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Water chestnut

(Trapa natans)

Photo of water chestnut
Photo credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Fast-growing, floating perennial herb. Cord-like plant stems can reach upwards of 16 ft. Exhibits great reproductive capacity, as it is an annual plant that over-winters entirely by seed. An individual seed can give rise to 10-15 rosettes, each of which can produce 15-20 seeds. Thus, one seed can give rise to 300 more new seeds in a single year! This is not the same species as is used in Asian cooking.

Overview

Other names for this plant include:
  • Common names: bull nut, European water chestnut, water nut
  • Scientific names: T. natans var. natans; T. natans var. bisponosa; T. bisponosa
Ecological threat:
  • It invades shallow to deep freshwater habitats in the northeastern United States.
  • Dense, floating mats restrict light availability, reduce the oxygen content and displace other emergent and floating vegetation. Limits boating, fishing, swimming and other recreational activities.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for water chestnut was based upon this literature review [PDF] developed by the department.

Identification

Leaves: Upper leaves are alternately arranged in clusters up to 50 cm across, shiny on the upper side and dull with fine hairs underneath and diamond-shaped with toothed edges; submersed leaves are oppositely arranged, long and narrow, with green feather-like structures that often replace the linear underwater leaves. Upper leaves are attached to the stem with an inflated petiole, which keeps them afloat.

Flowers: Small and solitary, four white or light purple petals on short, thick stalks that float among the upper leaves; the four sepals turn into the spines of the fruit. It begins to flower in mid to late July.

Fruits & seeds: Large (2.5 cm), variously-shaped nuts are swollen in the middle and have 2-4 sharp spines. Each nut contains a single, fleshy seed. Mature nuts sink to the bottom when dropped and may be able to produce new plants for up to 12 years.

Roots: Develop on shoots. Lower roots are unbranched and thread-like, while upper roots are sparsely branched and fibrous.

Similar species: Trapa bispinosa is considered a variant of T. natans. The nuts have two to four short spines compared to the two large spines of T. natans.

Distribution

Currently, there have been no reports of water chestnut in Wisconsin. Have you seen it? Send us a report.

Control

Mechanical: Manual or mechanical harvesting is effective at reducing larger populations and eradicating small populations.

Chemical: Aquatic approved 2, 4-D has been an effective herbicide for control.

Photos

View water chestnut pictures in our photo gallery!

Resources

Sources for content:
  • Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. University of Wisconsin Press. 2005. Pg. 148
  • Center for Invasive Species and Ecosystem Health, Invasive.org. Last updated on May 4, 2010. Water Chestnut.
  • Washington State Department of Ecology: Trapa natans [exit DNR]
  • New Hampshire Department of Environmental Services. Environmental Factsheet WD-BB-43. Water Chestnut Discovered in New Hampshire Waters [exit DNR]
  • Global Invasive Species Database: Trapa natans [exit DNR]. Last updated Sept. 28, 2005.
Links for more information: