Floating Leaved Rooted Plants  

 
Species: Potamogeton amplifolius Tuckerman, big-leaf pondweed
Family: Potamogetonaceae

Big-leaf pondweed has two leaf types: large, usually wavy-edged underwater leaves which are curved into a banana shape, and oval, leathery floating leaves that grow on or near the water surface. The underwater leaves often decay late in the growing season, making big-leaf pondweed easy to confuse with other floating-leaved pondweeds.


Leaf: Alternate leaves of two types. Submersed leaves: bright to dark green, translucent, 8-20 cm long and 2-7.5 cm wide, folded along the midrib, curved backwards into a banana-shape, and sometimes with wavy margins. They have short stalks (1-2cm) and 19-45 lengthwise veins. These leaves often decay in late summer. Floating leaves: often absent. The opaque, leathery, oval leaves taper at both ends and are 5-10 cm long and 2.5-5 cm wide. They have 25-45 veins and 3-10 cm long stalks that are generally longer than the floating leaves. Sheaths (stipules) are up to 10 cm long, whitish, translucent, occur at leaf bases, but are not attached to the leaves. They become stringy with age.

Stem: Few or unbranched stem (to 3 mm thick) to 5 m long.

Flower: Small flowers have 4 petal-like lobes. Up to 16 whorls of tightly clustered flowers are arranged into an up to 5 cm long spike on stalks rising above the water. The flower stalks are thicker than the stem and are 5-15 cm long.

Fruit: Seed-like achene is 3-5 mm long, has flattened sides, a 0.5-1 mm beak, and is orange to pinkish when ripe. The back is rounded or keeled when dry.

Root: Fibrous, from creeping underground rhizomes.

Propagation: Seeds, fragments, rhizomes.

Importance of plant: Plants may show rapid early season growth, with plants over 3 m tall observed in early May. Seeds and entire plant are good wildlife food and habitat.

Distribution: Throughout North America. Particularly common in western and northeastern Washington lakes.

Habitat: Lakes and ponds. Will grow in clear water as deep as 6 m.

May be confused with: Without floating leaves, may be confused with other broad leaved pondweeds such as Illinois pondweed (P. illinoensis), and white stem pondweed (P. praelongus). However these do not have as many leaf veins, and white stem pondweed has no leaf stalks. Big-leaf pondweed will hybridize with other pondweeds, forming plants with intermediate characteristics.

Photographs: Potamogeton amplifolius floating leaves, underwater leaves.

Line Drawings: Potamogeton amplifolius


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