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  • Writer's pictureDon Yurewicz

Water Chestnut - Exotic

Water Chestnut has invaded portions of the Connecticut and Nashua rivers in New Hampshire and is present in nearby states.

Source: Northeast Invasive Aquatic Nuisance Species Panel Click Here

Water chestnut is a rooted submerged aquatic plant with two distinct leaf types. The floating leaves are somewhat triangular (or fan shaped) in form, with conspicuously toothed margins along the outside edges. The upper surface of the leaf is glossy, the undersides covered with soft hairs. The leaves are arranged in a loose, radiating pattern or rosette and joined to the submersed stem by long leaf stems, or petioles (up to 15 cm long). Spongy inflated bladders in the petioles provide buoyancy for the rosette. The rosettes are anchored to the sediments on stems reaching lengths of up to 15 feet.

The first submersed leaves to emerge are alternate, linear and entire, but these give way as the plant develops to feather-like finely divided, leaf-like roots (or root-like leaves—there is ongoing debate as to which is correct). The upper leaf-roots contain chlorophyll, causing them to be greener. When water levels drop, those lower down anchor the plant to sediments.

Small white flowers appear above the rosettes in mid to late July, each emerging from its own stalk from the axils of the floating leaves. When the fruits form they submerse and dangle beneath the rosette. The fruits are woody and nut-like, typically with four sharp barbs.

Invasive water chestnuts in Oxbow Lake, East Hampton, Massachusetts

Water chestnut can quickly form dense floating mats and out-competes native plant communities. Its decay can deplete oxygen levels, leading to fish kills. Dense growths can interfere with swimming, boating, fishing, and waterfowl hunting. Water chestnut was first discovered in North America in the late 1800s, imported as a showy water garden plant. It escaped to New Hampshire, Connecticut, Massachusetts, New York, Vermont, and Rhode Island. It spreads by rosettes, woody seeds, and plant pieces that break off and float on water currents that can then be picked up by boaters.

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