Habitat: Water chestnut grows in the floating-leaf and submersed plant community. It thrives in the soft sediments of quiet, nutrient rich waters in lakes, ponds, and streams. The plant is well adapted to life at the water’s edge, and prospers even when stranded along muddy shores.
Description: Water chestnut has two distinct leaf types: floating and submerged. 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, while the underside is 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 up to 5 metres. 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. 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.
Origin and Range: Water chestnut is native to Europe, Asia, and tropical Africa. It is cultivated in Asia and other parts of the world where the fruit is eaten. It has not yet been found in New Brunswick. However, in Canada, it can be found in Quebec and Ontario and can be found in two New England states in the U.S, Vermont and Massachusetts.
Annual Cycle: Unlike most aquatic plants, water chestnut is a true annual. Plants sprout anew each year from seeds overwintering in the sediments. Submersed stems grow rapidly to the surface, where the floating rosettes form and the flowers and fruit develop. During the growing season rosettes may become detached and float to new areas. Water chestnut flowers from July to September. The fruit, or nuts, begin to appear by late summer. Each water chestnut seed can produce 15 to 20 new rosettes and each rosette can generate up to 20 seeds. At the end of the growing season, frost kills the plant and decomposition is rapid. The nuts fall and sink into the sediment where they overwinter and sprout in the spring. The nuts remain viable for up to 12 years but most germinate within 2 years. The nuts have sharp barbs that readily attach to boating gear and wildlife and are easily dispersed by natural and human processes.
Look Alikes: Water chestnut is easily distinguished from other aquatic plants in our region.
Impacts: Water chestnut can form extremely dense floating mats of vegetation that shade out native vegetation, decrease plant biodiversity, and make recreational activities like swimming, angling, and boating almost impossible in the infested areas. Reduced light penetration and plant growth beneath the water chestnut canopy, combined with a large amount of decomposing vegetation below, can lead to decreased dissolved oxygen levels, which can impact native species and cause fish kills.