Water chestnut leaves and flower

Water chestnut (Trapa natans)

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 which 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

Regulated areas of water chestnut
Water chestnut is Prohibited (Red counties)

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:

  • Invades shallow to deep freshwater habitats in the northeastern United States.
  • Dense, floating mats restrict light availability, reduce the oxygen contrent, and displace other emergent and floating vegetation. Limits boating, fishing, swimming, and other recreational activities.

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 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. Begin to flower in mid to late July.

Fruits & seeds: Large (2.5 cm.), variously-shaped nuts are swollen at 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

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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

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Last revised: Wednesday December 19 2012