California Aquatic Invasive Species (AIS)
University of California
California Aquatic Invasive Species (AIS)

Invasive Freshwater Plants and Rock Snot

Water Lettuce

Scientific Name

Pistia stratiotes - Visit ITIS for full scientific classification. 



  • Floating colonies with rosettes of ribbed, velvety, light green leaves.
  • Leaves up to 2" wide and 6" long with squared or slightly indented tips.
  • Inconspicuous flower spikes with separate male and female flowers. 
  • Still or slow-moving, warm, freshwater habitats, often with high nutrient levels.
  • Does not tolerate freezing. 
Invasion Pathways and Distribution
  • Introduced from tropical South America.
  • Found worldwide in tropical areas.
  • Can remove heavy metals from water but roots are killed by high levels of pollution.
  • Sold as an ornamental pond or aquarium plant and used for medicine in Asia.
  • It is spread by discarding plants from ponds and aquariums into waterways.
  • In California it is found along the Colorado River and is expected to spread. 
Life History
  • Seeds ripen in 4-6 weeks, need light to sprout, and can survive dry or freezing conditions.
  • Also spreads by stolons (a type of stem) that produce new plants.
  • Colonies become large and dense when conditions are right. 
  • Dense colonies reduce oxygen, light and pH levels in the water, outcompete other aquatic plants, interfere with water flow and recreational activities, increase siltation and provide mosquito habitat. 
References and Useful Links

For references by category and links to other useful AIS sites see our LEARN MORE page.

Top of page

Webmaster Email: