Identification: According to Rybicki et al. (2015):
Habit: floating, rooted, aquatic annual
Stems/Roots: submerged, flexuous stem and roots that anchor into the mud and extend upwards to the surface of the water
Leaves: rosette of floating, alternate, fan-shaped leaves, each leaf having a slightly inflated petiole (leaf stem) and biserrate (doubly serrated) leaf margins with leaf surfaces green above and red below; submerged leaves are opposite, linear, and die back to be replaced by roots
Flowers: solitary, small, white to pink flowers with four petals sprout in the center of the rosette; flowering typically in June
Fruits/Seeds: large drupe or "nut" with two, opposing, sharp spines that develop from hardened sepals and two pseudo spines where sepals senesce
Look-a-likes: Ludwigia sedioides (Humb. & Bonpl.) H. Hara has similar leaf shape, arrangement, and floating habit, but is much smaller than Trapa and the flowers are yellow.
Life history: Rosette numbers ranged between 34-70 per m2 with an average of five flowers per rosette (Rybicki et al. 2015). Plants senesce in the autumn due to frost, while seeds remain dormant in sediments for up to 10 years (Muenscher 1944). Germination occurs above 12°C (Muenscher 1944).
Habitat: Full sun, sluggish, eutrophic, fresh water, and soft sediment (Winne 1950) with water depths from 0.3-3.6 m (Muenscher 1937). Plant growth is associated with nitrogen-rich waters (Vuorela and Aalto 1982).
Tolerances: Trapa species appear to be affected by density dependence; at low densities, plants produced 10 times as many ramets as those at high density (Groth et al. 1996). Seeds germinate best in alkaline substrates with a pH of 7.9-8.3 and where salinity is below 0.1% or 1 ppt (Vuorela and Aalto 1982).
Community and environment: The leaf beetle, Galerucella nipponensis (Coleoptera: Chrysomelidae), was found to prefer vertically grown leaves of Trapa japonica (Ikeda and Nakasuji 2002).
Populations are established in the Middle Potomac drainage including many private and public ponds (N. Rybicki, pers. comm. 2018).
Status is unknown in the Lower Potomac drainage.
References: (click for full references)
Bickley, W.E., and E.N. Cory. 1955. Water caltrop in the Chesapeake Bay. Association of Southeastern Biologists Bulletin 2:27-28.
Chen, Y.Y., X.L. Li, L.Y. Yin, and W. Li. 2008. Genetic diversity of the threatened aquatic plant Ottelia alismoides in the Yangtze River. Aquatic Botany 88(1):10-16. https://doi.org/10.1016/j.aquabot.2007.08.002.
Chen, Y.Y., Q.X. Han, Y. Cheng, Z.Z. Li, and W. Li. 2010. Genetic variation and clonal diversity of the endangered aquatic fern Ceratopteris pteridoides as revealed by AFLP analysis. Biochemical Systematics and Ecology 38(6):1129-1136. https://doi.org/10.1016/j.bse.2010.12.016.
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(11):1406-1415. https://doi.org/10.1002/j.1537-2197.1996.tb13934.x.
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. http://apms.org/wp/wp-content/uploads/2012/10/v42p17.pdf.
Ikeda, K., and F. Nakasuji. 2002. Spatial structure-mediated indirect effects of an aquatic plant, Trapa japonica, on interaction between a leaf beetle, Galerucella nipponensis, and a water strider, Gerris nepalensis. Population Ecology 44(1):41-47. https://doi.org/10.1007/s101440200005.
Li, X.L., X.R. Fan, H.J. Chu, W. Li, and Y.Y. Chen. 2017. Genetic delimitation and population structure of three Trapa taxa from the Yangtze River, China. Aquatic Botany 136:61-70. https://doi.org/10.1016/j.aquabot.2016.09.009.
Mirick, P.G. 1996. Goose grief. Massachusetts Wildlife 46(2):15-16.
Muenscher, W.C. 1944. Aquatic Plants of the United States. Comstock Publishing Company, Inc/Cornell University, Ithaca, NY.
Rybicki, N., E. Striano, and R. Thum. 2015. A Novel Invasive Species of Water Chestnut Found in the Potomac River near Washington, D.C. Pages 42-43 in 55th Annual Meeting of the Aquatic Plant Management Society, July 12-15, 2015. The Aquatic Plant Management Society, Inc. Gainesville, FL.
Takano, A., and Y. Kadono. 2005. Allozyme variations and classification of Trapa (Trapaceae) in Japan. Aquatic Botany 83:108-118. https://doi.org/10.1016/j.aquabot.2005.05.008.
Tsuchiya, T., and T. Iwakuma. 1993. Growth and leaf life-span of a floating-leaved plant, Trapa natans L., as influenced by nitrogen flux. Aquatic Botany 46(3-4):317-324.
Vuorela, I., and M. Aalto. 1982. Palaeobotanical investigations at a Neolithic dwelling site in southern Finland, with special reference to Trapa natans. Annales Botanici Fennici 19(2):81-92. http://www.jstor.org/stable/23725192.
Winne, W.T. 1950. Water chestnut: A foreign menace. Bulletin to the Schools 36(7):230-234.
This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.