Water Chestnut

Water chestnut (Trapa natans) is an aggressive, invasive aquatic plant native to Europe, Asia and Africa. Though it only occurs in isolated areas in the Bay region, water chestnut spreads rapidly and can reappear after several years, which has resulted in a series of costly eradication efforts.

How was water chestnut introduced to the Bay watershed?

Water chestnut was first recorded in North America in 1859 near Concord, Massachusetts. It is believed to have spread from ornamental use in ponds. Wild populations have since become established in many locations throughout the northeastern United States, including the Hudson River, Lake Champlain region and Connecticut River valley.

In the Bay watershed, water chestnut was recorded in the Bird River in Baltimore County, Maryland, in 1955. The state of Maryland used an herbicide and mechanical removal to control the population. In 1964, it reappeared at the same site, along with 100 acres in the Sassafras River on Maryland's Eastern Shore. Populations were again removed in 1965 but reappeared in 1997.

Why is water chestnut considered to be invasive?

Water chestnut infestations have many negative ecological and economic impacts.

• Water chestnut threatens native underwater bay grasses by forming a complete canopy with up to three layers of leaves, blocking all sunlight from reaching bay grasses growing on the bottom.
• Thick water chestnut beds prevent nearly all recreational water use because they impede navigation.
• With four hard, half-inch spines that are sharp enough to cut through shoe leather, water chestnut seeds are a major hazard to beach activities.
• Water chestnut does not provide good habitat for native fish and invertebrates and has low food value for waterfowl.
• Beds of water chestnut are breeding grounds for mosquitoes.

Removal of water chestnut in the Potomac River from 1939 to 1945 cost the U.S. Army Corps of Engineers an estimated $3.7 million (in 2004 dollars), and had to be followed up with hand removal until at least 1965.

What is being done to manage water chestnut?

In response to the return of water chestnut populations in the Bird and Sassafras rivers in 1997, a massive mechanical and volunteer harvesting effort began in 1999 on both rivers. About 400,000 pounds of water chestnut were removed from the two rivers. However, since water chestnut seeds can remain viable for up to 12 years, follow-up surveys and removals continue to ensure a large population does not return.