Disclaimer:

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.

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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.gov

Identification: According to Crow and Hellquist (2000):

Habit: floating, rooted, aquatic annual

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

Leaves: heterophyllous; submerged pinnate or linear leaves 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).

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

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

Ecology:

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). Seed germination requires at least nine weeks of cold stratification below 30°C (Phartyal et al. 2018). Growth occurs in April and flowers and fruits develop from July to October at the end of the growth period (Palm et al. 2024). 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).

Status:

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...

Ecological	Economic	Human 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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Other Resources:

Ecology and Management of Invasive Plants Program

New York Invasive Species Clearinghouse

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Water chestnut in Maryland from MD DNR

US Fish and Wildlife Service Ecological Risk Screening Summary for Trapa natans

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

Peer Review Date: 11/4/2015

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.