Disclaimer:

The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

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Common name: water lettuce

Synonyms and Other Names: water-lettuce, water-cabbage, river-lettuce, water-bonnet, shell-flower

Taxonomy: available through www.itis.gov

Identification:  

Stem/Roots: Pistia stratiotes is a free-floating, herbaceous monocot with a rosette of gray-green leaves, resembling a head of lettuce (thus the common name), occurring as a single plant or connected to others by stolons (Dressler et al. 1987; Langeland and Burks 1998). Roots numerous and feathery.

Leaves: Leaves are ovate to obovate, up to 15 cm in length, without a leaf stalk, spongy near the leaf base, densely pubescent, with deeply furrowed parallel veins and wavy leaf margins (Godfrey and Wooten 1981; Dressler et al. 1987; Langeland and Burks 1998).

Flowers: Flowers inconspicuous, perfect, clustered in leaf axils with a single female flower and multiple male flowers (Langeland and Burks 1998).

Fruit/Seeds: Produces abundant seeds with high percentage of seed viability (Dray and Center 1989a, 1989b).

Look-a-likes: none

Size: Rosette generally 6 to 30 cm in diameter (Godfrey and Wooten 1981)

Native Range: The species is pantropical, occurring on all continents except Antarctica (Adebayo et al. 2011). The center of origin for P. stratiotes is unknown. Fossil records for this species can be found around the globe (Stoddard 1989). Ancient Egyptian hieroglyphics depict the plant and Greek botanists Dioscorides and Theophrastus described the plant floating on the Nile River more than 2,000 years ago, indicating African origin (Stoddard 1989). Pistia stratiotes in Brazil and Argentina host a large number of co-evolved specialist insect herbivores suggesting a South American origin (Center et al. 2002). In North America, both John and William Bartram described P. stratiotes as early as 1765 and 1773, respectively, along the St. Johns River in Florida, up to 300 river km upstream of the ocean inlet where any ballast material would likely have been deposited from trans-oceanic ships (Bartram and Harper 1942; Bartram and Harper 1943). Since plants were found so far upstream from known seaports, a rationale for Florida nativity has been suggested (Evans 2013). Late Pleistocene/early Holocene fossil records for this species in Florida lend support for this contention (Stoddard 1989; Evans 2013).

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Ecology: Pistia stratiotes is a free-floating, fast growing, obligate aquatic that can form vast, dense floating mats, covering the entire water surface of lakes and slow moving rivers (Langeland and Burks 1998). It is the sole species of this genus. In tropical and subtopical climates it is a perennial. In temperate regions the plant behaves as an annual, returning after the winter months from submersed seeds. This species does not tolerate freezing temperatures, although its seeds can survive submerged in water that is 4°C for at least 2 months (Parsons and Cuthbertson 2001). Pistia stratiotes has a low saline tolerance; plants cannot survive in waters with more than 2.5 ppt salinity (Sculthorpe 1967; Haller et al. 1974).

Pistia stratiotes reproduces rapidly by vegetative fragmentation from offshoots on short, brittle stolons. Seed production is also considered a major method of reproduction and dispersal (Dray and Center 1989a, 1989b). Plants can be solitary rosettes, or may have more than a dozen stolon-connected ramets or daughter plants.  Standing crop may be as high as 2 kg/m² at the peak of the growing season (Dray and Center 1992). Although vegetative reproduction is thought to be the primary means of propagation, seed crop has been reported to be in excess of 700 seeds/m² in a stand at a south Florida location, with greater than 80% seed viability (Dray and Center 1989a, 1989b).

Means of Introduction: The origin of Pistia stratiotes is contentious. Some argue the species is native to North America due to fossil evidence (Evans 2013), while others agree it was dispersed by transcontinental bird migrations (Stoddard 1989) or by dry ballast during early European colonization of North America (Stuckey and Les 1984; Schmitz et al. 1993; Dray and Center 2002).  

Pistia stratiotes was sold through aquarium and pond supply dealers, both online and in retail garden centers (Rixon et al. 2005); it is still offered for sale online and in several states (Rebecca Howard, USGS, pers. comm.). New introductions are probably the result of improper disposal of ornamental pond plants or waters, or when ponds adjacent to local water bodies overflow (Adebayo et al. 2011).

Status: Established in southern states (Alabama, Arizona, California, Florida, Louisiana, Mississippi, Texas) where plants may overwinter and also germinate from seed (Dray and Center 1989). Plants north of the Gulf states (Colorado, Connecticut, Delaware, Illinois, Kansas, Maryland, Michigan, Minnesota, Missouri, New York, North Carolina, Ohio, Rhode Island, South Carolina, and Wisconsin) likely do not overwinter, and are either extirpated, eradicated, or survive by seed production; the exception being Idaho where populations have established in a hot spring-fed river (Tom Woolf, ID Dept. of Ag., pers.comm.).

Impact of Introduction:

Summary of species impacts derived from literature review. Click on an icon to find out more...

Ecological	Economic	Human Health	Other

Dense populations of P. stratiotes can clog waterways and make fishing, swimming and boating difficult (Howard and Harley 1998). Thick colonies of water-lettuce block the air-water interface which reduces the amount of dissolved oxygen in the water making it less suitable habitat for fish species (Attionu 1976, Šajna et al. 2007, Sridhar and Sharma 1986). These dense mats can also block animal access to the water and may crowd or shade out native plants upon which other organisms depend for food or shelter (Sculthorpe 1967). 

Larvae and pupae of the mosquito genera Culex and Mansonia, found in the southeastern U.S., attach themselves to the root system of P. stratiotes (Lounibos and Escher 1985; Center et al. 2002). These mosquitoes are important vectors of St. Louis Encephalitis (Lounibos and Escher 1985; Petr 2000).

Pistia stratiotes has the fiber content, carbohydrate, and crude protein levels that are comparable with quality forages (Parsons and Cuthbertson 2001). Research has been conducted to utilize this species for biofuels and water remediation (Mishima et al. 2008; Lu et al. 2010).

References: (click for full references)

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Wisconsin Department of Natural Resources (WI DNR). 2010. Aquatic Invasives Data and Maps. Wisconsin Dept of Natural Resources, Madison, Wisconsin. http://dnr.wi.gov/lakes/invasives/BySpecies.aspx.

Yale University Peabody Museum. 2009. Botany Division, Yale Peabody Museum. Yale University, New Haven, CT. http://www.gbif.org/dataset/963f12d0-f762-11e1-a439-00145eb45e9a. Created on 03/13/2009. Accessed on 11/20/2015.

Other Resources:
University of Florida, Center for Aquatic and Invasive Plants - Pistia stratiotes

US Fish and Wildlife Service Ecological Risk Screening Summary for Pistia stratiotes

Author: Thayer, D.D., I.A. Pfingsten, V. Howard, J. Li., and J. Redinger

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.