Water Chestnut (Trapa natans)

Air bladders on leaf stems and woody fruit; Photo by Vermont DEC

Water chestnut is an annual, rooted aquatic plant with a leafy rosette that floats on the surface. Stems can reach up to 16 feet, supporting two types of leaves. Submersed leaves are feathery, whorled along the stem and up to 6 inches long. Leaves are triangular, 1 to 2 inches long, waxy and have toothed edges. Leaf stems have a bladder-like swelling that provides buoyancy. Flowers are small, white, have 4 petals and bloom from mid-July to fall frost. The fruit is a one-inch wide woody nut armed with 4 stout barbs. The sharp barbs are a painful hazard to swimmers and bare feet. Note: This is not the water chestnut used in Asian cooking.

Water chestnut, originally from Eurasia, is well established in a few water bodies in northeastern states but it is not yet known in Wisconsin. Each rosette can produce 15 nuts per season which sink to the bottom and remain viable for up to 12 years. The seeds germinate in spring and each new plant can divide into 10 - 15 rosettes over the summer. One acre of water chestnut can produce enough seeds to cover 100 acres the following year. Dense mats of stems and floating rosettes can deplete oxygen levels, choke out native species and hinder navigation. Water chestnut spreads by seed, by rosettes that break apart and float to new locations, and by fragments that attach to boats and trailers. It prefers calm, nutrient-rich lakes or streams with slightly alkaline waters.

Infestations can be eradicated by hand-pulling or through mechanical harvest, but sites must be monitored for many years to spot plants germinating from dormant seeds. Use of chemical herbicides in all Wisconsin aquatic environments (streams, lakes & wetlands) requires a permit from the DNR. Mechanical harvesting may require a permit.

USDA
USGS
Vermont Department of Environmental Conservation

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