Water chestnut (Trapas nantans L.) is an aquatic plant that bears a rosette of floating leaves at the top of the submerged and rooted stem.
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.
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) after the seeds have formed but before they are ripe (early August). Mechanical harvesters can be used as well but may be costly. Take care to remove all plant fragments as plants can regenerate.
Chemical Control: Larger infestations of water chestnut can be controlled by herbicides; use of chemical application is effective in controlling 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.
Title photo: Erica Burgeson pulling water chestnut. Credit: Katie Little, The Nature Conservancy.
Leaves photos: top photo, http://www.thesurvivalchannel.com/post-4617-WaterChestnutWaterCaltrop.html.
bottom (illustrated)photo, http://www.goodyearlakeny.org/our-lake.html
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