- Agricultural Invaders
- Aquatic Animals
- Aquatic Plants
- Pathogens & Parasites
- Terrestrial Animals
- Terrestrial Plants
Search our Education Materials Library for more: Library Links Factsheets
Water Chestnut (Trapa natans L.)
A Patch of floating water chestnut (Trapa natans) leaves.
If a shoreline property owner in New York or the Northeast complains to you about their water chestnut problem, don’t think they are talking about Chinese takeout. The European water chestnut (scientific name Trapa natans, or T. natans), an invasive aquatic plant released inadvertently into waters of the Northeast in the late 1800s, is slowly but inexorably spreading throughout New York State, clogging waterways, lakes and ponds and altering aquatic habitats.
It must be pointed out that this plant species is not the same as the water chestnut which can be purchased in cans at the supermarket. The fruits of T. natans, however, are used as a source of food in Asia and have been utilized for their medicinal (and claimed) magical properties.
T. natans is native to Europe, Asia and Africa. In its native habitat, the plant is kept in check by native insect parasites. These insects are not present in North America and the plant, once released into the wild, is free to reproduce rapidly. T. natans colonizes areas of freshwater lakes and ponds and slow-moving streams and rivers where it forms dense mats of floating vegetation, causing problems for boaters and swimmers and negatively impacting aquatic ecosystem functioning.
Common names: horned water chestnut, water caltrop
T. natans is native to Western Europe and Africa and northeast Asia, including eastern Russia, China, and southeast Asia to Indonesia. T. natans was first introduced to North America in the mid- to late-1870s, when it is known to have been introduced into the Cambridge botanical garden at Harvard University around 1877. It is known to have been planted in other ponds in that area, as well, and also in Concord, MA, in a pond near the Sudbury River. The plant escaped cultivation and was found growing in the Charles River by 1879. The plant was introduced into Collins Lake near Scotia, NY (in the Hudson River-Mohawk River drainage basin) around 1884, possibly as an intentional introduction for waterfowl food or possibly as a water garden escapee.
By the early part of the 1900s, water chestnut was established in the Hudson River. The first Great Lakes Basin introductions were sometime before the late-1950s when the plant was discovered growing in Keuka Lake (one of NY’s Finger Lakes). A major infestation of more than 300 acres exists throughout some 55 miles of Lake Champlain between New York and Vermont. Water chestnut can now be found throughout NY, from the Niagara Frontier through the Finger Lakes, from Lake Champlain to Long Island.
NYS Distribution of water chestnut as of January 2014. Numbers represent number of sightings at each location. © imapinvasives.org/nyimi
The North American distribution of water chestnut now extends throughout New England, south as far as Virginia, California, and in the Canadian Province of Quebec in a tributary of the Richelieu River. The plant has the potential to spread into the warmer regions of the U.S. as far south as Florida.
North American Distribution of water chestnut as of September 2014.
T. natans is a rooted aquatic annual herb that dies back at the end of each growing season. Re-growth is by means of seeds that germinate in the spring. Each seed produces 10 to 15 stems with submerged and floating leaves, terminating in floating rosettes. The feathery submersed leaves can be up to six inches (15 cm) long, and are alternate on the stem forming whorls around the stem. The three-quarter to one and a half inch (2 - 4 cm) glossy green floating leaves are triangular with toothed edges and form rosettes around the end of the stem. The floating leaves also have prominent veins and short, stiff hairs on their lower surface. The petioles (the stalks attaching the leaf blade to the stem; the transition between the stem and the leaf blade) of the floating leaves are two to eight feet (0.6 - 2.4 m) and contain spongy, buoyant bladders, allowing the rosettes to float on the surface of the water. Stems can reach lengths of up to 16 feet (4.9 m), although typical lengths tend to be in the six to eight foot (1.8 - 2.4 m) range. The stems are anchored to the bed of the waterbody by numerous branched roots. Single small, white flowers with four one-third inch (8.3 mm) long petals sprout in the center of the rosette.
