Mike Naylor, MD DNR
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Water Chestnut
Trapa natans
Origin: Europe, Asia and Africa
Background
Water chestnut was first observed in North America near
Concord, Massachusetts in 1859. The exact path for the
introduction is unknown.
Distribution and Ecological Threat
Water chestnut can grow in any freshwater setting, from
intertidal waters to 12 feet deep, although it prefers
nutrient-rich lakes and rivers. Presently, the plant is
found in Maryland, Massachusetts, New York and
Pennsylvania, with most problematic populations occurring
in the Connecticut River valley, Lake Champlain region,
Hudson River, Potomac River and the upper Delaware River.
Water chestnut can form dense floating mats, severely
limiting light -- a critical element of aquatic
ecosystems. This plant can also reduce oxygen levels,
which may increase the potential for fish kills. It
competes with native vegetation and is of little value to
waterfowl. Water chestnut infestations limit boating,
fishing, swimming and other recreational activities.
Further, its sharp fruits, if stepped on, can cause
painful wounds. Water chestnut has been declared a noxious
weed in Arizona, Massachusetts, North Carolina and South
Carolina. Its sale is prohibited in most southern states.
Mike Naylor, MD DNR
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Description and Biology
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Plant: an annual aquatic plant with a submerged stem;
stems can reach 12 to 15 feet in length; very fine
roots anchor the plant into the mud.
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Leaves: at the water's surface, the plant contains
a rosette of floating leaves. The saw-tooth edged
leaves are triangular in shape and connect to an
inflated petiole, which provides added buoyancy for the
leafy portion; additional, feather-like leaves can be
found along the submerged stem.
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Flowers, fruits and seeds: four-petaled white flowers
form in June and are insect-pollinated. The fruit is a
nut with four 1/2-inch, barbed spines. Seeds can remain
viable for up to 12 years, although most will germinate
within the first two years.
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Spreads: by the rosette and fruits detaching from the
stem and floating to another area on currents or by
fruits clinging to objects, birds and other animals.
Prevention and Control
Specialized methods of control are required to handle
water chestnut infestations. Because of the likelihood of
unintentional spread offsite and injury to those
attempting control, only trained and certified persons
should undertake management. Manual, mechanical and
chemical techniques are used in it's control. Complete
removal of plants is imperative, as floating, uplifted
plants and plant parts can spread the plant to new
locations. It is critical that any removal take place
prior to the July seed set. Eradication is difficult
because water chestnut seeds may lay dormant for up to 12
years. Biological controls are being investigated, but no
species have been approved for release.
Native Alternatives
Some aquatic nurseries carry native and non-invasive
alternatives. However, due to the similarity in appearance
among aquatic plants to the untrained eye, they are easily
confused. Contact your state natural resource agency,
native plant society or other resource (see
reference section) for assistance in locating species
appropriate to your location and site conditions.
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