Water Chestnut

What is Water Chestnut?  Water chestnut (Trapa natans) is an aquatic plant native to Asia. Introduced to North America near Concord, Massachusetts in 1859, water chestnut became established in locations throughout the northeast and by the early 20th century was moving southward. It can be found from Maryland all the way north to Maine and in some parts of Canada Read the Water Chestnut Eradication Report 1999-2009.

This aggressive species is a prolific reproducer. One acre of water chestnut can produce enough seeds to cover 100 acres the following year. With four, hard half-inch long spines that are sharp enough to penetrate shoe soles and large enough to keep people off beaches, these seeds are major hazards to water recreation. Additionally, water chestnuts can wipe out native bay grasses from some areas, create breeding grounds for mosquitoes, and provide poor habitat to native fish and birds. To help control its distribution, the sale of all species of water chestnut are banned from most of the Southern United States, including Maryland.

Water Chestnut History in Maryland
Water chestnut first appeared in Maryland in the Potomac River near Washington, D.C. as a two-acre patch in 1923. The plant spread rapidly, covering 40 river miles within a few years. By 1933, 10,000 acres of dense beds extended from Washington, D.C. to just south of Quantico, VA. This resulted in the loss of bay grasses, and the U.S. Army Corps of Engineers responded by conducting a massive removal effort from 1939 to 1945, with follow-up removal by hand until at least 1965. The cost was estimated at $2.8 million (converted from 1950 to 1992 dollars).
Water chestnut was recorded in the Bird River in Baltimore County for the first time in 1955. The Maryland Departments of Game and Inland Fish and Tidewater Fisheries used mechanical removal and an herbicide (2,4-D, the only fully-licensed herbicide that has been successfully used to control water chestnut) to control the population. However, in 1964 it reappeared in the Bird River and an additional 100 acres were discovered in the Sassafras River in Kent County. A combination of removal techniques were used once again in 1965, when 200 acres existed in the Sassafras. This effort was believed to have been successful, and no plants had been noted in vegetation surveys until summer 1997.

The Bird River water chestnut population spread from approximately 50 plants in summer 1997 to over three acres in 1998, and approximately 30 acres in 1999. A massive mechanical and volunteer harvesting effort began on both rivers in 1999, resulting in the removal of approximately 400,000 pounds of plants from the two rivers. In light of the potentially massive problems posed by water chestnut, the Maryland Department of Natural Resources (DNR) continued mechanical and hand removal efforts. Less than 1000 pounds of plants were discovered and removed from both rivers in 2000, indicating that the 1999 removal efforts were successful in reducing the total number of plants. In 2001, a large volunteer force was used instead of the mechanical harvesters, which was a significant milestone for the overall eradication effort.

Current Problems
In 2009, approximately 10 bushels of plants were removed from the Bird River in Railroad Creek and Taylor Creek (adjacent to Mariner Point Park). DNR biologists worked with volunteers from the Bay Program and CBNERR to survey and remove plants on the Sassafras River. Only 2 bushels of water chestnut were removed from the Sassafras in 2009, down from 60 bushels in the prior year.

Water chestnut seeds can remain viable in sediments for up to 12 years, requiring follow-up surveys each year. DNR will continue its annual survey to ensure the Bird and Sassafras remain free of this floating invasive species. If you have observed water chestnut in these or any other rivers in Maryland, please contact Mark Lewandowski at 410-260-8634 or email mlewandowski@dnr.state.md.us Read the Water Chestnut Eradication Report 1999-2009.