Each rosette is capable of producing up to 20 hard, nut-like fruits. Water chestnut starts to produce fruits in July; the fruits, which ripen in about a month, each contain a single seed. The 1 to 1.5 inch (2.5 - 4 cm) wide fruits grow under water and have four very sharp spines. Water chestnut seeds generally fall almost directly beneath their parent plants and serve to propagate the parent colony. Population overwintering is accomplished through mature, greenish brown nuts sinking to the bottom where they can remain viable in the sediment for up to 12 years. Some seeds, however, or plant parts (floating rosettes) that still contain nuts, may be moved downstream in currents. Ducks, geese and other waterfowl may also play a role in the nuts’ dispersal (the spiny nuts have been observed tangled in the feathers of Canada geese). Old nuts, black in color, will float, and are not viable. When deposited in shallow water or on the shore, water chestnut nuts can lead to injuries if stepped on.
Trapa rosette showing nuts and
inflated leaf petioles
Water chestnut has become a significant environmental nuisance throughout much of its range, particularly in the Hudson, Connecticut and Potomac Rivers, and in Lake Champlain. The plant can form nearly impenetrable floating mats of vegetation. These mats create a hazard for boaters and other water recreators. The density of the mats can severely limit light penetration into the water and reduce or eliminate the growth of native aquatic plants beneath the canopy. The reduced plant growth combined with the decomposition of the water chestnut plants which die back each year can result in reduced levels of dissolved oxygen in the water, impact other aquatic organisms, and potentially lead to fish kills. The rapid and abundant growth of water chestnut can also out-compete both submerged and emergent native aquatic vegetation.
Water chestnut infestation on Lake Champlain
Water chestnut has little nutritional or habitat value to fish or waterfowl and can have a significant impact on the use of an infested area by native species.
T. natans likely impacts non-native and invasive plant and animal species in the same manner it impacts natives. Some of the potentially impacted invasive plant species might include: Eurasian watermilfoil (Myriophyllum spicatum), curly pondweed (Potamogeton crispus), and Eurasian or brittle water-nymph (Najas minor). It is not yet known in a match up of T. natans or and hydrilla (Hydrilla verticillata, which invader would outcompete which. Because of its invasiveness and severity of its impacts, T. natans has been listed in federal regulations prohibiting interstate sale/transportation of noxious plants.
A massive riverine infestation of water chestnut.
Economic impacts result from T. natan’s impenetrable mats of vegetation which can impede swimming, boating, commercial navigation, fishing, and waterfowl hunting. Untreated populations of such an aquatic invasive species also can result in losses to shoreline property values and, as a result, to local government property tax revenues. As mentioned earlier, the sharp, spiny nuts can result in puncture injuries to swimmers and recreators walking along the shore of infested areas and can injure the feet of livestock and horses, as well.
One example of the cost of managing T. natans in a waterbody is the experience of the States of New York and Vermont on Lake Champlain. From 1982 through 2011, $9,600,000 has been spent on Trapa control in the lake with funding from a number of sources including: the two states; the U.S. Army Corps of Engineers; the U.S. Fish and Wildlife Service; the U.S. Department of Agriculture; Ducks Unlimited; the Lake Champlain Basin Program; and The Nature Conservancy. A combination of hand pulling and mechanical harvesting has been used on the lake since the early-1980s. Significant reductions of T. natans populations resulted from this prolonged annual control effort, however, every time that funds were reduced, rapid grow back of the species and extension of its range in the lake was observed.
Lake George, NY, water chestnut relative annual control costs, 1982 - 2011
Mechanical and Chemical Control
It is much easier and less expensive to control newly introduced populations of T. natans. Early detection of introductions and a rapid control response are key to preventing high-impact infestations. Because T. natans is an annual plant, effective control can be achieved if seed formation is prevented. Small populations can be controlled by hand pulling working from canoes or kayaks.
Large infestations usually require the use of mechanical harvesters or the application of aquatic herbicides. Regardless of treatment type, it should ideally take place before the fruit has ripened and dropped to the bottom forming a long-term seed bank. Because of the potential of unintentional spread of floating plant parts offsite, mechanical harvesting should be undertaken only by trained and certified equipment operators. Since water chestnut overwinters entirely by seeds that may remain viable in the sediment for up to 12 years, repeated annual control is critical to deplete the seed bank. Treatment generally is needed for five to twelve years to ensure complete eradication and can be very expensive (see Economic Impact, above).
