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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 illinoensis and Potamogeton nodosus

Aquatic Plant Identification Manual for Washington's Freshwater Plants

Floating Leaved Rooted Plants

Species: Potamogeton illinoensis Morong, Illinois pondweed
Potamogeton nodosus Poiret, long-leaf or American pondweed
Family: Potamogetonaceae

These perennial pondweeds have both floating and underwater leaves. The long, broad underwater leaves of Illinois pondweed are on short leaf stalks. The floating leaves are not always present and have stalks shorter than the leaf blade. Long-leaf pondweed has narrower, translucent underwater leaves and long leaf stalks. The leathery, oblong floating leaves are on leaf stalks longer than the leaf blade. Both species are common throughout North American waters.

Leaf: Alternate. When floating leaves are present, the submersed leaves are often decomposed. Illinois: submersed leaves: 6-20 cm long to 5 cm wide have 9-19 parallel veins with many cross-veins and are on short (0.5-2 cm) stalks. Sometimes transitional leaves are present. Floating leaves: sometimes absent, 4-13 cm long, 2-6 cm wide, have 13-29 longitudinal veins. The leaf is longer than the stalk. Sheaths (stipules) 2.5-7 cm long are free from the leaf base and persistent. Long-leaf: submersed leaves: 2-15 cm long, 1-4 cm wide, have tapered ends, 7-15 longitudinal veins, and are on long stalks (2-12 cm). Floating leaves: 5-13 cm long to 5 cm wide are on long stalks (5-20 cm). Sheaths (stipules) are 3-9 cm long, free from the leaf base, and often decomposed.

Stem: Simple or branched, more or less cylindrical.

Flower:  Small, greenish, whorled, in spikes to 6 cm long, on stalks thicker than the plant stem. Stalks to 15 cm.

Fruit: Achenes. Illinois: approximately 4 mm long, ridged, beak attached slightly below tip. Long-leaf: 3-5 mm long, back rounded, short beaked.

Root: Fibrous, from slender rhizomes. Illinois: forms thickened overwintering rhizomes. Long-leaf forms chains of tubers.

Propagation: Rhizomes, seeds, tubers.

Importance of plant: Provides important food and cover for aquatic animals. Tubers of long-leaf pondweed are an important waterfowl food.

Distribution: Illinois: North and Central America. Long-leaf: worldwide except Australia.

Habitat: Shallow to fairly deep water of lakes, ponds, rivers. Illinois: prefers alkaline water, more common east of the Cascades.

May be confused with: Illinois pondweed can be confused with other pondweeds having broad submersed leaves, especially whitestem pondweed (P. praelongus) or bigleaf pondweed (P. amplifolius). Long-leaf pondweed can be confused with grass-leaved pondweed (P. gramineus) or alpine pondweed (P. alpinus). Detailed measurements of the leaves can usually distinguish them. Many pondweeds form hybrids with intermediate characteristics.

Photographs: Potamogeton illinoensis floating leaves and fruits, Potamogeton nodosus floating leaves and fruits
Line Drawings: Potamogeton illinoensis, Potamogeton nodosus

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