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Washington State Department of Ecology > Water & shorelines > Freshwater studies > Lake water quality > Aquatic Plant Guide home > Free Floating Plants > Spirodela polyrrhza

Aquatic Plant Identification Manual for Washington's Freshwater Plants

Free Floating Plants

Species: Spirodela polyrrhiza (L.) Schleiden, giant-duckweed, great-duckweed, duckmeat
Family: Lemnaceae

Giant duckweed is only giant in relation to other duckweeds. This tiny plant lacks true leaves and stems. An individual plant consists of an oval leaf-like body called a thallus.  The underside of the thallus is typically reddish-purple with a cluster of 4-16 roots hanging in the water. Because of its colonial habit, giant duckweeds usually look like a mat of green pumpkin seeds floating on the surface of still or slow-moving waters. Giant duckweed grows extremely rapidly and can form extensive floating mats, especially in nutrient enriched waters.
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Leaf: There are no leaves. The plant body (thallus) is actually an expanded "stem" that functions as a leaf. It is oval to oblong, has 5-12 distinct veins, and is 4-10 mm long. The thallus is glossy green and smooth on the upper surface and reddish purple below. It may occur alone, or in clusters of 2-5.

Stem: No stems.

Flower: Tiny and rarely produced, occur in 2 pouches, usually 2 male flowers and 1 female flower in each pouch.

Fruit: A ribbed seed develops in a balloon-like bag (utricle).

Root: Clusters of 4 to 16 slender fibrous roots hang below the surface of the water from each plant. Each root ends with a pointed rootcap.

Propagation: Reproduces quickly by asexual budding, seeds, and overwinters as dark-green or brown buds on the sediments.

Importance of plant: Provides a high protein food source for ducks and geese, also eaten by certain fish. In Africa and Asia, giant duckweed has been harvested for cattle and pig feed. Grows quickly, especially if the water is warm and nutrient enriched. Has been used to reduce nutrients in sewage effluent.

Distribution: Worldwide. 

Habitat: Lakes, ponds, marshes and slow streams in areas sheltered from wind. Often grows with other members of the duckweed family.

May be confused with: Duckweeds (Lemna spp.) which are smaller and have only a single root per thallus. Water-meal plants (Wolffia spp.) are much smaller (1 mm in length) and have no roots at all. Mud midget plants (Wolffiella gladiata) are much longer than wide and have no roots. Mexican water fern plants (Azolla mexicana) are greenish-red with a fuzzy, nubby texture. Purple-fringed riccia (Ricciocarpus natans) which has a lobed plant body and many purple root-like structures. A related species, Spirodela punctata, is also found in Washington. It has fewer veins (5 to 7) and fewer roots (2 to 7).

Photographs: Closeup of Spirodela polyrhiza, closeup showing "roots"

Line Drawings: Spirodela polyrhiza

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