Herbs, to 1 m. Leaves emersed, petiolate; blade linear-lanceolate to broadly elliptic to ovate, to 30 × 1–12 cm. Inflorescences to 1 m. Flowers chasmogamous; sepals 1.7–3.2 mm; petals purplish white to purplish pink, 3.4–6.4 mm, margins ± erose, apex obtuse; anthers ellipsoid, 0.7–1.4 mm; style ± straight, 0.6–1.5 mm, exceeding ovary length. Fruiting heads 4–6.5 mm diam; achenes ovoid, 1.7–3.1 mm, abaxial keels broadly rounded, with 1 median abaxial groove, rarely 2, beak erect or nearly erect. 2n = 14 (Eurasian material).
Phenology: Flowering and fruiting late summer.
Habitat: Stream margins
Elevation: 200 m
Distribution
Introduced; Alaska, Eurasia.
Discussion
The name Alisma plantago-aquatica has been used in a variety of North American floras. We are following, however, the treatment of I. Björkqvist (1968), in which the native distribution of A. plantago-aquatica is restricted to Eurasia.