Swamp Smartweed

The extreme variability in Persicaria hydropiperoides is reflected in its extensive synonymy. Among the segregates most often recognized in floras and checklists is P. opelousana, which C. B. McDonald (1980) showed to be broadly sympatric and highly interfertile with P. hydropiperoides. Consistent with this conclusion, R. S. Mitchell (1971) found that P. hydropiperoides and P. opelousana are unique among native North American smartweeds in consistently possessing multicellular plate-glands on the abaxial surface of their leaves. Such glands also are found on P. maculosa, an introduced European species.

Herbarium specimens of Persicaria hydropiperoides sometimes are misidentified as P. maculosa, especially when the roots are missing. The former species may be distinguished reliably by its achenes all trigonous (trigonous and biconvex achenes are mixed in the inflorescences of P. maculosa) and bristles on the margins of the ocreae that average longer. M. L. Fernald (1922c) reported hybrids with P. robustior from Nova Scotia.

Plants perennial, 1.5-10 dm; roots also often arising from proximal nodes; rhizomes often present. Stems decumbent to ascending, usually branched, without noticeable ribs, glabrous or obscurely strigose distally. Leaves: ocrea brown, cylindric, 5-23 mm, chartaceous, base inflated, margins truncate, ciliate with bristles (2-)4-10 mm, surface glabrous or strigose, not glandular-punctate; petiole 0.2-2 cm, glabrous or strigose; blade without dark triangular or lunate blotch adaxially, broadly lanceolate to linear-lanceolate, 5-25 × 0.4-3.7 cm, base tapered or acute, margins antrorsely appressed-pubescent, apex acuminate, faces glabrous or appressed-pubescent along midveins and sometimes on faces, usually punctate abaxially. Inflorescences terminal, sometimes also axillary, erect, uninterrupted or interrupted proximally, 30-80 × 2-5 mm; peduncle 10-30 mm, glabrous or strigose; ocreolae overlapping distally, often not overlapping proximally, margins ciliate with bristles to 2(-3) mm. Pedicels ascending, 1-1.5 mm. Flowers bisexual or unisexual and staminate, 2-6 per ocreate fascicle, homostylous; perianth roseate proximally, roseate, white, or greenish white distally, not glandular-punctate or sometimes glandular-punctate with punctae on tubes and inner tepals, scarcely accrescent; tepals 5, connate ca. 1/ 1/ 2 their length, obovate, 2.5-4 mm in bisexual flowers, 1.5-2.5 mm in staminate flowers, veins prominent or not, not anchor-shaped, margins entire, apex obtuse to rounded; stamens 8, included or exserted in staminate flowers; anthers pink or red, elliptic to ovate; styles 3, connate near middle. Achenes included or apex exserted, brown to brownish black or black, 3-gonous, 1.5-3 × 1-2.3 mm, shiny, smooth.

B.C., N.B., N.S., Ont., Que.; Ala., Alaska, Ariz., Ark., Calif., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., Wash., W.Va., Wis.; Mexico; Central America; South America.

Flowering Jun-Nov.

Wet banks and clearings, shallow water, marshes, moist prairies, ditches; 0-1500m.

Polygonum hydropiperoides Michaux, Fl. Bor.-Amer. 1: 239. 1803; Persicaria opelousana (Riddell ex Small) Small; P. paludicola Small; Polygonum hydropiperoides var. adenocalyx (Stanford) Gleason; P. hydropiperoides var. asperifolium Stanford; P. hydropiperoides var. breviciliatum Fernald; P. hydropiperoides var. bushianum Stanford; P. hydropiperoides var. digitatum Fernald; P. hydropiperoides var. opelousanum (Riddell ex Small) W. Stone; P. hydropiperoides var. psilostachyum H. St. John; P. opelousanum Riddell ex Small; P. opelousanum var. adenocalyx Stanford

Persicaria hydropiperoides is a New World species of flowering plant in the buckwheat family known by the common names swamp smartweed and false waterpepper. It is widespread across much of North America and South America. It grows in moist and wet habitats, and is sometimes semi-aquatic.

Persicaria hydropiperoides is quite variable and is sometimes divided into several varieties, some of which may be better treated as species in their own right.

In general, Persicaria hydropiperoides is a rhizomatous perennial herb growing upright or erect and approaching a maximum height of one meter (40 inches). Roots may emerge from nodes on the lower stem. The bristly lance-shaped leaves are around 10 centimeters (4 inches) long. The leaves have sheathing stipules known as ochrea. The spikelike inflorescence produces many pinkish flowers each about 3 millimeters wide.