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Photo credit: © Bernard Dupont, https://flic.kr/p/biWPZt
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Species at a Glance
As its name implies, water lettuce is a floating perennial plant that resembles an open head of lettuce. It forms in colonies of rosettes that link together to blanket the water’s surface, blocking waterways and disrupting natural ecosystems.
Leaves: Thick, soft, light green leaves are formed in rosettes with no leaf stems. Rosettes can occur by themselves or connected to others by short stolons. Leaves are large, up to 16 cm (6 in) long with parallel ridges (veins) covered in short hairs. Leaf margins are wavy and the top margins are scalloped.
Flowers: Flowers are inconspicuous and hidden in clusters in the center amongst the leaves. They form on a small stalk with a single female flower and a whorl of male flowers above.
Fruits/Seeds: The fruit arises from the female flower as a many-seeded green berry.
Stems/Roots: Roots are light-colored and feathery and hang submersed beneath the floating leaves.
Water lettuce may be confused with water hyacinth (Eichhornia crassipes); however, water lettuce has large ribbed leaves and it doesn’t have the showy flowers characteristic of water hyacinth.
Mostly occurring in slightly acidic lakes, rivers, ponds, and canals in temperate climates, water lettuce has also been found to survive in mud. It is not winter hardy, requiring temperatures above 15°C (59°F) for growth.
While some believe water lettuce is a native species, others believe it was brought over in the ballast water of ships from the tropical and subtropical regions of Asia, Africa, and South America. It is also commonly sold through the aquarium industry and additional introductions have occurred as plants are intentionally released or escape from ponds and water gardens. This plant can spread by fragmentation, by daughter plants that form on the stolons of the mother plant, or by seeds.
Water lettuce has been present in Florida since as early as 1765; however, it has since spread throughout the southeastern United States north to New York and westward to Texas, Arizona, and California. It is also present in Hawaii. In the Mid-Atlantic, water lettuce is found in Delaware, New Jersey, New York, North Carolina, and Maryland.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
The large mats formed by water lettuce negatively impact boating, fishing, and swimming. They also make navigation difficult, hinder flood control efforts, and clog hydroelectric turbines. The colonies of rosettes decrease biodiversity by blocking sunlight and oxygen from penetrating into the water column, thereby altering the native plant and animal communities below.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).