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Washington State Department of Ecology > Water & shorelines > Freshwater studies > Lake water quality > Aquatic Plant Guide home > Floating Leaved Rooted Plants > Potamogeton alpinus and Potamogeton gramineus

Aquatic Plant Identification Manual for Washington's Freshwater Plants

Floating Leaved Rooted Plants  

Species: Potamogeton alpinus Balbis, alpine pondweed
Potamogeton gramineus L., grass-leaved pondweed
Family: Potamogetonaceae

Alpine and grass-leaved pondweed have both floating and underwater leaves. The underwater leaves of both species are long, fairly narrow, and attach directly to the stem. When present, the floating leaves of alpine pondweed are broader, but are similarly shaped to the underwater leaves, and are attached to the stem by short stalks. The floating leaves of grass-leaved pondweed are oval and on stalks longer than their leaf blades. Grass-leaved pondweed is extremely variable in appearance and may look differently depending on whether it is growing in deep or shallow water or stranded along shorelines by receding water.

Leaf: Alpine: submersed leaves: reddish, to 20 cm long and 1-2 cm wide, have rounded tips, usually 7 veins, and lack stalks. Transitional leaves are often present. Floating leaves: (often absent) to 6 cm long by 2.5 cm wide are on short stalks. Sheaths (stipules) are free from the leaf base, to 3 cm long, and break apart easily. Grass-leaved: submersed leaves: green-to-reddish, to 13 cm long by 1.2 cm wide, have pointed tips, 3-9 veins and no stalks. Floating leaves: 1.5-7 cm long and 1-3 cm wide are on stalks longer than the blades. Sheaths (stipules) to 3 cm long, are persistent and free of the leaf base.

Stem: Long, forms rhizomes. Alpine: reddish, few branched. Grass-leaved: many short branches.

Flower: Small, clustered on emergent spikes, each with 4 petal-like lobes. Alpine: spikes to 3.5 cm long on stalks 6-15 cm long. Grass-leaved: spikes to 4 cm long on long stalks to 20 cm.

Fruit: Achene. Alpine: 3-4 mm long, sharply ridged back, curved beak. Grass-leaved: 1.5-3 mm, back slightly ridged, straight to slightly curved beak.

Root: Fibrous, from rhizomes. Grass-leaved forms long tubers.

Propagation: Seeds, creeping rhizomes, tubers (grass-leaved pondweed only).

Importance of plant: Seeds, tubers, and vegetation provide important food and cover for aquatic animals and waterfowl.

Distribution: Both occur throughout the temperate Northern Hemisphere.

Habitat: Alpine: shallow cold water ponds and lakes. Grass-leaved: shallow to deep water; variable appearance depending upon growing conditions.

May be confused with: Other pondweeds with long narrow underwater leaves. Grass-leaved pondweed is a variable species that hybridizes freely. Much disagreement and confusion exists regarding this plant among taxonomists. Each will hybridize with other pondweeds, forming plants with intermediate characteristics.

Photographs: Potamogeton gramineus floating leaves, close-up of fruit

Line Drawings: None available 

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