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

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

U.S. distribution by state and HUC8 drainage and/or county:

Connecticut: Housatonic, Lower Connecticut (Les and Capers 2012), Lower Hudson (Gibbons 2011), Quinebaug (Reid 2016), Quinnipiac (L. Dodd, USACE-ERDC, pers. comm. 2017), and Shetucket (IPANE 2001) drainages

Delaware: Centreville in Brandywine-Christina drainage (Pace and Thiers 2016)

District of Columbia: ponds of U.S. [Fish Comission], B. St. NW in Middle Potomac-Anacostia-Occoquan drainage (Pace and Thiers 2016)

Maryland: Chester-Sassafras (Batuik et al. 1992), Gunpowder-Patapsco (Hummel and Kiviat 2004), Lower Potomac (Knox 2017), and Middle Potomac-Anacostia-Occoquan (Carter and Rybicki 1994) drainages

Massachusetts: Blackstone, Housatonic, Middle Hudson (Seidler 2014), Charles (Hummel and Kiviat 2004), Chicopee, Deerfield, Lower Connecticut, Westfield (Center for Invasive Species and Ecosystem Health 2017), Concord (Mills et al. 1993), Gulf of Maine/Bay of Fundy (Olmsted 2010), Merrimack River (National Park Service 2013), Middle Connecticut (Barrington et al. 2015), Narragansett (Open Space Committee 2008), and Nashua (Shnitzler 2006) drainages

New Hampshire: Black-Ottauquechee (A. Smagula, NH DES, pers. comm. 2016), Nashua (New Hampshire Department of Environmental Services 2015), and West (Center for Invasive Species and Ecosystem Health 2017) drainages

New Jersey: Hackensack-Passaic, Mullica-Toms, Raritan, Rondout (Center for Invasive Species and Ecosystem Health 2017), Middle Delaware-Musconetcong (Smith 2009), and Sandy Hook-Staten Island (Crouse 2011) drainages

New York: Chaumont-Perch, Chenango, Hackensack-Passaic, Hudson-Hoosic, Lake Ontario, Lower Genesee, Middle Delaware-Mongaup-Brodhead, Northern Long Island, Oneida, Rondout, Salmon-Sandy, Schoharie, Southern Long Island, Upper Delaware (S. Kishbaugh, NYS DEC, pers. comm. 2015), Conewango (Lundin 2013), Hudson-Wappinger (Seigler 2014), Irondequoit-Ninemile, Middle Hudson (Titus 1994), Lake Champlain (Countryman 1970), Lower Hudson (Philbrick 2016), Mettawee River, Mohawk (Madsen 1990), Niagara (iMapInvasives 2016), Oswego (Coin Glenn 2000), Seneca (Krings 2011), and Upper Susquehanna (Hummel and Kiviat 2004) drainages

Pennsylvania: Crosswicks-Neshaminy, Lower Susquehanna-Swatara, Middle Delaware-Mongaup-Brodhead (Pennsylvania Flora Database 2011), Middle Delaware-Musconetcong, Schuylkill (Center for Invasive Species and Ecosystem Health 2017), and Upper Allegheny (iMapInvasives 2016) drainages

Rhode Island: Blackstone, Narragansett, Quinebaug (State of Rhode Island Department of Environmental Management Office of Water Resources 2015), and Pawcatuck-Wood (DeGoosh 2009) drainages

Vermont: Black-Ottauquechee (Winters and Audette 2016), Hudson-Hoosic (Hunt 2006), Lake Champlain, Mettawee River (Countryman 1970), and Otter Creek (A. Bove, VT DEC, pers. comm. 2003) drainages

Virginia: Potomac River, near Mt. Vernon in Middle Potomac-Anacostia-Occoquan drainage (Wofford et al. 2016), and a pond at Waples Mill Meadow Park in Middle Potomac-Catoctin drainage (Center for Invasive Species and Ecosystem Health 2017)

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).

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).

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

Environmental	Socioeconomic

Trapa natans offers little nutritional value for wildlife (IPANE 2013, Pennsylvania Sea Grant 2012, VDEC 2002). Water chestnut is also capable of an allelopathic response that inhibits the growth of phytoplankton (Lui et al. 2010a). These two impacts may alter existing predator/prey relationships as native species go elsewhere to search for food.

Large infestations of T. natans can reduce water flow and even clog waterways (CANSWG 2006, Naylor 2003, Pennsylvania Sea Grant 2012). During the growing season, dense surface mats block the air exchange between the water’s surface and the atmosphere (Pennsylvania Sea Grant 2012). Caraco and Cole (2002) found that beds dominated by T. natans had dissolved oxygen levels below 2.5 mg/l about 40% of the time. Low levels of oxygen caused by the presence of this species, makes T. natans populations unsuitable for fish species and likely effects the redox reactions in bottom sediments (Caraco and Cole 2002). When water chestnut populations die and sink, the decomposition of this large amount a plant material reduces the dissolved oxygen level even further and in extreme cases, can cause fish kills (IN DNR 2012, OISAP 2013, Swearingen et al. 2002, VDEC 2002).

Potential:
Areas of stagnant water caused by dense stands of T. natans create breeding grounds for mosquitoes (Naylor 2003).

Trapa natans has a high socio-economic impact in the Great Lakes.
Realized:
Large infestations of T. natans can reduce water flow and even clog waterways (Group 2006, Naylor 2003, Pennsylvania Sea Grant 2012). Dense patches of T. natans can hinder commercial navigation (IN DNR 2012, IPANE 2013).

Infestations of water chestnut can also limit or even prevent recreational activities such as boating, fishing, and hunting (WI DNR 2012). The hard, spiny seeds can punctuate leather and can cause painful wounds to humans and animals that step on them (Haber 1999, Swearingen et al. 2002). These nuts can also wash up and accumulate along the shore; reducing the access to beaches (IN DNR 2012, OISAP 2013).

