Mike Naylor, MD DNR

Water Chestnut

Trapa natans L.
Water chestnut family (Trapaceae)

Origin: Europe, Asia and Africa

Background
Water chestnut was first observed in North America near Concord, Massachusetts in 1859. The exact path for the introduction is unknown. It has been declared a noxious weed in Arizona, Massachusetts, North Carolina and South Carolina and its sale is prohibited in most southern states.

Distribution and Habitat
Water chestnut can grow in any freshwater setting, from intertidal waters to 12 feet deep, although it prefers nutrient-rich lakes and rivers. Presently, the plant is found in Maryland, Massachusetts, New York and Pennsylvania, with most problematic populations occurring in the Connecticut River valley, Lake Champlain region, Hudson River, Potomac River and the upper Delaware River.

Ecological Threat
Water chestnut can form dense floating mats, severely limiting light – a critical element of aquatic ecosystems. Once established, it can reduce oxygen levels, increasing the potential for fish kills. It competes with native vegetation and is of little value to waterfowl. Water chestnut infestations limit boating, fishing, swimming and other recreational activities. Further, its sharp fruits, if stepped on, can cause painful wounds.

Description and Biology

• Plant: annual aquatic plant with a submerged stem; stems can reach 12-15 ft. in length; very fine roots anchor the plant into the mud.
• Leaves: rosette of floating leaves at the water’s surface; saw-tooth margins; triangular in shape and connect to an inflated petiole which provides added buoyancy for the leafy portion; additional, feather-like leaves can be found along the submerged stem.
• Flowers, fruits and seeds: flowers are four-petaled and white; form in June; are insect-pollinated; fruit is a nut with four ½-inch, barbed spines; seeds can remain viable for up to 12 years, although most will germinate within the first two years.
• Spreads: by the rosette and by fruits detaching from the stem and floating to another area on currents or by clinging to birds and other floating objects.

Prevention and Control
Specialized methods of control are required to handle water chestnut infestations. Because of the likelihood of unintentional spread offsite and injury to those attempting control, only trained and certified persons should undertake management. Manual, mechanical and chemical techniques are used in its control. Complete removal of plants is imperative, as floating, uplifted plants and plant parts can spread the plant to new locations. It is critical that any removal take place prior to the July seed set. Eradication is difficult because water chestnut seeds may lay dormant for up to 12 years. Biological controls are being investigated, but no species have been approved for release.

Native Alternatives
Aquatic plant species are difficult to tell apart to the untrained eye. Contact your state natural resource agency, native plant society or other resource (see References) for assistance.

Leslie J. Mehrhoff, University of Connecticut