WaterLettuce

Water lettuce
(Pistia stratiotes)

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

Current invasive species rule revisions propose to regulate this species as Prohibited.



Overview

Regulated areas of Water Lettuce
This species is listed as Proposed Prohibited (Red counties)

Other names for this plant include:

  • Common names: tropical duckweed
  • Scientific names: Pistia spathulata

Ecological threat:

  • 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 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.

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 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 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

Pistia_stratiotes
Counties in WI where water lettuce has been reported (as of April 2013). Both vouchered and unvouchered reports included.

Do you have water lettuce in your county but it isn't shaded on the map? Have you seen it? 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 flowering and seed set.

Chemical: Registered aquatic herbicides can provide temporary control of water lettuce in small scale applications. 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

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Links for More Information

Last revised: Thursday October 31 2013