The major economic costs associated with water chestnut populations are mechanical or chemical control efforts (Naylor 2003). The Pennsylvania Department of Conservation and Natural Resources (n.d.) states that this species costs hundreds of thousands of dollars to control.

Potential:
Millions of dollars have been spent on mechanical harvesting and manual removal of T. natans populations; these programs have had limited success (Wu and Wu 2006). Vermont spent almost $500,000 in 2000 to mechanically remove water chestnut (Pennsylvania Sea Grant 2012). From 1982-2005 various state organizations spent over $5 million to control T. natans in Lake Champlain (IPANE 2013).

In Vermont, many previously fished bays of southern Lake Champlain are now inaccessible, and floating mats of T. natans can create a hazard for boaters. Large stands of water chestnut may also restrict fish farming and batfish harvesting (Gunderson and Kinnunen 2004).

Trapa natans has a moderate beneficial impact in the Great Lakes.
Realized:
This ornamental plant has been used in ponds and outdoor water gardens (Liu et al. 2010).

The fruit has historically been used to treat conditions such as rheumatism and sunburn (Lui et al. 2010a). Once cracked open, the flesh inside the nut-like fruit can be cooked, eaten raw, or used in other foods (Lui et al. 2010a, Magness et al. 1971).

Potential:
Even though this is not the water chestnut typically found in Asian cuisine, T. natans is a food source typically used in Asia (O’Neill Jr. 2006). Dried nuts can be ground into flour for baking (Sturtevant and (ed) 1972).

In an experimental study, extracts from T. natans (combined with extracts from other species) decreased pain for patients suffering from shingles (Hijikata et al. 2005). In another study, an herbal mixture containing T. natans brought symptom relief to those suffering herpes genitalis and labialis outbreaks (Hijikata et al. 2007). A peptide contained in T. natans has anti-fungal properties (Mandal et al. 2011).

The husks from T. natans can be transformed into iron-modified activated carbon; an adsorbent compound that is able to remove chromium (VI) from wastewater (Lui et al. 2010b). In experiments in India, T. natans was able to remove a significant amount of mercury from paper mill effluent (Mishra et al. 2013). Trapa natans is also able to remove nitrite from the water (Rawat et al. 2012). Trapa natans can remove metals from contaminated water (Baldisserotto et al. 2007, Rai and Sinha 2011). Unfortunately, this species stores the toxic compounds in the edible parts of the plant; reducing the ability of this species to be utilized as a food source (Rai and Sinha 2011).

Strayer et al. (2003) found increased diversity in epiphytic and benthic macroinvertebrates in T. natans populations, compared to stands of native vegetation in the Hudson River (New York). Even with this increase in biodiversity, Strayer et al. (2003) concluded that these macroinvertebrates were not available to fish because of the low oxygen concentrations. Surveys conducted by Kornijów et al. (2010) also found dense, diverse benthic communities under floating mats of T. natans containing insects, oligochaetes, crustaceans, and other taxa. However, Kornijów et al. (2010) determined that water chestnut beds provided valuable habitat for invertebrate biodiversity and production, and may contribute substantially to fish production.

The Great Lakes Life & Wildlife Commission have not found T. natans in their ceded territories, but recommended immediate control upon detection (Falck and Garske 2003).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
An integrated management plan that incorporates multiple methods of control will be most effective at controlling populations of T. natans. Invaded habitats should be monitored for up to 12 years after control measures are complete to ensure that the seed bank is exhausted (PA DCNR n.d., Swearingen et al. 2002).

Biological
In its native range in China, the leaf beetle Galerucella birmanica has significant negative impacts on T. natans populations (Ding et al. 2006). However, this species has many other host species in the U.S., making it unsuitable for use as a biocontrol agent (Maryland Sea Grant 2012).

Physical
Smaller populations can be controlled by hand harvesting or raking because the roots are easily uplifted from the sediment (Naylor 2003). Larger populations, including those thick enough to clog waterways, may require the use of a large aquatic plant harvester (PA DCNR n.d.). Harvesting methods should be conducted before plants set seeds-- typically in July (Maryland Sea Grant 2012). All plant fragments, especially those containing roots, should be removed to prevent the expansion of the T. natans population (Swearingen et al. 2002). Plant fragments should be disposed of far from the water, preferably in a plastic bag (PA DCNR n.d.).

260,000 lbs. of water chestnut were removed by mechanical means and the help of over 60 volunteers from the Sassfras River (Maryland) during a three day harvest in 1999 (Naylor 2003). Mechanical removal methods have been used annually in Sodus Bay, New York since the 1960s, but the T. natans population persists (USEPA 2000). However, mechanical removal followed by an application(s) of 2,4-D was able to eradicate a population of T. natans in Maryland (Naylor 2003).

Laboratory and greenhouse studies by Wu and Wu (2006) demonstrated that ultrasonic waves of 20 kHz, aimed directly at water chestnut stems and petioles, for 10 seconds resulted in 100% plant death.

Chemical
Herbicides containing 2,4-D (both the amine and butoxy-ethyl ester formulations) have been effective in controlling T. natans (GLMRIS 2012, WI DNR 2012) Applying 2,4-D just as plants are reaching the surface of the water, in early summer, will provide the best results (USACE 2011). This compound causes minimal adverse effects on neighboring wildlife (Maryland Sea Grant 2012).

Herbicides containing triclopyr are also effective at controlling T. natans, but it is non-selective and may harm other plant life (GLMRIS 2012).

The growth and expansion of water chestnut populations can also be repressed if light attenuating dyes are applied prior to plant germination (GLMRIS 2012, USACE 2011).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

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).