Polygonum hydropiper is an annual herb to 60 cm tall, roots fibrous, stems erect to sprawling, branching freely at base, plant peppery tasting, stems often reddish; leaf sheaths rough, margins with bristles to 0.6 cm long, turning brown and disintegrating with age, nodes swollen, concealing small self fertile flowers.
Leaves alternate, narrow, to 16 cm long, 2 cm wide
Flowers greenish-white to pink, radially symmetrical; petals 5 sprinkled with tiny yellowish glandular dots; inflorescence linear, spike-like (racemes); blooming and fruiting July-Oct. (Hough 1983).
Fruit dry, achenes, dark brown usually 3-angled, sometimes lens-shaped, surface dull. Similar to P. punctatum. Seeds eaten by birds (Martin et al. 1951).
Wetland status: OBL.
Frequency in NYC: Very infrequent.
Habitat: Wet, open areas, high nutrient, organic soil, pH 6.0, seedlings tolerant of flooding (Sultan et al. 1998).
Notes: Sap acrid, may cause photosensitization (Kingsbury 1964).