Shoreline Plants  

 
Species: Polygonum amphibium L., water smartweed
Polygonum hydropiperoides Michaux, waterpepper
Family: Polygonaceae

Water smartweed and waterpepper are erect to sprawling perennials with willow-like leaves and clusters of small flowers. The stems tend to be reddish and are usually swollen at the base of each leaf stalk, similar to bamboo. The flower clusters of water smartweed are compact and oblong, while waterpepper flowers are spaced along the flower stalk. Both plants can form mats along the edges of lakes and ponds, but generally are not considered a nuisance

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Leaf: Floating or upright with papery sheaths (stipules) at the base. Alternately-arranged. Water smartweed: large, up to 35 cm long, 6 cm wide with a rounded or pointed tip. The leaf stalk is up to 5 cm long. Waterpepper: can be 10 cm long, but often less, tapering to a sharp point. Leaf stalks short, less than 5 mm.

Stem: Jointed, to 2 m long (more in floating mats of water smartweed), with an underground rhizome and an aboveground stem that sometimes sprawls on or just below the water surface. Water smartweed: stout (to 1 cm across), usually lying flat, but sometimes erect. Waterpepper: slender, usually erect along the upper portion.

Flower: Small, each flower 4-5 mm long. Clustered to form oblong spikes at the tips of flower stalks. Water smartweed: pink flowers form dense clusters, 1-15 cm long. Waterpepper: greenish, pinkish, or white flowers form loose, elongated spikes, 6-10 cm long, 2-5 mm wide.

Fruit: Dry, hard, shiny brown achene contains one seed. Water smartweed: 2.5 to 3 mm, round and flattened. Waterpepper: 2 mm, three-sided.

Root: Fibrous, arising from rhizomes and stem joints in contact with the ground or water.

Propagation: Seeds; spreads by rooting from the trailing stems.

Importance of plant: Seeds provide food for waterfowl, marsh birds, song birds, and upland game birds. Important medicinal plants, especially water smartweed. Used as an antiseptic and as a poisoning cure.

Distribution: Nearly world-wide. Locally, Water smartweed is most common in north-central Washington. Waterpepper is most common west of the Cascades.

Habitat: Shallow water along the margins of lakes, ponds, and streams.

May be confused with: Other plants lack the swollen joints and sheathing stipules of smartweed. However, there are many other Polygonium species in Washington, some of which may grow in wet conditions. Refer to technical keys to be sure of species.

Photographs: Polygonum amphibium, Polygonum hydropiperoides

Line Drawings: Polygonum hydropiperoides


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