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Species: Lemna minor L., lesser duckweed
Lemna trisulca L., start duckweed or ivy duckweed
Family: Lemnaceae

Duckweeds are among the world’s smallest flowering plants. Individual lesser duckweed plants are tiny, round, bright green disks, each with a single root. They are found scattered among emergent plants or massed together in floating mats. Star duckweed is much less commonly observed. Individual nonflowering plants are longer and narrower than lesser duckweed, commonly floating in masses beneath the water surface. Flowering plants more closely resemble lesser duckweed.

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Leaf: No true leaves. The leaf-like body is called a thallus. Lesser: nearly circular to oval, 2-5 mm in diameter; occur as single plants or up-to-five plants may be connected. Star: two types: non-flowering plants are elongate or spatula-shaped (6-10 mm long), tapered to a stalked base, connected in branched chains of 8- 30 plants, and submersed beneath the water surface. Flowering plants are more oval-shaped with a separate margin and a shorter stalk at the base. They float on the water surface.

Stem: None.

Flower: Tiny, rarely seen. Arises from a pouch in the thallus.

Fruit: Inconspicuous, usually 1 seeded.

Root: Lesser: Single short rootlet hangs from the underside of each plant. Star: Often rootless.

Propagation: New plants bud from pockets on either side of the parent plant and eventually break apart. Overwinters as winterbuds on the lake bottom, but rarely reproduces from seeds. Distributed by wind and on the bodies of birds, and aquatic animals. A single lesser duckweed plant can reproduce itself about every 3 days under ideal conditions in nutrient-rich waters.

Importance of plant: Food for fish and waterfowl and habitat for aquatic invertebrates. Because of its high nutritive value, duckweeds have been used for cattle and pig feed in Africa, India, and southeast Asia. Sometimes used to remove nutrients from sewage effluent.

Distribution: Throughout much of the temperate and subtropical regions of the world.

Habitat: Still and slow moving waters in many freshwater habitats. Often found along the shoreline after water levels have dropped.

May be confused with: Giant-duckweed (Spirodela polyrrhiza) which has a cluster of 6 to 18 roots from each thallus and is green on top and brown on the bottom, watermeal (Wolffia spp.) which is smaller and rootless, mud midget (Wolffiella gladiata), which is much longer than broad and lacks rootlets, and Mexican water fern (Azolla mexicana), which is greenish red with a fuzzy, nubby texture. Another duckweed species (Lemna turionifera) also occurs in Washington, but is very difficult to differentiate from lesser duckweed. Lemna turionifera is reddish on the under surface, produces overwintering turions, and has a slightly different vein pattern. 

Photographs: Closeup of Lemna minor, massed along shoreline
Lemna trisulca closeup

Line Drawings: Lemna minor


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