An Invasive Species Success Story

Contact: Jonathan McKnight, Maryland Department of Natural Resources | 410-260-8539 |

Photo: Alfred Cofrancesco, U.S. Army Corps of Engineers,

ANNAPOLIS, MD (June 18, 2004) – There are few success stories in the history of the introduction of invasive exotic species to North America. After all, a true success is a species that is prevented from ever entering a North American ecosystem, and no one ever hears about it. However, there are some species that have been effectively controlled by aggressive action, and in Maryland, the Asian Water Chestnut is a great example.

The water chestnut (Trapa natans) is an aquatic plant native to Asia. Introduced into North America in 1859, the water chestnut became established in locations throughout the northeast and by the early 20th century was moving southward.

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, compared to native aquatic grass beds, provide poor habitat to native fish and birds.

The 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 downstream from Washington, D.C. The proliferation of water chestnut resulted in the loss of native aquatic grasses, and the U.S. Army Corps of Engineers responded by conducting a massive and well-funded removal effort from 1939 to 1945, even as war raged in Europe.

The 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 to control that 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 was used once again in 1965, to remove 200 acres from the Sassafras. This effort was believed to have been successful, and no plants had been noted in vegetation surveys until the summer of 1997.

The Bird River water chestnut population spread from approximately 50 plants in 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 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 will continue to be necessary.

In light of the potentially massive problems posed by the water chestnut, the Maryland Department of Natural Resources (DNR) continued mechanical and hand removal efforts in 2000, 2001 and 2002. As a result of these efforts, 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 help control its spread, and to prevent the establishment of new Maryland populations, the sale of all species of water chestnut are banned from most of the Southern United States, including Maryland.

The near-elimination of the water chestnut from the wild is a testimony to what determined action in the face of invasive species incursions can accomplish. And the US Army's aggressive assault on this foreign invader in the Potomac River, fought between the time when Hitler invaded Czechoslovakia and Poland and the final liberation of Europe, demonstrates that firm resolve and concerted action can accomplish ecological victories, even in the face of competing priorities.

For more information about water chestnut and other Invasive Species of Concern, visit the Maryland Invasive Species Council.