Potential negative impacts to non-target species and public perceptions regarding the use of chemicals in recreational waters have limited chemical control of T. natans except as a treatment of last resort and usually only in still or sluggishly flowing waters. The herbicide 2,4-D has been tested and shown to be non-adverse on non-target species. 2,4-D has been used widely in the U.S. Another herbicide that is effective on T. natans is glyphosate. Application of aquatic herbicides requires both a licensed pesticide applicator and a permit from your state environmental regulatory agency.
The unfortunate fact is that for large infestations of water chestnut (i.e. those too large to be controlled by hand-pulling) over the long-term mechanical and chemical control measures have proven to be impractical to provide an economically sustainable control of water chestnut. Scientists have now turned to the potential of biocontrol agents to serve as a long-term solution to water chestnut infestations.
A number of potential biological control agents were found in field surveys in the native European and Asian ranges of water chestnut. The most promising biocontrol species appeared to be the leaf beetle Galerucella birmanica. Unfortunately, field observations in China suggested that G. birmanica may also attack native water shield (Brasenia schreberi) in addition to Trapa natans. This host non-specificity could be problematic to the use of the beetle for biocontrol in North America.
Laboratory and field tests initially indicated that out of 19 different plant species in 13 different families, G. birmanica laid eggs and completed development only on species of Trapa and B. schreberi. Adult G. birmanica in the field and lab indicated that the beetles showed a strong preference for T. natans. This preference continued even after the water chestnut was completely defoliated; adults resisted migrating to nearby water shield. While this is very promising news, additional studies on host specificity with additional North American aquatic plants are on-going. [Ding, et. al., 2006]
Blossey B, Schroeder D, Hight S, Malecki R. 1994. Host specificity and environmental impact of two leaf beetles (Galerucella calmariensis and G. pusilla) for biological control of water chestnut (Trapa natans). Weed Science 42:134-140.
Deck J, Nosko P. 2002. Population establishment, dispersal, and impact of Galerucella pusilla and G. calmariensis, introduced to control water chestnut in central Ontario. Biological Control 23: 228-236.
Ding J, Blossey B, Du Y, Zheng F. 2006. Galerucella birmanica (Coleoptera: Chrysomelidae), a promising potential biological control agent of water chestnut, Trapa natans. Biological Control. Vol.36, Issue 1, Pages 80–90
Fernald ML. 1950. Gray’s Manual of Botany. 8th ed. American Book Company, N.Y.
Gleason HA. 1957. The New Britton and Brown Illustrated Flora of the Northeastern U.S. and Adjacent Canada. New York Botanical Gardens, N.Y.
Methe BA, Soracco RJ, Madsen JD, Boylen CW. 1993. Seed production and growth of water chestnut as influenced by cutting. J. Aquat. Plant Manage. 31: 154-157.
Mills EL, Leach JH, Carlton JT, Secor CL. 1993. Exotic species in the Great Lakes: A history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19: 1-54.
Mullin BH. 1998. The biology and management of water chestnut (Trapa natans L.). Weed Technology 12:397-401.
Pemberton RW. 2002. Water Chestnut. In: Van Driesche R., et al. 2002. Biological Control of Invasive Plants in the Eastern United States. USDA Forest Service Publication FHTET-2002-04.
Rawinski T. 1982. The ecology and management of water chestnut (Trapa natans L.) in central New York. M.S. thesis, Cornell University.
Vermont Invasive Exotic Plant Fact Sheet Series: Water Chestnut. Vermont Agency of Natural Resources and The Nature Conservancy, Vermont Chapter. June, 1998.
Hunt T, Marangelo P. 2012. 2011 Water Chestnut Management Program: Lake Champlain and Inland Vermont Waters, Final Report. Vermont Department of Environmental Conservation. May 2012.
