Aquatic Plant Identification Manual for Washington's Freshwater Plants
Floating Leaved Rooted Plants
epihydrus Raf., ribbon-leaf pondweed
Potamogeton natans L., floating-leaved pondweed
Both these perennial pondweeds have oval floating leaves and long, narrow, underwater leaves. The ribbon-like underwater leaves of ribbonleaf pondweed have a broad light green central stripe, and the floating leaves are often oppositely arranged. The underwater leaves of floating-leaved pondweed are so narrow they appear to be stiff leafless stalks, and the floating leaves often have slightly heart-shaped bases.
Leaf: Ribbon-leaf: submersed leaves: alternate, limp, ribbon-like, 5-25 cm long, 0.2-1 cm wide, light-green with a light colored center stripe (1-1.5 mm broad), lack stalks. They may decay later in the growing season. Floating leaves: leathery, appear opposite, 2-8 cm long and 1.5-3.5 cm wide on long flattened leaf stalks. Transitional leaves are sometimes present. Delicate sheaths (stipules) up to 4 cm long are free of leaf bases. Floating-leaved: submersed leaves: alternate, stiff, to 50 cm long and up to 2 mm wide. Floating leaves: alternate, dark green to copper-colored, 6-11 cm long, to 6 cm wide, have slightly heart-shaped bases and stalks longer than the leaf blade. Persistent sheaths (stipules) 6-8 cm long are free from the leaf base.
Stem: Ribbon-leaf: simple or few-branched, somewhat flattened, to 2 m. Floating-leaved: generally unbranched, nearly cylindrical, to 2 mm thick.
Flower: Small, inconspicuous, with 4 petal-like lobes. Ribbon-leaf: in spikes 1-4 cm long on stalks to 16 cm long. Floating-leaved: in compact spikes less than 5 cm long on stalks to 12 cm long.
Fruit: Achene. Ribbon-leaf: 2-4 mm long, circular, flattened, ridged on back, beak inconspicuous. Floating-leaved: 3.5-5 mm, back rounded or faintly ridged with a 0.5-1 mm beak.
Root: Fibrous from rhizomes.
Propagation: Seeds, sometimes by large winter buds. Floating- leaved: sometimes produces tubers.
Importance of plant: Seeds, tubers, and vegetation provide food and cover for aquatic animals and waterfowl.
Distribution: Ribbon-leaf: most of North America, locally in western and mountainous eastern Washington. Floating-leaved: North America, Eurasia.
Habitat: Ponds, lakes, or slowly flowing waters to 0.5-3 m deep. Floating-leaved tolerates brackish water.
May be confused with: Ribbon-leaf: water star-grass (Heteranthera dubia) and flat-stem pondweed (P. zosteriformis), which have similar underwater leaves. Look for the broadly striped leaves and roundish stem. Floating-leaved: a similar species, P. oakesianus, is known from southwest British Columbia. Many pondweeds form hybrids with intermediate characteristics.
Potamogeton natans floating leaves