water lettuce

Pistia stratiotes L.

Description from Invasive.org

Appearance: Pistia stratiotes is a sprawling, semi-woody shrub that invades wetlands in Florida. The green stems are up to 15 ft. (4.6 m) long and prickly.
Foliage: Leaves are flashy and arranged in a rosette. They measure 0.75-6 in. (2-15 cm) in length. The leaves are green to grayish-green, and have dense white hairs and parallel veins on their surface. As its common name indicates, the plant resembles a floating head of lettuce.
Flowers: This plant flowers year-round in southern Florida, but peaks during summer and early autumn.
Fruit: The hydrosoil under a Pistia stratiotes infestation was shown to hold 4,196 seeds/m2. Mature seeds in fruits had an 84% germination rate.
Ecological Threat: Pistia stratiotes, forms extensive mats that can block navigational channels, impede water flow in irrigation and flood control canals, and disrupt submersed animal and plant communities (Sculthorpe, 1967; Attionu, 1976; Bruner, 1982; Sharma, 1984). Pistia stratiotes is among the world’s worst weeds (Holm et al., 1977). It has been placed on prohibited plant lists in Florida (FDEP, 2000), Louisiana (LDWF, 2000), Mississippi (MDAC, 1997), and Texas (TPWD, 2000), and is considered a noxious species (an invasive species of concern, but not regulated) in South Carolina (SCDNR, 2000) and Delaware (DDFW, 2000).

Resources

Biological Control of Invasive Plants in the Eastern United States - USDA Forest Service
Global Invasive Species Database - Invasive Species Specialist Group
Images, Video and Information - University of Florida - Center for Aquatic and Invasive Plants
Fact Sheet - Ontarios Invading Species Awareness Program

Selected Images from Invasive.org

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Plant(s)
USDA ARS , USDA Agricultural Research Service, Bugwood.org
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Foliage
Graves Lovell, Alabama Department of Conservation and Natural Resources, Bugwood.org
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Plant(s)
USDA APHIS PPQ - Oxford, North Carolina , USDA APHIS PPQ, Bugwood.org
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Plant(s) exotic invasive
Troy Evans, Great Smoky Mountains National Park, Bugwood.org
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Plant(s) in water garden
Forest and Kim Starr, Starr Environmental, Bugwood.org
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Foliage in water garden
Forest and Kim Starr, Starr Environmental, Bugwood.org
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Foliage leaf
Forest and Kim Starr, Starr Environmental, Bugwood.org
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Plant(s)
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
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Flower(s)
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
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Plant(s)
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
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Infestation
Karen Brown, University of Florida, Bugwood.org
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Infestation
USDA APHIS PPQ - Oxford, North Carolina , USDA APHIS PPQ, Bugwood.org
Additional Resolutions & Image Usage