Invasive Species in the Chesapeake Watershed
What is Trapa natans
Trapa comes from the Latin term calcitrapa referring to a spiked iron ball used as an ancient weapon. The descriptive species name natans means "floating." Trapa natansis commonly known as bull nut or water chestnut. (Note: Eleocharis dulcis, the Chinese water chestnut, is often confused with European water chestnut, but is unrelated to the Trapacae family.)
Trapa 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. At the surface, the terminus of 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 of the plant. The inflated petioles are approximately 15 cm long, and the leaves are found to be 2 to 4 cm long. The long stems can reach 4-5 m in length. Additional feather-like leaves can be found along the submerged stem. The submersed leaves, structurally similar to adventitious root hairs, are sessile and photosynthetic. The very fine roots anchor the plant into a muddy substrate.
The white flower of water chestnut, which forms in late June from the axils of the leaf rosettes, has four petals, approximately 8 mm long, and is entomophilus (insect pollinated). The water chestnut fruit is a caltrop shaped nut, having four half-inch barbed spines.
Native to Europe, Asia and Africa, water chestnut 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). Naturalized populations can be found in Australia and various locations of the northeastern United States.
Water chestnut flowering for the northeastern United States begins in June and continues until mid-July. The ovary is two chambered each with an ovule, though generally only one seed per flower will develop (Groth et al. 1996). After fertilization, the fruit develops barely submerged beneath the rosette. The four triangular sepals of the flower develop into the barbed spines of the mature fruit. The fruit is technically a drupe and takes about one month to ripen. Mature fruits are thereophytes, which eventually fall to the lake or river bottom to overwinter. Lateral dispersal can occur when water chestnut plants are uprooted and float downstream. Seeds can remain viable for up to 12 years, although most will germinate within the first two years.
The seeds will germinate from the sediment substrate, firmly lodged by lateral roots; germination occurs in the late spring, and the first leaves reach the surface in mid-May. As a young plantlet, the submersed leaves develop and provide absorptive surface area for nutrients as well as a photosynthetic surface. Once the primary stem has developed and produced the first floating leaves, secondary offshoots begin to develop at a rapid rate. The rapid vegetation of water chestnut in low-density conditions contributes greatly to its success as an aquatic invader.
Each water chestnut seed can potentially create 15 to 20 rosettes. Each rosette can generate up to 20 seeds. The prolific plant can cover a given area in a mat-like manner often creating a canopy that reduces growth of other aquatic species and interrupts the passage of light needed to maintain a well-functioning aquatic ecosystem. The abundant detritus created by water chestnut could affect other aquatic organisms and also reduce oxygen levels in shallow habitats.
Water chestnut in North America was first observed near Concord, Massachusetts, in 1859 (Worobel 1996). The exact path or reason for the introduction is a mystery. Harvard botanist Asa Gray cultured the organism in his botanical garden in 1877. Its escape to local waters occurred by 1879 (Worobel 1996) and populations were documented in New York by the late 1800s. Further spread occurred through waterways and into Vermont and Massachusetts.
The first population of water chestnut in Maryland was documented in 1923 in a two-acre patch on the Potomac River outside of Washington D.C. Within a few years, the plant had spread over 40 river miles on the Potomac. The 10,000-acre coverage of water chestnut reaching past Quantico, Virginia, prompted removal efforts by the Army Corps of Engineers in 1939. Water chestnut was found in the Bird River, Baltimore County, in 1955 and subsequently in the Sassafras River, Kent County, in 1964.
The most problematic populations currently occur in the Potomac and Hudson rivers and in Connecticut River valley, Lake Champlain region. In 1998, water chestnut was found in the South River in Quebec, which is connected to the Lake Champlain outlet via the Richelieu River. Its spread has continued because of the suitability of habitat; in 2001, for example, water chestnut was discovered in the Pike River, which flows into Misssissquoi Bay.
Water chestnut has been declared a noxious weed in Arizona, Massachusetts, New Hampshire, North Carolina and South Carolina, and sale is prohibited through most Southern States (USDA).
The dispersal of water chestnut by human hands to the United States (see USGS map) and other parts of the world can be attributed to its status as an ornamental plant having medicinal and nutritional value. In many parts of Asia, the fruit is a staple food source and used for livestock feed. The fruit has been used medicinally to treat elephantitus, pestilent fevers, rheumatism and skin complaints (Worobel 1996).
In Europe, water chestnut populations have been dwindling in Belgium, Holland and Sweden; it has been listed as a strictly protected species by the Bern Convention March 1998 (Council of Europe, convention on the conservation of European wildlife and natural habitats); in 1981 Germany issued stamps featuring four aquatic plants including water chestnut.
Where is Trapa natans Found in the Chesapeake Bay Watershed?
Water chestnut is presently found on the Sassafras and Bird rivers of Maryland, and in a number of ponds including a non-tidal pond above Lloyds Creek and in Urieville Lake in Kent County, Maryland. Pennsylvania has reported populations in the Lower Susquehanna, areas around Philadelphia, and in isolated lakes. Most recently, a population was reported in the Upper Delaware River. Attached is a map showing Trapa natans distribution in the Chesapeake Bay Watershed.
Maryland has a harvesting program that has been in effect since 1999. The program has focused on the water chestnut populations on the Bird and Sassafras rivers.
Water chestnut was recorded in the Bird River in Baltimore County for the first time in 1955. The Maryland Departments of Game and Inland Fish and Tidewater Fisheries used mechanical removal and the herbicide 2,4-D, the only fully-licensed herbicide that has been successfully employed for controlling water chestnut populations. However, in 1964 it reappeared in the Bird River and an additional 100 acres were discovered in the Sassafras River, of which 30 acres were mechanically removed. A combination of removal techniques was used once again in 1965, when 200 acres were found in the Sassafras River This effort was believed to have been successful, and no plants had been noted in vegetation surveys until summer 1997 (MDNR web).
The Bird River water chestnut population spread from approximately 50 plants in summer 1997 to over three acres in 1998, and at least 20 acres in 1999. The Sassafras population is slightly larger, though determining its exact size has been difficult due to its remote location. A massive mechanical and volunteer harvesting effort was undertaken in both rivers in 1999 and resulted in the removal of approximately 400,000 pounds of plants from the two rivers.
Despite the discovery in 2001 of several new locations in which water chestnut grew, less than 500 pounds were harvested this year – about enough to fill the bed of a small pick-up truck. This was about half of the approximately 1,000 pounds last year, and a tiny fraction of the 200,000 pounds in 1999. With declining weights of plants harvested, and declines in plant density in the most affected areas, it seems that the eradication efforts to date have been successful.
Water chestnut infestations have been identified in isolated areas. There are no coordinated efforts to remove water chestnut in Pennsylvania at this time (Tim Block, Betsey Lyman pers. comm.).
Virginia has no known populations of water chestnut (Keith Heffernan pers. comm.).
What Research and Control Efforts are Currently Underway for Trapa natans?
Biological control possibilities were investigated in the early 1990s. Surveys were conducted by the U.S. Department of Agriculture in 1992 and 1993 that sought natural enemies of water chestnut in Northeast Asia; in 1995, the survey was continued in Europe (Pemberton 1996). The prime candidate, Galerucella birmanica, a beetle that consumes up to 40% of water chestnut leaf tissue (Ahmad 1998) was found to have various other plant hosts, thereby making it unsuitable for bio-control purposes in the U.S. Other insects that fed exclusively on water chestnut were identified but not found to be damaging (Pemberton 1996). Predators found in the warmer climate of India have potential but could not withstand the cooler temperatures of water chestnut-infested Northeast regions of the United States.
Hand removal is an effective means for eradication of smaller populations: water chestnut roots are easily uplifted. Their removal is imperative as floating uplifted plants can further spread seeds downstream. The potential for water chestnut seeds to lay dormant for up to 12 years makes total eradication difficult. Nonetheless, hand-harvesting from canoes and raking have been useful and they are a means to promote community involvement.
For large-scale control of water chestnut populations, which can form dense, thick mats capable of covering miles at a time, herbicides and mechanical harvesting can both be effective. Aquatic plant harvesting boats are often employed in instances where waterways are blocked. For example, mechanical harvesting in 1999 on the Sassafras River removed an estimated 260,000 pounds of water chestnut (Naylor 1999). Unfortunately mechanical harvesting boats cannot operate in some of the shallow areas that water chestnut can inhabit. For this reason, mechanical harvesting has been complemented by hand harvesting in Maryland on the Bird and Sassafras rivers.
Herbicide 2,4-D has been tested, and deemed safe for use by federal and state agencies. Used widely in the U.S., it has not shown adverse affect on neighboring wildlife. Maryland and Virginia used 2,4-D in the 1960s to eradicate Eurasian watermilfoil populations in the Bay. Due to public perception, the use of herbicides is seen as a last resort option. Integrating all possible methods for water chestnut removal will be the most effective course for eradication.
Who Is Involved in the Management of Trapa natans in the Chesapeake Bay?
Water chestnut infestations create havoc for boating and recreational areas. The dense mats make navigation difficult, while the spiky seeds, capable of puncturing shoe leather, are a danger to bathers and beach users. Private land owners, boat owner groups and park groups are active in eradication efforts where water chestnut occurs. The following groups were active in Maryland's 2000 eradication efforts: the State Highway Administration, U.S. Department of the Interior, C&O Canal National Historic Park, Echo Hill Environmental Education Center, USFWS, Baltimore Department of Environmental Protection, Horsehead Wetlands Center, Wildfowl Trust of North America, and Bird River Beach Community Association (Naylor, 2000).
Water chestnut has been listed as a noxious weed in South Carolina, New Hampshire and Arizona.
Ahmad, Syed Hasib 1998. The Freshwater Aquatic Fruit: Water Chestnut. Aquaculture & Fisheries Institutional Finance & Programme Implementation Department, Government of Bihar, India
Block, Tim. The Morris Arboretum. University of Pennsylvania
Groth, A.T., Lovett-Doust, L. and 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.
Heffernan, Keith Virginia Division of Game and Inland Fisheries
Heywood, V.H.1978 Flowering plants of the world. Mayflower Books, New York.
Hutchinson, J. 1969. Evolution and phylogeny of flowering plants. Dicotyledons: Facts and theory. Academic Press, London.
Lyman, Betsey The Nature Conservancy of Pennsylvania, Conservation Programs.
The Metropolitan District Commission. 1994. A survey of the aquatic plant community of the Charles River Lakes District, Waltham and Newton, Massachusetts. Report by Aquatic Control Technology, Inc. and Fugro East, Inc., Northborough, MA.
Methe, B.A., Soracco, R.J., Madsen, J.D. and Boylen, C.W. 1993. Seed production and growth of water chestnut as influenced by cutting. J. Aquat. Plant Manage. 31: 154-157.
Murty, A.V.S.S.S. and Subrahmanyam, N.S. 1989. A textbook of economic botany. Wiley Eastern Limited, New Delhi.
Naylor, Mike. Maryland Department of Natural Resources
Pemberton, Robert. 1997. Natural Enemies of Trapa Species in Northeast Asia and Europe. TEKTRAN United States Department of Agriculture, Agricultureal Research Service
Worobel, Elizabeth 1996 University of Manitoba. Canada eworob@cc.Umanitoba.CA
USDA Agricultural Resource Service, Invaders Database System
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Last modified October 10, 2012