Water chestnut (Trapa natans) is an invasive aquatic annual plant that bears a rosette of floating leaves at the top of a submerged and rooted stem.  T. Natans is not the same as the water chestnut that can be found in supermarkets and used in Asian Cuisine. 

Origin/Introduction: Water chestnut is native to Eurasia. It was intentionally introduced at the Cambridge Botanical Garden and in a pond in Concord, Massachusetts in the 1800s where it escaped cultivation and spread throughout the northeast U.S.

Habitat: Water chestnut can be found in freshwater lakes and ponds and prefers slow-moving shallow waters high in nutrients with a neutral or alkaline pH.


Water chestnuts form large-spanning floating mats that shade out native aquatic vegetation and impede water recreation. The pointy seeds of water chestnut can penetrate and injure feet if stepped on. Additionally, the decay of dead water chestnut populations can result in areas with reduced dissolved oxygen levels which affects fish and other aquatic wildlife.   

European water chestnuis an aquatic plant, which is usually rooted in the mud; it bears a rosette of floating leaves at the tip of the submersed stem. Although it grows best in shallow, nutrient-rich lakes and rivers, it can also grow on wet, mucky substrates. Stems – Stems are elongate, flexuous and typically about a meter long but may reach as much as 5 meters in length.
Leaves – The conspicuously toothed leaf blades are ½-1 inch long and rhombic in outline (with four sides); the spongy, inflated leaf stalks are up to 3 inches long and provide the buoyancy to keep the terminal leafy portion of the plant floating at the water surface. Green, feather-like, submersed leaves (considered by some to be modified roots) with very fine segments are present on the underwater portion of the stem.
Roots – Numerous finely branched roots develop along the lower stem and assist in anchoring the plant to the substrate.
Flowers – The inconspicuous flowers with their four white petals, each about 1/3 inch long, are borne singly on erect stalks located in the central area of the leafy rosette.
Fruit – The fruit is a black, four-horned, nut-like structure, about an inch wide, that develops under water


Leaves float on the surface forming a rosette. Floating leaves are each 3/4th to 1 ½ inches in length, waxy, triangular, and toothed. Green, feather-like leaves up to six inches in length form in alternate pairs along the submerged portion of the stem.

Stems are elongated and typically about a meter long but may reach as much as 5 meters in length. 


Petioles have a bladder-like swelling filled with air and spongy tissue that provides buoyancy to the floating rosette. 


Flowers are inconspicuous with their four white petals, each about 1/3 inch long, and are borne singly on erect stalks located in the central area of the leafy rosette. Blooms occur in mid to late July and continue until frost. 


Seeds also called nutlets grow along submerged stems and have a nut-like structure that is about an inch wide with 4-5 pointed edges. Nutlets begin forming in mid-summer, typically in July, they are green when immature and turn black as they age. Each nutlet can produce 10 – 15 new plants; in turn, each rosette can produce 15 – 20 seeds.  Nutlets are easily dispersed in water and can remain viable in sediment for 12 years. 



Because water chestnuts produce a large number of seeds, treatments may have to be repeated until the seed bank in the sediment is exhausted.

Physical/Mechanical Control: Depending on the size of the infestation, water chestnut populations can be managed by hand pulling rosettes (floating portion) before seeds have become fully mature; depending on your location, June -July and sometimes into early August is a good time of year to remove water chestnut by hand.

Since water chestnut is an annual plant that regenerates by seed, you do not need to remove the entire plant to control it. You can snip off the floating rosettes of the plant and remove any visible nutlets. Removing just the floating rosettes of water chestnut also reduces disruption to the sediment and limits the accidental removal of native plants during a manual removal effort.

Mechanical harvesters can be used to remove larger infestations.

Chemical Control: Larger infestations of water chestnut can be controlled by herbicides; the use of the chemical application is effective in controlling the annual growth of the plant but not the ripened seeds (which remain viable up to 12 years!), therefore, herbicides should be applied before seeds are produced.

Permits are required for herbicide use and must be applied by a licensed applicator.

To quantify water chestnut removal efforts, identify gaps in management, and track the effectiveness of removal efforts, it is important to survey the treatment area prior to a removal effort and to report treatment and post-treatment data to iMapInvaives using this step-by-step guide

For more information:

Photo Credits:

Leaves photos
Illustrated photo 

Petioles photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org.

Flower photo: Ohsaka City University Botanical Garden, http://flowers.la.coocan.jp/Lythraceae/Trapa%20natans%20quadrispinosa.htm

Seeds photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org.

Stem photo: Roberta Hill, Maine Volunteer Lake Monitoring Program (VLMP) 2007, http://www.mainevlmp.org/mciap/herbarium/WaterChestnut.php


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