Water Smartweed is an extremely variable species. It is a native, prostrate to erect perennial aquatic forb growing from 1 to 3-1/2 feet tall. It can be found as a floating or a terrestrial plant, with several flowering stems. Stem nodes may be enlarged and have a brownish-purple color and hair, where the leaf joins.

The leaves are alternate, much longer than wide, up to 8 inches x 3 inches, lanceolate (broadest in the lower half) to ovate, with smooth margins, tapering to a pointed tip, the leaf base tapered to rounded, sometimes slightly heart-shaped, and with a short stout stalk that forms a sheath at the stem (an ocrea), although some leaves, particularly upper leaves, may be without a stalk. The upper leaf surface does not have the darker triangular or crescent shaped blotch as found on P.maculosa. Leaf blade margins are usually smooth, but sometimes undulating, and have rough forward curving short stiff hair (antrorse hair); surfaces are green on both sides, although the upper surface may have appressed hairs. Underside mid-veins are very prominent. The leaf stem ocrea is tan to dark brown, cylindric in shape with truncate to oblique margins. Ocrea margins may or may not have fine ciliate hairs. The ocrea surface is sometimes smooth but usually with appressed dense hairs, without glands.

The inflorescence is one or two terminal spikes that vary from cone shaped to cylindric in shape, 3/4 to 6 inches long. It is not interrupted as in P. punctata except perhaps near its base.

The flowers are about 1/16 inch wide, consisting of 5 rosy-pink tepals (sepals and petals combined) that form a cup shape, each bluntly rounded at the tip. Tepals are united at their bases for 1/3 their length. Tepals have prominent veins but without the anchor shape as found in P. lapathifolia. The perianth is without hair or glands. There are 5 stamens with pink anthers that may be exserted from the cup or contained within, and two styles that are united for 2/3 rds of their length. Flowers may be bisexual or male and female flowers may be on separate plants. The upper part of the flowering stem turns darker, to a purplish-brown with maturity.

Seed: Fertile flowers produce an ovoid, somewhat flattened dark colored seed 2.2-3 × 1.8-2.6 mm in size. They may be shiny or dull, with a smooth surface or very minutely glandular.

Habitat: Water Smartweed grows in the loamy or silty soils of shallow marshes, meadows and moist riparian areas where water does not flow with much current. It withstands periodic flooding, but not drought. It grows from long rhizomes which will produce colonies of plants.

Names: There has been much consolidation of species in this plant family - see notes at bottom of the page for more discussion. The current genus, Persicaria is from the Latin meaning 'peach-like', derived from persica for 'peach' and aria, meaning 'pertaining to', and thought to be referring to the leaves being similar to one of the peaches. This name was applied in times past to the Knotweed family (smartweeds). The species amphibia, refers to being adapted to growing in water or on land - much the same connotation as the word 'amphibian' means in the animal family.

The author names for the plant classification are as follows: First to classify in 1753, assigning the former name of Polygonum amphibium was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was amended in 1822 by ‘Gray’ which is for Samuel Frederick Gray (1766-1828) British botanist and pharmacologist whose most important work was a Supplement to the Pharmacopoeia in 1818 and The Natural Arrangement of British Plants in 1821.

Comparisons: Another smartweed that grows in moist environments, but not in water is Spotted Lady's-thumb, Persicaria maculosa Gray. The leaves there have a dark triangular blotch. Another that likes wet environments is Dotted Smartweed, Persicaria punctata (Elliott) Small, but the flowers are greenish-white with glandular dots and the inflorescence is interrupted.