Water chestnut (Trapa natans) is an invasive aquatic annual herb that bears a rosette of floating leaves at the top of a submerged and rooted stem. T. natans
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.
This fast-growing, aquatic plant forms large mats that shade out native aquatic vegetation and has the ability to completely dominate surface waters. It reduces oxygen levels for fish and encourages sedimentation by restricting silt movement. Hard, pointy seeds can penetrate shoe leather if stepped on.
Leaves: Float on the surface forming a rosette and are waxy, triangular, and toothed. Green, feather-like, submersed leaves on the underwater portion of the stem are paired and are considered by some to be modified roots.
Petioles: Have a bladder-like swelling filled with air and spongy tissue that provides buoyancy to the floating rosette.
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. Blooms occur in mid to late July and continue until frost.
Seeds: a black, four-horned, nut-like structure, about an inch wide, that develops underwater. Each seed can produce 10 – 15 new plants; in turn, each rosette can produce 15 – 20 seeds. Seeds are easily dispersed in water and can remain viable in sediment for 12 years.
Stem: stems are elongate, typically about a meter long but may reach as much as 5 meters in length.
Note: Because water chestnut produces 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 (June).
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.
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