Water Lettuce - Pistia stratiotes

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. In the southern U.S., this floating plant 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. Like water hyacinth, water lettuce is not yet established in Sudbury, Assabet, and Concord watershed and is not considered winter-hardy although both species have been found in the Assabet River. In Massachusetts, water gardeners and aquarium hobbyists can unintentionally spread these plants by releasing them to natural waterways.

Water lettuce found on the Assabet River

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.

  • 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

Because this species is not yet established in Massachusetts, the best mangement options are prevention and early detection. The International Waterlily & Water Gardening Society compiles a comprehensive list of state-regulated invasive aquatic plants. Check here before buying any plant for water gardening, and never release non-native plants to our local ecosystem.

In Florida, where water lettuce is a significant problem, they have used mechanical harvestors and chopping machines remove water lettuce from the water and transport it to disposal on shore; two insect biocontrols which are believed to be helping keep water lettuce under maintenance control; and registered aquatic herbicides provide temporary control.

University of Florida Center for Aquatic and Invasive Plants
Invasive Plant Atlas of New England