Polygonum hydropiper L. (1753)

Flowering and fruiting: March-July

Erect, 30-45 (-50) cm high with tufted roots branched from base or above, or sometime simple, annual to perennial herb. Stem glabrous, brown, sometimes shining. Leaves 1.5-8 (-10) x 0.4-1.5 (-2) cm, linear lanceolate to lanceolate, acuminate, margin ciliate, surface glabrous or scabrous or slightly pubescent beneath on the midrib and reddish punctate gland-dotted on both surfaces, petiole 0.2-0.4 cm long. Ochreae 0.25-1.5 (-2.0) cm long, glabrous, ciliae at the mouth of ochrea 1-4 mm long. Inflorescence 3-7 cm long, lax, flowers ± distant, erect, not drooping pedunculate raceme; peduncles 1.0-6.5 cm long. Flowers 1.0-2.0 mm across, pedicellate; pedicel 0.5-1.25 mm long. Ochreolae 1.0-2.0 mm long, ovate, dentate-entire, gland-dotted. Tepals 5, pink, 1.0-3.0 x 0.75-1.5 mm, obovate, obtuse, red gland-dotted. Stamens 6, filaments long, unequal. Ovary 0.5-1.5 x 0.5-0.75 mm, trigonous, elliptic with 3 or sometimes 2 styles united in the upper half, then free; stigma capitate. Nuts 2.5-3.5 (-4) x 1.5-2.0 mm, mostly trigonous, sometimes biconvex within the same raceme, dark brownish, pubescent.

Herbs annual. Stems erect, 40-70 cm tall, much branched, glabrous, dilated at nodes. Leaves with peppery taste; petiole 4-8 mm; leaf blade lanceolate or elliptic-lanceolate, 4-8 × 0.5-25 cm, both surfaces glabrous, densely brown punctate, sometimes appressed hispidulous along midvein, base cuneate, margin ciliate, apex acuminate; cleistogamous flowers present in many leaf axils; ocrea tubular, 1-1.5 cm, membranous, sparsely appressed hispidulous, apex truncate, shortly ciliate. Inflorescence terminal or axillary, spicate, pendulous, interrupted below, usually lax, 3-8 cm, slender; bracts green, funnel-shaped, 2-3 mm, margin membranous, sparsely shortly ciliate, each 3-5-flowered. Pedicels longer than bracts. Perianth greenish, white or pink above, 5(or 4)-parted, brownish pellucid glandular punctate; tepals elliptic, 3-3.5 mm. Stamens 6, rarely 8, included. Styles 2 or 3. Achenes included in persistent perianth, black-brown, opaque, ovoid, biconvex or trigonous, 2-3 mm, densely small pitted. Fl. May-Sep, fr. Jun-Oct. 2n = 18-22.

This is one of the many weedy smartweeds in Illinois, which can be difficult to identify. Waterpepper can be distinguished from other smartweeds primarily by its racemes of flowers, which have the following key characteristics