European Water Chestnut

  • European Water Chestnut

    (Trapa natans)

    European Water Chestnut is an invasive aquatic plant native to Europe, Asia and Africa. Imported as an ornamental water garden plant, it was first observed in North America near Concord, Massachusetts in 1859. Since its arrival in North America, it has become a serious nuisance by forming dense mats of floating vegetation that shade out native vegetation, decrease plant biodiversity and seriously affect swimming, angling and boating in affected areas. The hard nuts have barbed spines which may cause injury when stepped on.

    The European Water Chestnut is an annual plant that reproduces by vegetative means and seeds, which remain viable for up to 12 years. Containing hundreds of seeds, the nuts sink to the water bottom where the seeds germinate in the spring. One acre of water chestnuts can produce enough seeds to expand to 100 acres the next year. European Water Chestnut plants may be spread by re-rooting of leaf pieces and distribution of nuts downstream when individual plants are uprooted. Due to the size and weight of the nuts, it is unlikely they are spread by waterfowl or water currents.

    Because the plant is an annual, control methods focus on removing the plant before it blooms and spreads seeds. Plants are pulled out and disposed of far away from water. Destroying one plant will prevent up to 120 plants from growing next year.

    Range

    European Water Chestnut is established in the northeastern United States, with isolated populations in southern Quebec and on the Ottawa River within Voyageur Provincial Park.

  • Identification

    • The European Water Chestnut does not look like any other water plant in Ontario.
    • It has floating green, glossy roundly triangular leaves with sharply toothed edges that form a densely crowded circular arrangement called a rosette up to 30 cm in diameter.
    • The plant is rooted underwater with a stem reaching up to 4-5 metres in length with submerged feathery leaves and a spongy section that keeps the rosette afloat. The rosettes form dense floating mats in freshwater bodies, less than 4-5 metres deep.
    • Flowers emerge in mid-to-late July, with four small, white petals about 8 mm long and attached to the centre of the stem.
    • Hard nuts about 3-4 cm wide ripen a month later with four short, sharp spines that can pierce shoes. Viable nuts are greenish brown while older nuts are black and not viable. Viable nuts float, nonviable nuts sink.

    Impact

    • The European Water Chestnut can form dense floating mats of rosettes that choke a water body by limiting light and oxygen for native plants. They alter the habitats of native plants and offer little nutritional value for wildlife.
    • The dense mats also restrict boating routes and make swimming impossible while the nuts can cause painful puncture wounds.
    • Controlling European Water Chestnut is difficult and expensive; a combination of pulling plants by hand, rake and mechanical harvesters is used on the Ottawa River.

    How can you help?

    • Learn to identify European Water Chestnut and how to prevent spreading the plants and seeds.
    • Do not plant European Water Chestnut in your water garden; use only native or non-invasive plants.
    • Do not release aquarium plants or pets into any waterbody; return or donate to a pet store, garden centre, school or friend.
    • Inspect your boat and equipment for any plants or animals before leaving a water body.
    • Avoid boating on the Ottawa River within Voyageur Provincial Park if possible.
    • If you see European Water Chestnut or a plant that you think might be this invader, report it on the toll-free Turtle Island Invaders hotline at 1-844-872-2348 (1-844-TRACE-IT) and report it to your local Lands or Natural Resources officer. Be sure to include understandable directions to the location.
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