 2018).

Community interactions: Flowers are fertilized by generalist insects from Coleoptera and Hemiptera (Kadono and Schneider 1986).

Means of Introduction: Trapa natans was originally introduced by intentional ornamental plantings, followed by escape from ornamental ponds, hitchhiking on waterfowl, and dispersal downstream. It spreads either by the rosettes detaching from their stems and floating or carried by boats and trailers to another area, or more often by the nuts being swept by currents or waves to other parts of the lake or river (Bickley and Cory 1955; Mirick 1996; Hummel and Kiviat 2004). There is speculation that waterfowl may vector the nuts via their plumage, but doubt is placed on the likelyhood of long-distance dispersal by waterfowl due to the weight of the nuts containing viable seed (Les and Mehrhoff 1999).


Eradicated from the Potomac River in the District of Columbia and Virginia (Carter and Rybicki 1994).

Likely cultivated in Delaware in 1874 (see collection by Albert Commons at NY2376848).

Reports from Kentucky and West Virginia USACE reservoirs were likely mistaken identities (L. Dodd, USACE-ERDC, pers. comm. 2017).

Impact of Introduction:
Summary of species impacts derived from literature review. Click on an icon to find out more...

EcologicalEconomicHuman Health

Ecological impacts

Trapa natans creates nearly impenetrable mats across wide areas of water (Countryman 1978, Winne 1950, Kiviat 1993). Dense mats of T. natans severely limit the passage of light into the water, a critical element of a well-functioning aquatic ecosystem, reducing oxygen levels, which may increase the potential for fish kills (Caraco and Cole 2002, Kornijow et al. 2010). Trapa natans decreases dissolved inorganic nitrogen, potentially reducing eutrophication from runoff waters (Tsuchiya and Iwakuma 1993). It also accumulates heavy metals such as cadmium, nickel, and cobalt (USEPA 1989). Invertebrate, fish, bird, and mammal foraging habitat may be provided by T. natans (Findlay et al. 1989, Yozzo and Odum 1993, Coote et al. 2001, Winne 1950, Kiviat 1993).

Economic and human health impacts

The presence of T. natans mats restricts recreational activities such as swimming, fishing from the shoreline, and the use of small boats are eliminated or severely impeded (Bickley and Cory 1955, Bove and Hunt 1997). Mature T. natans pose a health threat, as the plants’ nuts drift to shore where their sharp spines may hurt bare feet (Gwathmey 1945).

Remarks: Trapa natans has been valued for its nutrional and medicinal qualities in India, China, Hong Kong, Malaya, Thailand, and Russia (Hummel and Kiviat 2004). It was also tested and approved as a cattle feed supplement (Besha and Countryman 1980). Unfortunately, an unrelated edible aquatic plant, Eleocharis dulcis (Burm.f.) Trin. ex Henschel, a sedge in the Cyperaceae family, is also called water chestnut. The corm of E. dulcis is the familiar water chestnut, or Chinese water chestnut, sold in cans and commonly served in Chinese restaurants.

A number of animal species and one fungus species were found to consume Trapa (Pemberton 1999). At least three species of Coleoptera were documented feeding on Trapa leaves at high enough amounts to be considered for biocontrol: the leaf beetles, Galerucella birmanica Jacoby (=G. nipponensis Laboissiera) and G. nymphaeae L., and the weevil, Nanophyes japonica Roelofs (Ban 1983, Kadono and Schneider 1986, Ikeda and Nakasuji 2002, Ding et al. 2006).

Originally placed in Trapaceae, T. natans is now considered in the Lythraceae family based on molecular evidence (Graham 2005).

The Genus Trapa is derived from the Latin calcitrapa meaning "heel snare" (Les 2018).

References: (click for full references)

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Author: Pfingsten, I.A., L. Cao, L. Berent. L.O. Wishah, and C.R. Morningstar

Revision Date: 2/15/2022

Peer Review Date: 11/4/2015

Citation Information:
Pfingsten, I.A., L. Cao, L. Berent. L.O. Wishah, and C.R. Morningstar, 2024, Trapa natans L.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=263, Revision Date: 2/15/2022, Peer Review Date: 11/4/2015, Access Date: 4/20/2024

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [4/20/2024].

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