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Washington State Department of Ecology > Water & shorelines > Freshwater studies > Lake water quality > Aquatic Plant Guide home > Submersed Plants > Zannichellia pualustris

Aquatic Plant Identification Manual for Washington's Freshwater Plants

Submersed Plants

Species: Zannichellia palustris L., horned pondweed
Family: Zannichelliaceae

Horned pondweed is a delicate underwater branching perennial that can grow to a length of 1 m. It has opposite, thread-like leaves that emerge in such a way as to give the plants a uniform shape. Unlike many look alike aquatic plants which have flowers that emerge from the water on spikes, horned pondweed has inconspicuous underwater flowers and fruits located at the leaf bases. Look for this plant in brackish or alkaline streams, ponds, ditches, and lakes.
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Leaf: The submersed thread-like, smooth edged leaves are oppositely-arranged (occasionally appearing whorled) and each leaf has a central vein. The leaves are 2-10 cm long and less than 1 mm wide. A flared, transparent, membranous sheath surrounds the stem at the leaf base. 

Stem: The completely submersed weak stems are branched, thin, and thread-like. 

Flower: The flowers are small, lack sepals and petals, and are solitary or clustered at the leaf bases. Male and female flowers are separate, but grow on different parts of the same plant, although often both occur together in the leaf bases. The female flowers are surrounded by a sheathing bract. Because the flowers remain entirely underwater, pollination occurs in the water. Flowers from June through August. 

Fruit: Tiny banana shaped achenes are 2 to 4 mm long and have a conspicuous hooked beak measuring 1-1.5 mm long. A distinctive toothed ridge develops along the outer edge of the achene.

Root: Roots from slender creeping rhizomes.

Propagation: By seeds and rhizomes. 

Importance of plant: Fruits and entire plants are eaten by waterfowl and other birds. Provides good habitat for small aquatic animals.

Distribution: More or less worldwide and common throughout North America. In Washington, horned pondweed is most common in hard water lakes of the Columbia Basin and the Okanogan region.

Habitat: Shallow freshwater, alkali or brackish lakes, ponds, ditches, and streams. 

May be confused with: Wigeongrass (Ruppia maritima), which has alternately arranged, more densely spaced leaves. Flowers of wigeongrass are located at the end of long stalks, rather than clustered at the base of the leaves. Water-nymphs (Najas spp.) which have tooth edged opposite leaves with a widened base, and oval fruits. Narrow leaf pondweeds (Potamogeton spp.) which are more robust and have flowers and fruits on long stalks.

Photographs:  http://www.dnr.state.md.us/bay/sav/key/horned_pondweed.asp

Line Drawings: Zannichellia palustris 

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