Practical information to identify and manage non-native, invasive plants and animals
The Quiet Invasion:
A Guide to Invasive Species of the Galveston Bay Area
Water lettuce
Pistia stratiotes

USDA Symbol:PIST2
ITIS TSN:42542
Presence:Current Invaders
Habitat:Aquatic
Native Range:Uncertain, but may be native to Africa and South America due to the existence of water lettuce-associated insects found on those continents. Water lettuce may also be native to Asia based on the existence of ancient medicinal remedies using the species.
Toxic Characteristics
Produces calcium oxalate crystals plus unknown toxins. It is harmful when eaten in large quantities. May irritate mouth and throat and can cause nausea, vomiting, and diarrhea.
Geographic Distribution
Primarily found in California, Arizona, Texas, and the Southeastern United States; also reported in New Jersey and surrounding states; Ohio and Missouri. Found in water bodies of the Lower Galveston Bay watershed, including Lake Houston and Armand Bayou.
Introduction Pathways
Identified in Florida as early as the 1765. Disposal of this aquarium plant and ballast from boats greatly contributes to its spread. Also used by some cultures for its medicinal qualities.
Specific Primary Habitats
Prefers stagnant, slow-moving freshwater environments. Grows optimally in temperatures from 72-86 Fahrenheit. It is found in wetlands, small ponds, lakes and slow-moving streams. May also be found in rice fields.
Identifying Characteristics
Roots are submerged, extremely feathery, and grow up to 3 feet in length. Leaves grow in lettuce-like rosets up to 6 inches in diameter and are distinguished by their wavy edges and many longitudinal ridges and veins. They are a light, lime green with velvet-like hairs. Leaf bases, which are stemless, meet underwater. When mature, the roset encloses a very small female flower and whorl of male flowers above. Fruit is a small green berry.
Reproduction Characteristics
Primarily reproduces vegetatively. Lateral stolons (floating stems) extend out from the central axis of the plant to initiate daughter plants. Can also reproduce via water-dispersed seeds.
Growth Characteristics
Water lettuce grows and vegetatively spreads very rapidly. Mats of stolon-connected plants can completely cover a small body of water in a short period of time.
Ecological, Economic, or Social Impact
Leaves of water lettuce form extensive mats on surface water, reducing light penetration and dissolved oxygen levels in the water. Alters native aquatic plant and fish communities. Impedes boat traffic and can clog irrigation canals.
Control
Contact herbicides such as Endothall and Diquat will act quickly, while systemic herbicides such as Rodeo act slowly. Take care to minimize impacts of herbicide application on native species. Decaying plant material may cause dissolved oxygen depletion.

Native Species Alternatives
Below is a list of alternative plants that are native to the area:
  • American lotus (Nelumbo lutea)
  • White water lily (Nymphaea odorata)
  • Floating bladderwort (Utricularia radiata)
This species belongs to the following lists:
Images
To view a larger version of an image, click on the thumbnail.
Severe infestation of water lettuce in an open-water wetland. Photo courtesy of USDA ARS Archives, USDA Agricultural Research Service, www.forestryimages.org; Image Number 0002016.
Severe infestation of water lettuce in an open-water wetland. Photo courtesy of USDA ARS Archives, USDA Agricultural Research Service, www.forestryimages.org; Image Number 0002016.
Close-up of a water lettuce leaf. Photo courtesy of USDA APHIS Archives, USDA APHIS, www.forestryimages.org; Image Number 1148054.
Close-up of a water lettuce leaf. Photo courtesy of USDA APHIS Archives, USDA APHIS, www.forestryimages.org; Image Number 1148054.
A single water lettuce rosette. Photo courtesy of USDA APHIS Archives, USDA APHIS, www.forestryimages.org; Image Number 1149036.
A single water lettuce rosette. Photo courtesy of USDA APHIS Archives, USDA APHIS, www.forestryimages.org; Image Number 1149036.
Infestation of water lettuce in a partially wooded wetland. Photo courtesy of Richard Carter, Valdosta State University, www.forestryimages.org; Image Number 1451191.
Infestation of water lettuce in a partially wooded wetland. Photo courtesy of Richard Carter, Valdosta State University, www.forestryimages.org; Image Number 1451191.
Mat of water lettuce infestation clogging a drainage ditch. Photo courtesy of Ken A. Langeland, University of Florida, www.forestryimages.org; Image Number 1624022.
Mat of water lettuce infestation clogging a drainage ditch. Photo courtesy of Ken A. Langeland, University of Florida, www.forestryimages.org; Image Number 1624022.
Water lettuce growing among cypress trees at the Wallisville Lake Project near the Trinity River. Photo courtesy of Lisa Gonzalez.
Water lettuce growing among cypress trees at the Wallisville Lake Project near the Trinity River. Photo courtesy of Lisa Gonzalez.
Page Updated/Reviewed: 07/14/2010 8:32 AM
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