Patch of floating water chestnut (Trapa natans) leaves. John M. Randall, The Nature Conservancy, www.forestryimages.org
NYS Distribution of water chestnut as of January 2014. © 2014 imapinvasives.org/nyimi The Nature Conservancy. Accessed January 2014
North American distribution of water chestnut as of September 2014. Plants Database. USDA Natural Resources Conservation Service
Drawing of floating and submerged leaves and fruit (nut). Connecticut River Coordinator's Office, US Fish & Wildlife Service
Hand holding water chestnut rosette. Alfred Cofrancesco, U.S. Army Corps of Engineers, www.forestryimages.org
Water chestnut infestation on Lake Champlain. Alfred Cofrancesco, U.S. Army Corps of Engineers, www.forestryimages.org
Riverine infestation of water chestnut. Leslie J. Mehrhoff, University of Connecticut, www.forestryimages.org
Chart of Lake George, NY, water chestnut annual control costs, 1982 - 2011. Data from Hunt T, Marangelo P. 2012.
Water chestnut harvesting machine. US Fish & Wildlife Service, Silvio Conte National Fish and Wildlife Refuge
Angiosperm (The) Phylogeny Group. 1998. An ordinal classification for the families of flowering plants. Annals of the Missouri Botanical Garden 85: 531-553.
Batra HN. 1962. First record of Haltica cyanea Weber and Bagous species of singhara crop. Indian Journal of Entomology 23: 66-68.
Bharadwaj K, Chandra V. 1980. Water chestnut (Trapa natans): supplement to cereals and a conserver of riverine waste lands. Biological Memoirs 5: 5-12.
Bogucki DJ, Gruendling KG, Madden M. 1980. Remote sensing to monitor water chestnut growth in Lake Champlain. Journal of Soil and Water Conservation 35: 79-81.
Cantelo WW. 1965. A Host List of the Insects of Thailand. Department of Agriculture, Royal Thai Government and the U.S. Operation Mission to Thailand, Bangkok, Thailand.
Cook CDK. 1978. Trapaceae, p. 156. In Heywood, V. H. (ed.). Flowering Plants of the World. Mayflower Books, New York.
Cook CDK., Gut BJ., Rix EM., Schneller J., Seitz M. 1974. Water Plants of the World. Dr. Junk Publishers, The Hague, The Netherlands.
Cozza R, Galanti G, Bitonti MB, Innocenti AM. 1994. Effect of storage at low temperature on the germination of the waterchestnut (Trapa natans L.) Phyton 34:315-320.
Cronquist A. 1981. An Integrated Classification of Flowering Plants. Columbia University Press, New York.
Crow EC, Hellquist CB. 2000. Aquatic and Wetland Plants of Northeastern North America, Vol. 1. University of Wisconsin Press, Madison, Wisconsin, USA.
Egorov A, Gratshev B. 1990. New Bagous spp. from the Soviet Far East, pp. 32-39. In Lelei, A. S. (ed.). News of Insects Systematics of the Soviet Far East. Institute of Biology and Pedology, Far East Branch, Academy of Sciences of the USSR, Vladivostok, Russia.
Fassett NC. 1957. A Manual of Aquatic Plants. University of Wisconsin Press, Madison, Wisconsin, USA.
Groth AG, Lovett-Doust L, Lovett-Doust J. 1996. Population density and module demography in Trapa natans (Trapaceae), an annual, clonal aquatic macrophyte. American Journal of Botany 83: 1406-1415.
Hayashi M, Morimoto K, Kimoto S. 1984. The Coleoptera of Japan. Hoikuska Publishing Company, Tokyo, Japan. (in Japanese)
Horn GH. 1893. The Galerucini of boreal America. Transactions of the American Entomological Society 20: 73-81.
Khatib MH. 1934. The life-history and biology of Galerucella birmanica Jac. (Coleoptera, Phytophaga, Chrysomelidae, Galerucine) and the external morphology of larvae and pupae, part I. Indian Journal of Agricultural Science 4: 715-729.
Kiviat E. 1993. Under the spreading water-chestnut. Hudsonia 9: 1-6.
Kunii H. 1988. Longevity and germinability of buried seed in Trapa sp. Memoirs of the Faculty of Science of Shimane University 22: 83-91.
