PISTIA STRATIOTES L.
water lettuce, laitue d'eau
pronounced: pis-tee-a / stra-tee-o-teez
from: pistos (G.): water
stratiotes (G.): a soldier (Discordies name for an Egyptian water plant)
a water plant
Water lettuce is a floating plant. Experts disagree as to whether water lettuce is native to the U.S.: it has been present in Florida since as early as 1765 when the explorer, William Bartram, described and drew the plant in Lake George. This floating plant commonly forms large infestations which prevent boating, fishing and other uses of lakes and rivers. Water lettuce occurs in lakes, rivers and canals, occasionally forming large dense mats.
As its name implies, water lettuce resembles a floating open head of lettuce. Water lettuce has very thick leaves. The leaves are light dull green, are hairy, and are ridged. There are no leaf stalks. Water lettuce roots are light-colored and feathery. Its flowers are inconspicuous.
- water lettuce is a floating perennial
- floating, "obligate" (requiring a wet habitat)
- linked plants form dense mats in the water
- will halt boat traffic on rivers; will cover a lake surface from shore to shore
- an aquatic weed worldwide in rivers, lakes and ponds of temperate climates
- temperature tolerance: water lettuce is not winter-hardy; its minimum growth temperature is 15o C (59o F); its optimum growth temperature is 22-30o C (72-86o F); its maximum growth temperature is 35o C (95o F) (Kasselmann 1995)
Pistia stratiotes L.
- monocot, perennial
- free-floating except when stranded in the mud; singly or massed in large numbers; mother and daughter plants attached by short stolons
- thick soft leaves are formed in rosettes, with no leaf stems; leaves to 6 in. long; light green; with parallel ridges (veins), covered in short hairs; leaf margins wavy, top margins scalloped
- flowers inconspicuous (not observed in Florida till the 1980s though they had been flowering all along); nearly hidden in the center amongst the leaves; on small stalk, single female flower below and whorl of male flowers above
- roots hanging submersed beneath floating leaves; feathery, numerous
- fruit a green berry
Pistia stratiotes is not likely to be confused with any other floating plant.
- Experts disagree as to whether or not Pistia stratiotes is native to the U.S. After all, it was observed, described and drawn by William Bartram in 1765 during his explorations of Florida. He wrote that he saw:
- "...prodigious quantities of the pistia, which grows in great plenty most of the way [along the St. Johns River, Florida], and is continually driving down with the current, and great quanitites lodged all along the extensive shores of this great river and its islands, where it is entangled... and... all matted together in such a manner as to stop up the mouth of a large creek, so that a boat can hardly be pushed through them, though in 4 foot water..."
- Some experts believe the plant's origins are in Africa.
Distribution in the U.S.
Water lettuce is present in the states of the southeast U.S. and north to New Jersey and New York, and westward to Texas, Arizona and California. Also presentin Hawaii, Puerto Rico and the Virgin Islands.
The best way to track the spread of invasive aquatic plants may be to identify the drainage basins (watersheds) they have been discovered in. Drainage maps give useful information to eco-managers because drainage maps show precisely where the plants are, making it easier for managers to infer where the plants might go next, and thus where to take preventive measures.
How it got here
- Pistia stratiotes, water lettuce, is believed by some to be a native plant and by others to be a non-native plant that arrived in the ballast of explorer's sailing ships.
- Pistia stratiotes continues to be sold through aquarium supply dealers; it is not on the U.S. Federal Noxious Weed List.
Potential to spread elsewhere in U.S.
- water lettuce is found globally in the tropics and subtropics, but its spread is limited by severe cold (Holm et al. 1977); its leaves regrow after moderate freezes
- water lettuce reproduces vegetatively and sexually; new daughter plants are formed on stolons which grow from the mother plants; seedlings are produced in mild climates (Penfound & Earle 1948)
- Pistia stratiotes mats clog waterways, making boating, fishing and almost all other water activities, impossible
- water lettuce mats degrade water quality by blocking the air-water interface and greatly reducing oxygen levels in the water, eliminating underwater animals such as fish
- water lettuce mats greatly reduce biological diversity: mats eliminate native submersed plants by blocking sunlight, alter emersed plant communities by pushing away and crushing them, and also alter animal communities by blocking access to the water and/or eliminating plants the animals depend on for shelter and nesting
- in Florida, water lettuce has never been the problem that water hyacinth has been; water lettuce is essentially under "maintenance control" in Florida
Due to decades of university, state and federal research and experience with Pistia stratiotes in the U.S., several methods have been developed to help in its management:
the action of mechanical harvestors and chopping machines remove water lettuce from the water and transport it to disposal on shore; chopping machines grind the plant into bits and spray the slurry across the water
Years of research to find insect biocontrols has resulted in the successful introduction of two insects which are believed to be helping keep water lettuce under maintenance control in many places; however biocontrol fish which are able to control submersed plants are ineffective against the floating water lettuce.
registered aquatic herbicides do provide temporary control of water lettuce
What can you do?
First, clean your boat before you leave the ramp! Transporting plant fragments on boats, trailers, and in livewells is the main introduction route to new lakes and rivers.
But, there's plenty more you can do to help.
Laws and lists
- is "state-listed" in Arizona, Florida, Puerto Rico and South Carolina
- is on the Florida Prohibited Plants list, Florida Department of Environmental Protection:
- is not on the Federal List of Noxious Weeds (USDA/APHIS, 2000)
- is on the Florida Exotic Pest Plant Council list:
Category I - "plants invading and disrupting native plant communities in Florida"
Want to know more?
The information contained on this wep page was extracted from published scientific literature and agency reports. It is important to know that plant research, like most areas of scientific research, is still relatively young and incomplete--much may have been published about the physiology of one plant but not about its management; much may have been published about how to culture and grow another plant but not about its natural ecology. Thousands of research articles may have been published about one invasive plant, but perhaps only a dozen about another.
If you want to read the research yourself, perhaps to clarify or expand an area of information contained here, or to help determine your own line of research, you are welcome to query the world's largest collection of international scientific literature about aquatic, wetland and invasive plants, the APIRS bibliographic database, which contains more than 54,000 citations and their content keywords. Or you might want to ask us to do it for you and mail or e-mail the search results to you.
This is the literature aboutPistia stratiotes that was used to develop this web page. More research items about this plant may be found at APIRS:
- Holm LG, Plucknett DL, Pancho JV, Herberger JP. 1977. The world's worst weeds: distribution and biology. Honolulu: University Press of Hawaii. 609 pp.
- Kasselmann C. 1995. Aquarienpflanzen. Egen Ulmer GMBH & Co., Stuttgart. 472 pp. (In German)
- McCann JA et al. 1996. Nonindigenous aquatic and selected terrestrial species of Florida-Status, pathway, and time of introduction, present distribution, and significant ecological and economic effects. Southeastern Biological Science Center, Gainesville, 256 pp.
- Van TK, Steward KK. 1982. Evaluation of chemicals for aquatic plant control. Annual Report 1981, Ft. Lauderdale, FL, 66 pp.
- Vandiver VV. 1999. Florida aquatic weed management guide. Univ. of FL, IFAS, Cooperative Extension Service, Publ. SP-55, 130 pp.
See more information about water lettuce as contained in the Langeland/Burks book, Identification & Biology of Non-Native Plants in Florida's Natural Areas
For brief control information, see Efficacy of Herbicide Active Ingredients Against Aquatic Weeds by K. Langeland, M. Netherland, and W. Haller.
This web page was authored in June, 2001, by Victor Ramey (Center for Aquatic and Invasive Plants, University of Florida), with significant contribution from Barbara Peichel (Sea Grant, University of Minnesota). The information contained herein is based on the literature found in the APIRS database.
1. From Identification and Biology of Nonnative Plants in Florida’s Natural Areas – Second Edition, by K.A. Langeland, H.M. Cherry, et al. University of Florida-IFAS Pub SP 257. 2008.
2. From Strangers in Paradise, Impact and Management of Nonindigenous Species in Florida, Chapter 2: Florida’s Invasion by Nonindigenous Plants: History, Screening, and Regulation, by D.R. Gordon and K.P. Thomas, pp. 21-37. Island Press, Washington, DC, 1997.
3. From Identification and Biology of Nonnative Plants in Florida’s Natural Areas – Second Edition, by K.A. Langeland, H.M. Cherry, et al. University of Florida-IFAS Pub SP 257. 2008.