Trapped Again? A New Species of Water Chestnut Discovered in the Potomac

Trapped Again? A New Species of Water Chestnut Discovered in the Potomac
Trapa sp. 2-horned fruits.

ANNAPOLIS, MD (June 1, 2018) – Eurasian water chestnut (Trapa natans) has been present in the United States since the latter half of the 19th century and is a well-known invasive species.  Once water chestnut shows up in a water body, it can quickly spread to cover large areas and, if allowed to reproduce, can spread far beyond the initial site.  Colonies produce extensive floating mats that cover the water surface and shade out submerged aquatic vegetation. A suspected new introduction of a relative of Eurasian water chestnut was discovered within the freshwater reaches of the Potomac River in Virginia in 2014.  It was recognized as new by fruits that have 2 horns in contrast to the 4-horned fruits associated with T. natans.  This new species occurred in the Potomac River and in ponds in the Potomac River watershed as recently as the summer of 2017.  It has been given the name “Trapa sp.” until its specific identity can be determined.  Because Trapa sp. poses a great threat to waterways, presumably similar to its Eurasian cousin, it has been chosen as the Maryland Invasive Species Council’s June Invader of the Month.

Trapa sp. 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 to reveal a large, hard, nut-like fruit 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, plants flower after mid-June and may produce fruit from late June until a hard frost.

In 2014, Trapa sp. was found in Pohick Bay (Gunston Cove, Fairfax County) in Lorton, Virginia, which is within the tidal portion of the Potomac River.  It has been harvested each year since to prevent its spread in tidal water.  By 2017, it was found in additional waterways such as storm-water management ponds and other ponds in Fairfax and Prince William Counties.  The barbed seeds can disperse long distances by floating or by adhering to the feathers and fur of wildlife.  Seeds may have hitchhiked among ponds via resident Canada geese. Evidence suggests that Trapa sp. has been spreading cryptically in the Potomac watershed for over a decade.   Nancy Rybicki, Ryan Thum, Lynde Dodd, Kadiera Ingram, and other scientists have analyzed characteristics of the fruits and flowers as well as the genetics of all known colonies in the Potomac River watershed in an effort to determine which species are in this system.

Trapped Again? A New Species of Water Chestnut Discovered in the Potomac
Trapa sp. leaves and seed

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 U.S., the introduction of any Trapa to the U.S. 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 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 ( and (or) the U.S. Geological Survey’s Nonindigenous Aquatic Species program phone app (  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.

For further information:

Aquatic Nuisance Species Task Force, Water Chestnut Management Plan

Dodd, L., Rybicki, N., Thum, R., Kadono, Y., and Ingram, K. Genetic and morphological differences of water chestnut (Myrtales: Lythraceae: Trapa) populations in the Northeastern United States, Japan, and South Africa (2018, in review, USACOE Special Report) corresponding author, Lynde Dodd,

Interstate Commission on the Potomac River Basin – The Past, Present and Future of Potomac Aquatic Plants and Why They Matter.

Maryland Department of Natural Resources, Water Chestnut Eradication Report

U.S. Geological Survey, Calendar of events for submersed aquatic vegetation and Trapa natans in the tidal Potomac River and transition zone of the Potomac Estuary

U.S. Geological Survey, Nonindigenous Aquatic Species Database

Phone Apps

Mid Atlantic Early Detection Network (MAEDN) (phone app for reporting any colonies observed is available at this address)

U.S. Geological Survey, Nonindigenous Aquatic Species Reporting (phone app for reporting a sighting is available at this address)

Photo credits

Nancy Rybicki.  Trapa sp., Pohick Bay, Gunston Cove, Lorton, VA, August 2014


Nancy Rybicki, US Geological Survey, Reston, VA. Contact:

Jil Swearingen, Invasive Species Consultant, In the Weeds, Cheverly MD. Contact: