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   Trapa natans (aquatic plant)     
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      Flowers (Photo: Leslie J. Mehrhoff, University of Connecticut) - Click for full size   Single rosette showing horned fruits and the inflated leaf petioles which enables the rosette to float (Photographer: Alfred Colfrancesco, US Army Corps of Engineers) - Click for full size   Floating rosettes (Photo: Leslie J. Mehrhoff, University of Connecticut) - Click for full size   Infestation on Lake Champlain, New York (Photo: Alfred Colfrancesco, US Army Corps of Engineers) - Click for full size
    Taxonomic name: Trapa natans L.
    Synonyms:
    Common names: bull nut (English), European water chestnut (English), water chestnut (English), water nut (English)
    Organism type: aquatic plant
    Trapa natans, commomly known as water chestnut, is an annual plant introduced from Asia and has become abundant in the northeastern United States where it creates a nuisance in lakes, ponds, canals and other slow-moving water. Trapa natans grows best in shallow, nutrient-rich lakes and rivers and is generally found in waters with a pH range of 6.7 to 8.2 and alkalinity of 12 to 128 mg/L of calcium carbonate. Trapa natans out-competes native plants for sunlight and spreads either by the rosettes detaching from their stems and floating to another area, or more often by the nuts being swept by currents or waves to other parts of the lake or river.
    Description
    According to Methe et al. (1993, in CBP, 2002), T. natans is an annual aquatic plant with a submerged flexuous stem that anchors into the mud and extends upward to the surface of the water. T. natans features a rosette of floating, fan-shaped leaves, each leaf having a slightly inflated petiole (stem); the roots are fine, long and profuse. The small, 4-petalled flower is white and the fruit is a large nut with four sharp spines.
    Similar Species
    Trapa bispinosa

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    Occurs in:
    lakes, water courses, wetlands
    Habitat description
    Trapa natans can be found in lakes, ponds, canals, and slow water. T. natans grows best in shallow, nutrient-rich lakes and rivers and is generally found in waters with a pH range of 6.7 to 8.2 and alkalinity of 12 to 128 mg/L of calcium carbonate (Methe et al. 1993, in CBP 2002).
    General impacts
    According to VDEC (2002), T. natans is a fierce competitor in shallow waters with soft, muddy bottoms. Uncontrolled, it creates nearly impenetrable mats across wide areas of water. In Vermont, USA, many previously fished bays of southern Lake Champlain are now inaccessible, and floating mats of T. natans can create a hazard for boaters. It is also a human nuisance because mature T. natans nuts drift to shore where their sharp spines may hurt bare feet (Haber, 1999). VDEC (2002) states that this noxious plant also severely limits the passage of light into the water, a critical element of a well-functioning aquatic ecosystem. It reduces oxygen levels, which may increase the potential for fish kills. T. natans outcompetes native vegetation and is of little value to wildfowl.
    Uses
    Trapa natans produces a nut-like fruit that can be cooked, eaten out of hand, or used in other foods (Magness et al. 1971).
    Introduction pathways to new locations
    Floating vegetation/debris: The plant spreads when rosettes detach from the stems and float to another area (Haber, 1999).


    Local dispersal methods
    Water currents: Currents or waves carry nuts to other parts of water bodies (Haber, 1999).
    Management information
    Physical: According to CBP (2002), hand removal is an effective means for eradication of smaller populations; T. natans roots are easily uplifted. Their removal is imperative because floating uplifted plants can further spread seeds downstream. The potential of T. natans seeds to lay dormant for up to 12 years makes total eradication difficult. Hand harvesting from canoes and raking have been effective and are a means to promote community involvement.

    Chemical: For large-scale control of T. natans populations, which can form dense, thick mats capable of covering miles, herbicides and mechanical harvesting can both be effective. Aquatic plant harvesting boats are often employed in instances where waterways are blocked and herbicide (2,4-D) has been tested and deemed safe for use by federal and state agencies.

    Reproduction
    According to Haber (1999), flowers are produced singly on stalks arising from the center of the floating rosette of leaves. Each flower is bisexual, bearing a two-chambered ovary, four stamens and four white petals. Four triangular sepals surround the flower and develop into barbed spines in the mature fruit. Once the ovules of the insect pollinated flowers are fertilized, the flower stalks curve downward with the result that the fruit develops under water. The fruit matures into a nut-like, barbed spiny fruit. VDEC (2002) states that the single-seeded woody fruits produced from the previous year germinate in early spring. A single seed may give rise to 10 to 15 plant rosettes. Each rosette can produce up to 15 to 20 seeds.
    Lifecycle stages
    VDEC (2002) states that as a true annual, T. natans reproduces by overwintering seeds. Ungerminated seeds may remain viable for up to 12 years. However, most seeds probably germinate in the first two years.
    Reviewed by: Anon
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Wednesday, 28 September 2005


ISSG Landcare Research NBII IUCN University of Auckland