Lu Z., Zhu J, Zhu S, Chen Z. 1984. Preliminary studies on the beetle (Galerucella birmanica Jacoby) – an insect pest of waterchestnut and watershield. Scientia Agricultura Sinica 5: 73-76. (in Chinese)
Madsen J. 1994. Invasions and declines in submersed macrophytes in Lake George and other Adirondack Lakes. Lake and Reserve Management 10: 19-23.
Mantovani R, Galanti G, Nocentini A. 1992. Biological observations on Bagous rufimanus Hoffmann (Coleoptera, Curculionidae) with description of its immature stages. Aquatic Insects 14: 117-127.
Mills EL, Leach JH, Carlton J, Secor CL. 1993. Exotic species in the Great Lakes: a history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19: 1-54.
Oliver D. 1871. Flora of Tropical Africa, Vol. 2. Reeve, London.
Papastergiadou E, Babalonas D. 1993. The relationships between hydrochemical environmental factors and the aquatic macropytic vegetation in stagnant and slow flowing waters. 1.Water quality and distributions of aquatic associations. Archives of Hydrobiology Supplement 90: 475-491.
Prashad B. 1960. A new aquatic weevil from India, Bagous trapae, n. sp. (Curculionidae: Coleoptera). Indian Journal of Entomology 22: 298-301.
Pemberton RW. 1999. Natural enemies of Trapa spp. in northeast Asia and Europe. Biological Control 14: 168-180.
Schmidt KA. 1985. The life of the chrysomelid beetle Pyrrhalta nymhaeae (Galerucinae) on water chestnut, Trapa natans, pp. 1-38. In Coper, J. C. (ed.). Polgar Fellowship Reports of the Hudson River National Estuarine Sanctuary Program National Oceanic, Atmospheric Administration, U.S. Department of Commerce. Washington, D.C.
Sculthorpe CD. 1967. The Biology of Aquatic Vascular Plants. Edward Arnold Publishers Limited, London.
Singh GP, Lal S. 1965. A new leaf-spot disease of singhara (Trapa bispinosa) caused by Bipolaris tetramera. Indian Phytopathology 18: 85-87.
Soil Conservation Service. 1982. National list of scientific names. Vol. 1. Soil Conservation Service Technical Publication 159. U.S. Department of Agriculture, Soil Conservation Service. Washington, D.C.
State of Florida, Department of Environmental Protection. 1996. Aquatic plant importation, transportation, non-nursery cultivation, possession and collection. Rules of the State of Florida, Chapter 16C-52.
Tanaka T. 1976. Tanaka’s Encyclopedia of Edible Plants of the World. Keigaku Publishing Company, Tokyo, Japan.
Tsuchiya T, Iwaki H. 1984. Seasonal changes in phytosynthesis and primary production of a floating leaved plant, Trapa natans L., community in Lake Kasumiguara, Japan. Japanese Journal of Ecology 34: 367-374.
Tutin TG, Had VH, Barges NA, Moire DM, Valentine DH, Waiters SM, Web DA. 1968. Flora of Europe, Vol. 2. Cambridge University Press, Cambridge,
Voroshilov BH. 1982. Key to the plants of the Soviet Far East. Science Publications, Moscow, Russia. (in Russian)
Wibbe JH. 1886. Notes from Schenectady. Bulletin of the Torrey Botanical Club 13: 39.
Invasive Plant Atlas of New England (IPANE) water chestnut factsheet. Converted from eddmaps.org, January 2014
Vermont Lakes & Ponds Section - Water Chestnut. http://www.anr.state.vt.us/dec/waterq/lakes/htm/ans/lp_wc.htm#top
Water Chestnut Invasive Exotic Plant Fact Sheet. Departments of Environmental Conservation, Fish and Wildlife, and Forests, Parks and Recreation of the Vermont Agency of Natural Resources, and The Nature Conservancy of Vermont. Spring 1998; revised Winter 2003.
Water Chestnut (Trapa natans) in the Northeast. NYSG Invasive Species Factsheet Series: 06-1. Charles R. O’Neill, Jr., Invasive Species Specialist. New York Sea Grant. February 2006
USDA Natural Resources Conservation Service Plant Profile http://plants.usda.gov/java/profile?symbol=TRNA