water-lettuce, water-cabbage, river-lettuce, water-bonnet, shell-flower

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

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).

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).

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

Environmental	Socioeconomic	Beneficial

Potential pathway(s) of introduction: Dispersal, hitchhiking/fouling, unauthorized release, stocking/planting/escape from recreational culture

Pistia stratiotes occurs in close proximity to the Great Lakes basin. It is found during spring through the fall in Lake St. Clair (Adebayo et al. 2011), Detroit River, and inland waters in Ontario, Ohio, New York, and Minnesota (Cochran et al. 2006).

Pistia stratiotes spreads via vegetative fragmentation and water dispersal. Fragments or seeds of P. stratiotes may potentially be introduced to the Great Lakes by dispersal. Pistia stratiotes can be unintentionally transported to the Great Lakes by hitchhiking on boats and recreational equipment.

This species is common in the aquarium trade (Parsons and Cuthbertson 2001). According to a study on aquarium and pet stores near Lakes Erie and Ontario, 20% of stores surveyed carried Pistia stratiotes (Rixon et al. 2005). Pistia stratiotes may be released into the Great Lakes when aquarists dispose this plant into waterways. This species is planted in water gardens and may be unintentionally introduced to the Great Lakes. It is unknown whether this species is commercially cultured in the Great Lakes region. Pistia stratiotes is not known to be taken up or transported by ballast water.

Pistia stratiotes has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Pistia stratiotes inhabits tropical and subtropical lakes, reservoirs, and slow-flowing streams (Parsons and Cuthbertson 2001). Pistia stratiotes does best in warm waters, as it is killed by frost. Pistia stratiotes exhibits optimal growth at water temperatures of 22-30°C (Kasselmann 1995). It can tolerate temperatures as low as 15°C and as high as 35°C. This species has been observed to overwinter in the Erft River, Germany; the water temperature in that river is abnormally warm (>11°C) and only leaves that remained submerged survived (Hussner et al. 2014). Its seeds can survive for at least 2 months in water at 4°C and for a few weeks in ice at -5°C (Parsons and Cuthbertson 2001); its seeds have the potential to overwinter in the Great Lakes. Pistia stratiotes reproduces rapidly though seed production and vegetative fragmentation. Its short, brittle stolons that are involved in vegetative fragmentation may aid its establishment in the Great Lakes.

Pistia stratiotes has a widespread distribution encompassing 40 countries (Holm 1991), and occurs on every continent except Antarctica. This species is capable of expanding its distribution quickly. After 1 year, this species had rapidly spread and covered an entire lake (Sridhar and Sharma 1980, Venema 2001).

The native and introduced ranges of P. stratiotes have somewhat similar climatic and abiotic conditions as the Great Lakes; the Great Lakes may have lower air temperatures and lower water temperatures. The effects of climate change may make the Great Lakes a more suitable environment for the establishment of P. stratiotes. Warmer water temperatures and shorter duration of ice cover may aid the establishment of P. stratiotes. Freshwater lakes and slow-flowing streams in the Great Lakes may provide suitable habitats for P. stratiotes.

Pistia stratiotes has outcompeted other species where it has been introduced. Three years after it 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).

Surveillance and management efforts are currently underway to detect, control, and/or eradicate this plant in Michigan (MI DEQ 2013) and Wisconsin (Falk et al. 2010). However, a basin-wide monitoring program is lacking (Dupre 2011).

Pistia stratiotes has the potential for high environmental impact if introduced to the Great Lakes.

Pistia stratiotes may have detrimental impacts on other species and the environment. Pistia stratiotes produces α-asarone, a phenylpropanoid with antialgal activity (Aliotta et al. 1991), so it may interfere with the growth processes in algae. Pistia stratiotes causes high evapotranspiration rates where it occurs (Sharma 1984). By growing in dense mats, P. stratiotes can shade out and reduce the amount of light available to submerged macrophytes and planktonic algae (Attionu 1976). In addition, its dense cover may reduce water temperature, reduce pH, promote stratification, and inhibit mixing of the water by wind (Attionu 1976). As a result of its inhibition of hydrophyte and algal growth, the respiratory activity of its roots, decomposition when it dies, and the restriction of wind-generated mixing, P. stratiotes can reduce the amount of dissolved oxygen where it occurs (Attionu 1976, Šajna et al. 2007, Sridhar and Sharma 1986). It is suspected that the oxygen and light limitations caused by P. stratiotes may have killed native plants, fish, and wildlife (FL DEP 2007). Three years after P. stratiotes was first observed in Slovenia, there was a decline in native freshwater plants (Ceratophyllum demersum, Myriophyllum spicatum, Najas marina, and Trapa natans) (Šajna et al. 2007).

Pistia stratiotes has the potential for high socio-economic impact if introduced to the Great Lakes.

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 mosquitos, such as malaria vectors Anopheles and Mansonia (FL DEP 2007, Lounibos and Dewald 1989, Parsons and Cuthbertson 2001, Rejmankova et al. 1991). Mansonia larvae perforate leaves and roots of P. stratiotes to reach air chambers (Lounibos and Dewald 1989). Taeniorhynchus (Mansonioides) africanus and Anopheles gambiae breed in ponds and streams that are clogged with P. stratiotes (Philip 1930).

Pistia stratiotes causes damages 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, damages flood control structures, and can create dams against bridges (FL DEP 2007). Pistia 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 has the potential for moderate beneficial impact if introduced to the Great Lakes.

This plant has the fiber content, carbohydrate, and crude protein levels that are comparable with quality forages (Parsons and Cuthbertson 2001). This plant can be fed to pigs, but cows find it unpalatable. Pistia stratiotes is valued as an ornamental plant in water gardens. Research has been conducted to utilize this species for biofuels and water remediation (Lu et al. 2010, Mishima et al. 2008). Pistia stratiotes is used in Ayurvedic medicine for its diuretic, antidiabetic, and antidermatophytic, antifungal and antimicrobial properties.

This species is prohibited in Illinois. It is not prohibited in Indiana, Michigan, Minnesota, New York, Ohio, Ontario, Pennsylvania, Quebec, or Wisconsin (Great Lakes Panel on Aquatic Nuisance Species 2012).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological
Research has been conducted to investigate biological control techniques such as the utilization of specialist herbivores (e.g. Spodoptera pectinicornis, Neohydronomus affinis) (Cilliers 1991, Wheeler et al. 1998). A South American beetle, Neohydronomus affinis was released in Australia and reduced P. stratiotes infestations by 40% or more within 12-18 months (Harley et al. 1990).

Physical
Mechanical harvestors and chopping machines can help remove water lettuce from the water and dispose after grinding the plant down to bits (Ramey 2001). The resulting slurry is then sprayed over the water.

Chemical
In general, the most common herbicides used to control water lettuce are herbicides with the active ingredient diquat or glyphosphate (Howard and Harley 1998). Herbicides may cause weed die-off and subsequent decomposition that may remove dissolved oxygen from the water. Herbicides might not be able to kill the seeds of the floating aquatic weed.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.