Species Overview

Scientific: Pistia stratiotes 

Common: water lettuce, waterlettuce, water-lettuce, pistia, shell-flower, tropical duckweed, water bonnet, water cabbage, water fern, water lily 

Family: Araceae (aroid or arum family) 

Origin: Native to the Paleotropics and Neotropics, including peninsular Florida [14, 69]. 

Introduction History: Widely introduced through ornamental and aquaculture trade [49]. The popularity and availability of this weed, combined with improper disposal of plant material have allowed for proliferation in Florida, especially in abnormally high-nutrient freshwater habitats [37, 49, 53]

Species Characteristics

General Description: An aggressive, free-floating aquatic herb that forms rosettes resembling open heads of lettuce. Leaves are pale green, velvety, and strongly ribbed, while long feathery roots hang beneath the water surface. Rapid reproduction can form dense mats covering ponds, lakes, canals, and slow-moving waterways. 

Habitat: Stagnant or slow-moving aquatic habitats (both lotic and lentic systems). Typically freshwater bodies including lakes, ponds, rivers, springs, canals, drainage ditches, and seasonally flooded wetlands [41, 43, 63, 64, 69]. Can tolerate short-term exposure to mild salinity, suggesting some capacity to persist in transitional or disturbed aquatic environments [68]. 

Invaded Range: Widely distributed in tropical and subtropical regions globally [46, 63]. Reported from FL, LA, TX, Mexico, West Indies, Central America, South America, Asia, Africa, Pacific Islands, Australia [64]. In the southeastern United States, populations are most persistent in Florida, with northern distribution constrained by freezing winter temperatures [36, 43, 48, 54, 66, 67]. However, there have been documented instances of water lettuce persistence through temperate climate winters due to thermally altered waters (such as warm effluent outlets) and viable seed production [23]. Water lettuce is considered historically native in Florida based on fossil evidence and genetic research [14, 15, 32, 59, 60].  It is still commonly treated as a nuisance weed species due to prolific growth and recalcitrance in managed waterways [37, 69]. Subtropical climates have typically mild and brief winter low temperatures. This climate, combined with vast wetland and freshwater habitats, seasonal flooding, and abundant water control infrastructure in Florida, promote the presence of water lettuce throughout the state [72].

Do you know what exactly invasive means? How about nuisance? Read this article.  

Still curious about the native status of water lettuce? Read this blog. 

Leaves: Form a basal rosette of thick, spongy, obovate to wedge-shaped overlapping leaves (0.5-6 inches long). Conspicuous parallel venation (7-15 veins) and ridged leaf surface with dense, short trichomes producing a dense pubescence (velvety texture) that repels water. Leaf color is green, gray-green, or pale green [13, 29, 44, 64]. 

Roots: Primary roots extend into water column, as long as 20 inches, with numerous, short branching fine roots giving a feathery appearance when suspended in water [64]. 

Inflorescence: Inconspicuous spathe inflorescence. White to pale green. Produced in late summer to early winter [64, 69]. 

Flowers: 6-8 staminate flowers formed in a single whorl around a central stalk; pistillate flower solitary [64]. 

Fruit: Small, green inconspicuous berry fruit with thin pericarp [64]. 

Seeds: Cylindric, light brown seeds [64]. Water-dispersed and able to withstand suboptimal conditions and germinate in water temperatures above 68°F [41, 43]. Seed germination occurs within the aquatic soil profile, and the first leaves produced allow it to float to water surface [22]. 

Other (Reproduction): Plants reproduce rapidly via vegetative fragmentation of stolons and through seed production, both contributing to dispersal and persistence [11, 12, 13, 41]

Impacts

Regulation/Categorization: Florida Noxious Weed, FISC Category “I”, UF/IFAS Assessment “Prohibited". 

• Rapidly forms dense mats which decrease light-availability for other submerged plants [11, 20, 52, 61, 66, 67]. Waterbodies with high abundance of floating aquatic plants, such as water lettuce, are associated with decreased native plant richness and abundance [28]. The floating habit and unanchored roots of water lettuce, like many other problematic aquatic macrophytes, allows invasion not limited by water depth or transparency, both of which influence light availability below the surface [6, 7].  

• Although floating aquatic macrophytes such as water lettuce can initially reduce nutrients within the invaded waterbody, they also may increase susceptibility to eutrophication [16]. Waterbodies with monocultures of floating vegetation may be more susceptible to algae blooms than those with mixed plant community structures due to lowered interspecific nutrient competition [3, 52]. 

• Heavy infestations reduce dissolved oxygen by limiting gas exchange, shading the water column, and increasing oxygen demand during decomposition, with concentrations beneath dense mats declining to levels critical for fish survival  [3, 16, 17, 23, 61, 63, 67]. 

• Rapid biomass production and subsequent deposition of organic material increase sedimentation rates, contribute to internal nutrient loading, and can alter waterbody quality and aquatic ecosystem structure [3, 17, 21, 43, 61, 73]. 

• Can impede waterway access and navigation and damage or interfere with water control infrastructure [18, 20, 21, 53, 65, 66, 67]

• Presents a human-health risk by providing habitat for disease-vector mosquitoes within the vegetative mats [31, 39, 71]. 

Do not sell, propagate, or move plants. Thoroughly clean boats and equipment before leaving infested waterways.

Small infestations can be hand pulled. Water drawdowns can be used to “strand” and desiccate water lettuce on exposed shorelines, but the time needed to effectively dry large mats of plants can be long.

Mechanical harvesting can be done but the process should be designed to prevent the spread of water lettuce plantlets to other parts of the water body.

The water lettuce leaf moth (Spodoptera pectinicornis) was imported from Thailand and released in Florida in 1990 but failed to establish. The water lettuce leaf weevil (Neohydronomus affinis) was imported from South America to the US in mid-1980s and is now established throughout Florida, but its effect on water lettuce growth in Florida has been negligible.

Herbicide selection is based on water use, selectivity to reduce damage to non-target native plants and cost. Several herbicides can be used as foliar sprays to selectively control water lettuce. Contact herbicides such as diquat, carfentrazone and flumioxazin are quickly absorbed by plant tissue and cause obvious damage within a few days, whereas systemic herbicides such as imazapyr, penoxsulam and bispyribac provide slower but very effective control. Submersed application of the contact herbicide flumioxazin is currently being evaluated for selective control of water lettuce, as are topramezone and the ALS herbicides.

Source and additional details: Biology and Control of Aquatic Plants: Chapter 2:12

Consult your local UF IFAS Extension for further assistance with management recommendations and planning.