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: Pistia stratiotes is a free-floating aquatic macrophyte that resembles an open head of lettuce (Ramey 2001a,b, EPPO 2017). It is also a perennial moncotyledon (Ramey 2001a,b). P. stratiotes tend to form small colonies and can be found in dense floating mats (EPPO 2017). Morphological variation is strongly influenced by environmental conditions and population density (Thompson 2007). P. stratiotes is only member of the Pistia genus worldwide, and so it is unlikely to be confused with other species (Cahill et al. 2018). Several characteristics help distinguish P. stratiotes (Ramey 2001a,b, Langeland and Burks 1999, CABI 2014, Cahill et al. 2018):

Leaves: gray-green, about 10-20 cm long, widest at the apex, spongy near the base, with dense white hairs, 7-15 prominent parallel veins, and arranged in rosettes (i.e. circular leaf arrangement).
Stolons (i.e. stem or runner): arise from leaf axis, up to 70 cm long, and entangle to create floating mats.
Roots: covered with fine root hairs which give them a feather-like appearance, dense, and hang unbranched about 50 cm below the water.
Flowers: inconspicuous and clustered on small, fleshy stalks that are nearly hidden in the leaf axils. Flowers are single sexed with male: 3-9 flowers whorled above a single female flower.
Fruit: green berry with many seeds.

Size: Rosette generally 6 to 30 cm in diameter (Godfrey and Wooten 1981), 15 cm length (Langeland and Burks 1999).

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: HABITAT: Pistia stratiotes is an aquatic, floating perennial plant that is native to tropical and subtropical lakes (Pieterse 1981). Distribution is limited by sensitivity to cold temperatures. However, overwintering by seeds could account for population recurrence in temperate regions with appropriate substrate and hydrologic conditions. This species most commonly inhabits slow moving water - ponds, lakes, swamps, irrigation canals, or ornamental ponds.

Parameter description	Numeric value	Reference
Temperature tolerance	15° C (59 °F) - 35° C (95 °F)	Rivers 2002
Optimal germination temperature	20 °C - 25 °C	Parsons and Cuthbertson 2001
Thermal minimum for seeds	4 °C and a few weeks in ice at –5 °C;	Pieterse 1981
Salinity tolerance	>20mM NaCl	Vázquez et al. 2021

 

FOOD WEB: This species is an autotroph. It serves as a food source for many invertebrate species and is commonly associated with the salvinia stem-borer moth (Samea multiplicalis), the leafhopper (Draeculacephala inscripta), and the aphid (Rhopalosiphum nymphaeae) (Dray et al. 1988). The West Indian manatee (Trichechus manatus) is also known to consume P. stratiotes in the southern coastal United States (Allen and Keith 2015).

LIFE HISTORY: Pistia stratiotes reproduces asexually and sexually. It can spread rapidly by vegetative fragmentation from offshoots on short, brittle stolons. Also, seed production is now considered a major method of reproduction and dispersal (Dray and Center 1989a,b). P. stratiotes possess both male and female flowers. Seeds are wind and insect pollinated and mature 30 days after fertilization (Parsons and Cuthbertson 2001). Seeds can disperse by water and sink into the mud or stream bed, and germinate in light at temperatures above 20 °C. Seedlings float to the surface once the primary leaf has developed. Each plant produces several stolons that are about 60 cm long that terminate in rosettes and can become fragmented to produce new plants.

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

Pistia stratiotes can have detrimental effects on the environment and other species due to its production of allelochemicals, which inhibits algae growth thus resulting in food web disruption and water chemistry changes (Aliotta et al., 1991). Additionally, it causes high evapotranspiration rates (Sharma, 1984) and forms dense mats that reduce light availability for submerged macrophytes and planktonic algae (Attionu, 1976). These factors, along with its impact on water temperature, pH, stratification, and oxygen levels, can lead to adverse consequences for native plants, fish, and wildlife (Attionu, 1976; Šajna et al., 2007; Sridhar and Sharma, 1986, FL DEP 2007). Pistia stratiotes has outcompeted other species where it has been introduced. Three years after P. stratiotes was first observed in Slovenia, it had covered the whole water surface and populations of native freshwater plants, Ceratophyllum demersum, Myriophyllum spicatum, Najas marina, and Trapa natans, had declined (Šajna et al. 2007).

Pistia stratiotes is among the world’s worst weeds (Holm 1991) and has received significant media attention (e.g. de la Cruz 2014, Spear 2014).

Pistia stratiotes mats provide habitat for disease carrying mosquitoes, such as malaria vectors Anopheles and Mansonia (FL DEP 2007, Lounibos and Dewald 1989, Parsons and Cuthbertson 2001, Rejmankova et al. 1991).

Pistia stratiotes causes damage to infrastructure. Infestations of this species can block waterways, reducing the efficiency of irrigation and hydroelectric power (Howard and Harley 1998). Dense mats of P. straiotes reduce water flow, damage flood control structures, and can create dams against bridges (FL DEP 2007). P. stratiotes may impact recreation, as it interferes with navigation and fishing (Labrada and Fornasari 2002). Florida spent about $1.4 million dollars in 2005-2006 to treat P. stratiotes (FL DEP 2007).

Pistia stratiotes is also used in traditional medicine for its therapeutic properties (Tripathi et al. 2010, Saddam et al. 2018, Abubakar et al. 2020).

Research has been conducted to utilize this species for biofuels (Lu et al. 2010, Mishima et al. 2008).

Pistia stratiotes is a potential candidate for the removal of heavy metals from contaminated water (Odjegba and Fasidi 2004, Ali et al. 2020, Leblebici et al. 2019, Vieira et al. 2019). Numerous experiments P. stratiotes could be used to remediate runoff from a variety of sources including nursery, greenhouse operations, domestic sewage, textile factories, rice mills, paper mills, and sugar mills  (Sridhar and Sharma 1980, Polomski et al. 2009,  Kumar et al. 2018, Ferreira et al. 2019, Schwantes et al. 2019, Kumar et al. 2020,Ekanayake et al. 2021). Several experimental trials indicate that P. stratiotes can dissipate herbicides, and so this plant could decontaminate waters near agricultural areas (Barchanska et al. 2019, Escoto et al. 2019, Alencar et al. 2020, Alonso et al. 2021).

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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., J. Redinger, and K.M Reaver

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.