Persicaria hydropiper

Annual herb.

20–70 cm high.

The stem is erect or ascending, branched below the middle, and typically rather robust.

The leaf arrangement is spiral. The connate stipules form an ochrea, which encircles the stem. The margin of the ochrea is typically short-fimbriate. The leaves are nearly sessile. The lamina is typically 5–10 cm long, simple, entire, narrowly ovate and not maculate.

The inflorescence is spicate. The flowers are clustered in small cymes, and are small and bisexual. The spike is narrow, lax until the summit and drooping. There are solitary flowers or cymes also in the lower leaf axils. The perianth is 3–5 mm long, corolla-like, connate at the base and greenish or reddish.

The fruit is a trigonous or slightly compressed ovoid achene, which is 2.5–3.2 mm long, opaque and dark brown.