Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut

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T. bispinosa foliage, roots, and 2-horned fruits.

ANNAPOLIS, MD (July 1, 2019) – Eurasian water chestnut (Trapa natans) has been present in the United States since the 1880s and is a well-known invasive species.  In 2014, a new introduction of a relative of T. natans was discovered in northern Virginia. It was recognized as new by fruits that have 2 horns in contrast to the 4-horned fruits associated with T. natans. By 2019, an investigation of numerous species of Trapa from around the world revealed that this species is Trapa bispinosa Roxb. var. iinumai Nakano. Its DNA and morphology matched samples of this morphotype in Taiwan. Because the US distribution of this two-horned morphotype, Trapa bispinosa, has expanded but is still potentially manageable, it has been chosen as the Maryland Invasive Species Council’s July Invader of the Month. Literature review and current data suggest that early detection and rapid response efforts could prevent its spread and further establishment in area waterways.

The newly recognized T. bispinosa has pink flowers, a fruit and seed with two sharp horns and no crown, and leaves that are reddish underneath. In contrast, T. natans has white flowers, four horns and a prominent crown, and leaves that are green underneath.

Trapa bispinosa was verified in 33 Northern Virginia locations by fall 2018. The number of sites has doubled each year for the last three years. The USGS Non-indigenous Aquatic Species database ( includes verified locations and information on the first year of colonization, current size of colonies, recent management efforts, and verification photos. All of the recent reported locations (2000 to 2018) are in Fairfax and Prince William counties in Virginia. Herbaria specimens showed it was in Westmoreland and Stafford counties in Virginia in 1995, but it was not reported in those counties after that. 

Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
The barbed seeds cling onto waterfowl plumage and seeds may be transported short distances by waterfowl in flight. Canada Geese are one species known to forage in and spread water chestnut.
Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut

Scientists conducting research on this 2-horned type of Trapa have not found this type of water chestnut in other states in the Northeast US. Reported occurrences are currently only in Virginia within the Potomac River watershed.  Trapa bispinosa is an annual, herbaceous aquatic plant with floating and submersed (underwater) leaves. The main stems of seedlings branch repeatedly to form a surface canopy of leaves arranged in a rosette. The floating leaves are alternate with triangular, toothed leaf blades and inflated petioles. Leaves of the rosette are green above and reddish below. The submersed leaves are opposite, linear, and fall off early to be replaced by fine, pinnately branched, leaf-like adventitious roots. The flowers are borne in the axils (the point of attachment of the leaf petiole to the stem) of the floating leaves and have four pink petals. The fruit has a green exterior that disintegrates, once the fruit falls into the water, to reveal a large, hard, nut-like seed 1½ – 2 in. (30 to 50 mm) wide, with two sharp upper horns that are barbed and two lower false or “pseudo” horns.  In the mid-Atlantic region, it sprouts in April, is at the surface by May, and spreads over the water surface and flowers in June. It may produce fruit from June until a hard frost. Invasive Trapa populations in the US are typically managed with hand-pulling or treatment with herbicides. In both cases, the ideal time of control for this annual, aquatic plant is before fruits are produced. Management between May and early-July has been very successful in eradicating the plants, but it may take several years of effort if seeds drop prior to harvest or lay dormant.

Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
T. bispinosa covers the water surface in this lake in Fairfax county, Virginia.  This species was verified in 8, 18, and 33 water bodies in 2016, 2017, and 2018, respectively.
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In August, 2014, the Virginia Department of Game and Inland Fisheries (VDGIF) found a patch (0.3 acre) of T. bispinosa in Pohick Bay (Gunston Cove, Fairfax County) in Lorton, Virginia, which is within the tidal portion of the Potomac River. It was originally mis-identified as T. natans.  In the past, the US Army Corps of Engineers eradicated T. natans from a 40-mile reach of the fresh, tidal, Potomac River using mechanical harvesters. Therefore, to manage the new colony of Trapa, VDGIF has coordinated a harvest each summer. The number of pounds harvested has diminished each year since 2015 when the harvesting period was adjusted to July, before the plants initiate fruit production.  

Field surveys in local waterways showed that T. bispinosa had spread, un-noticed until recently, into private and public ponds, including a pond upstream of Pohick Bay. The barbed seeds can disperse long distances by floating or by adhering to the feathers and fur of wildlife. Seeds may have hitchhiked among some ponds via resident Canada geese. 

Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
Illustration of the new Trapa bispinosa (drawing by Gabe Westergren)

All species of Trapa are native to the Old World and foreign to the New World (North America, Central America, and South America). Based on historic experience with the highly invasive Eurasian water chestnut in the US, the introduction of any Trapa to the US poses a serious risk requiring urgent attention. Therefore, all Trapa species qualify as “Early Detection, Rapid Response” invasive species. Volunteers are needed to report any colonies observed via the U.S. Geological Survey’s Nonindigenous Aquatic Species program ( or the University of Georgia, Center for Invasive Species and Ecosystem Health’s Early Detection and Distribution Mapping System (EDDMapS), using the Mid Atlantic Early Detection Network (MAEDN) phone app ( The iNaturalist ( can also be used. Reports should include pictures of the overall colony and close-ups of the leaves, seeds, fruits, flowers, and rosette to aid in identification of the species. Expert verifiers will review submitted reports with close-up photos to confirm or correct the identification.

Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
Foliage and pink flower of T. bispinosa.
Teaming up to Tackle Two-horned Trapa: A Highly Invasive New Species of Water Chestnut
These photographs show a comparison of the fruits of Trapa bispinosa (left) and Trapa  natans (right).

Photo credits:

Nancy Rybicki.  


Nancy Rybicki, US Geological Survey, Reston, VA (,

Jil Swearingen, Invasive Species Consultant, In the Weeds, Cheverly MD (, John Odenkirk, Virginia Game and Inland Fisheries, VA (

For further information:

Chorak, G. M., Dodd, L. L., N. Rybicki, K. Ingram, M. Buyukyoruk, Y. Kadono, Y. Y. Chen,  and R. A. Thum, 2019. Cryptic introduction of water chestnut (Trapa) in the northeastern United States, Aquatic Botany.

Maryland Department of Natural Resources, 2018. Water Chestnut Eradication Report.

Naylor, M., 2003, Water Chestnut (Trapa natans) in the Chesapeake Bay Watershed: A Regional Management Plan. Maryland Department of Natural Resources.

Rybicki, N. and J. Swearingen, 2018. Trapped Again? A New Species of Water Chestnut Discovered in the Potomac River – Invader of the Month (June, 2018). Maryland Invasive Species Council.

United States Department of Agriculture, 2016. Weed Risk Assessment for Trapa natans L. (Lythraceae) – Water chestnut.

Web resources:
US Trapa bispinosa Distribution Map: