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Trapa natans

Biological Category 
Plants
Species Type 
Aquatic Invasives
LHPrism Status 
Tier 3 - Established

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Description 

An aquatic, annual plant with floating rosettes of leaves, often forming dense mats. Floating leaves have toothed margins and prominent veins on the undersides, and each leaf has a swollen petiole to keep it afloat. The plants are rooted in bottom sediments, and feathery, submerged leaves occur along the stem.

Introduction History 
Water chestnut is native to Eurasia and Africa, and was introduced to North America in the 1870s. In the US, it is invasive in the northeastern states south to Virginia, and in the Great Lakes basin.
Ecology and Habitat 
Water chestnut is found in ponds, lakes, wetlands, slow-moving parts of rivers, and estuaries, where water is fresh to slightly brackish and 0.2-3.6 m deep. It prefers full sun, soft bottom sediments, and sluggish, nutrient-rich fresh water.1 Interestingly, water chestnut may have reached the limit of suitable habitat in the freshwater tidal Hudson River, as over a 10-year period water chestnut expanded at the expense of submerged aquatic vegetation only slightly more than the opposite, suggesting that coverage of both types is controlled by other factors.2
Reproduction and Phenology 
Plants emerge in spring from seeds in bottom sediments and remain rooted there; rosettes are submerged as stems lengthen in May; rosettes surface in June and plants bloom in JulyAugust. Each flower forms a one-seeded fruit with four barbed spines, which ripens starting in midlate July. Its fleshy exterior soon disintegrates and the hard, woody nut is heavy enough to sink to the bottom and overwinter in sediment. Seeds remain viable for up to 12 years.1 Seeds are generally spread by humans, boats, or water birds. Clonal reproduction is also common: each water chestnut plant can produce many ramets (stems), each of which is capable of surviving and producing nuts if detached from the main stem.6
Impacts of this species 

Once established in a water body with suitable conditions, water chestnut can spread very rapidly. Under ideal conditions it forms a thick mat, completely covering the water’s surface and intercepting 95% of sunlight.1 In other conditions it persists at lower densities, along with other aquatic plants. Dense water chestnut beds completely shade out submerged aquatic vegetation (SAV) and reduce dissolved oxygen (DO) to levels lethal to fish and other aquatic organisms.1,3 Low levels of DO may also result in the release of significant amounts of methane, a greenhouse gas, into the atmosphere.4 SAV provides food for ducks and other waterfowl and  supports a more diverse fish assemblage through much higher densities of microorganisms, algae, and (at times) macroinvertebrates than water chestnut. Fish found in water chestnut beds are those species tolerant of water pollution, low DO, and high turbidity. Human recreational and commercial use of shallow waters and access to deeper water is limited or prevented where dense water chestnut beds occur.1,3  

Nevertheless, water chestnut provides an important ecosystem service – the low DO in water chestnut beds results in the removal of large amounts of inorganic nitrogen to the atmosphere (e.g., water chestnut in the Hudson downstream of Albany removes the equivalent of all of that city’s wastewater nitrogen that enters the river).5 The plants can also accumulate polluting heavy metals.1 Water chestnut supports a high richness and abundance of invertebrates (although different in composition than that supported by SAV) and may enhance total fish production in the Hudson (although favoring only certain species).1,3 Water chestnut beds are used by snapping turtle, blue crab, and various marsh and water birds, and many mammals consume the seeds, including beaver, muskrat, and red squirrel.1 

Management Methods 

Manual or Mechanical Control
This page focuses on manual best management practices only. Please contact invasives@nynjtc.org with specific questions on chemical or biological control options.

  • In July, before any fruits mature: Hand-pull plants, being sure to remove roots, entire stem, and all plant parts (rooted or unattached stems can regrow, and even small rosettes can produce fruits). This is usually best done by canoe or other small boat with cargo space. Use heavy gloves to protect from spiny fruits.7,8 
  • All plant parts should be piled carefully at an upland site 15 m or more from water to prevent unintentional spread and the return of nutrients from the rotting material. 
  • To eliminate a bed, this will need to be repeated annually for 5-12 years (to exhaust the seed bank).  
  • For large, dense beds in large bodies of water, water-chestnut can be removed using large mechanical harvesters, transport barges, and dump trucks, and then composted. This method has worked at some sites to reduce densities enough to continue management by hand harvesting only.7 
  • Check soon for the availability of a biocontrol option: a leaf beetle (Galerucella birmanica) from water chestnut’s native range is in the final stages of host specificity testing (http://www.nyisri.org/resources/biocontrol/). Classical biocontrol can be a useful component of integrated weed management, but does not always work, and in some cases has adverse impacts on nontarget plants.  
Summary of Best Managment Practices 

Management Goals

  • Eliminate small occurrences by annual pulling until seed bank is depleted. This must be done in the first year the plant is noticed, and repeated annually until entirely eliminated.  
  • Reduce the density of large invasions by annual pulling. 
  • Prevent introduction into new areas by removal and proper disposal of all plant parts.
  • Check boats, nets, and other equipment to prevent dispersal of seeds.  
Additional Information 

REFERENCES

  1. Hummel, M., and E. Kiviat. 2004. Review of world literature on water chestnut with implications for management in North America. Journal of Aquatic Plant Management 42:17-28. 
  2. Findlay, S.E.G., D.L. Strayer, S.D. Smith, and N. Curri. 2014. Magnitude and patterns of change in submerged aquatic vegetation of the tidal freshwater Hudson River. Estuaries and Coasts 37:1233–1242.  
  3. Strayer, D.L. 2010. Alien species in fresh waters: Ecological effects, interactions with other stressors, and prospects for the future. Freshwater Biology 55 (Suppl. 1):152-174. 
  4. Pierobon, E., R. Bolpagni, M. Bartoli, and P. Viaroli. 2010. Net primary productivity and seasonal CO2 and CH4 fluxes in a Trapa natans L. meadow. Journal of Limnology 69:225-234. 
  5. Tall, L., N. Caraco, and R. Maranger. 2011. Denitrification hot spots: Dominant role of invasive macrophyte Trapa natans in removing nitrogen from a tidal river. Ecological Applications 21: 3104-3114.  
  6. Groth, A.T., L. Lovett-Doust, and J. Lovett-Doust. 1996. Population density and module demography in Trapa natans (Trapaceae), an annual, clonal aquatic macrophyte. American Journal of Botany 83:1406-1415. 
  7. VDEC (Vermont Department of Environmental Conservation). 2016. Water chestnut harvest program 2015: A report on 2015 water chestnut mechanical and hand harvest activities in Lake Champlain and other waterbodies in Vermont. VDEC, Watershed Management Division, Montpelier, VT. 48 p. 
  8. Chris Doyle (Solitude Lake Management) and Ann Bove (Watershed Management Division, Vermont Department of Environmental Conservation), pers. comm.