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Water lettuce

(Pistia stratiotes)

Photo of water lettuce
Photo credit: Troy Evans, Great Smoky Mountains National Park, Bugwood.org

A floating perennial, resembling a floating head of lettuce. It is disputed as to whether or not it is native to the United States since it was observed and documented in William Bartram’s 1765 explorations of Florida. It has high rates of vegetative reproduction, so it is able to rapidly form dense mats on the surface of water bodies.

Overview

Other names for this plant include:
  • Common names: tropical duckweed
  • Scientific names: Pistia spathulata
Ecological threat:
  • It invades freshwater lakes, reservoirs, ponds, marshes and slow-flowing streams and rivers making boating, fishing and almost all other water activities difficult.
  • Degrades water quality by blocking the air-water interface and greatly reducing oxygen levels in the water, impacting underwater animals such as fish.
  • Greatly reduces biological diversity: mats block sunlight, preventing the growth of submerged and emersed plant communities and also alter animal communities by blocking access to the water and/or reducing plants the animals depend on for shelter and nesting.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for water lettuce was based upon this literature review [PDF] developed by the department.

Identification

Leaves: Soft and thick, formed in rosettes; have no leaf stems; leaves 6 in. long; light green; parallel ridges (veins); covered in short hairs; leaf margins wavy, top margins scalloped.

Flowers: Inconspicuous; nearly hidden in the center amongst the leaves; on a small stalk, single female flower below and whorl of male flowers above. Flowers in late summer to early winter.

Fruits & seeds: A green berry.

Roots: Hang submersed beneath floating leaves; feathery, numerous.

Similar species: Not likely to be confused with any other plant.

Distribution

See the reported locations of water lettuce in Wisconsin.

Do you know of additional populations? Send us a report.

Control

Mechanical: Removal by hand or machine is a practical control method often used for small areas or when numbers are low. Physical removal should be completed before the flowering and seed set.

Chemical: Registered aquatic herbicides can provide temporary control of water lettuce in small scale applications. The application of aquatic herbicide requires a permit.

Biological: Only two insects have been released into the United States as biological control agents against this weed; the South American weevil N. affinis and the Asian moth S. pectinicornis.

Photos

View water lettuce pictures in our photo gallery!

Resources

Sources for content: Links for more information: