Water Chestnut

What is Water Chestnut?  Water chestnut (Trapa natans) is an aquatic plant native to Asia. In the Americas, water chestnut is an invasive species known for its aggressive growth habits. Read the Water Chestnut Eradication Report 1999-2007.

One acre of water chestnut can produce enough seeds to cover 100 acres the following year. With four, hard half-inch spines that are sharp enough to penetrate shoe leather and large enough to keep people off beaches, water chestnut seeds are major hazards to water contact recreation. Additionally, water chestnut can wipe out native bay grasses from some areas, prevents nearly all water use where it occurs, creates breeding grounds for mosquitoes, and provides only marginal habitat to native fish and birds.

Water Chestnut History in Maryland
Water chestnut was first recorded in North America near Concord, Massachusetts in 1859. Wild populations have since become established in many locations in the Northeastern United States. 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 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 at an estimated cost of $2.8 million (converted from 1950 to 1992 dollars), with follow-up removal by hand until at least 1965.

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, of which 30 acres were mechanically removed. 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.

Current Problems
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. The Sassafras population was slightly larger, but determining its exact size has been difficult due to its remote location. A massive mechanical and volunteer harvesting effort began on both rivers in 1999, and resulted in the removal of approximately 400,000 pounds of plants from the two rivers. As impressive as the 1999 effort was, the fact that water chestnut seeds can remain viable in sediments for up to 12 years, means that follow-up efforts are necessary.

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. Mechanical and manual harvesting took place again in 2001 and by 2002 there were not enough plants present at any of the locations to justify using a mechanical harvester. This was a significant milestone for the overall eradication effort. Since then control efforts have been performed on both rivers by small teams of DNR staff using personal watercraft (2003-2007).

Water chestnut populations on the Bird and Sassafras Rivers have been dramatically reduced. Despite these successes, the threat posed by the remaining small, but still significant populations requires that removal efforts continue to keep the population in check and prevent any further proliferation of water chestnut populations. Read the Water Chestnut Eradication Report 1999-2007.