Herbs perennial, slender, 1–4 dm, green, glabrous. Stems from creeping rhizome, rooting at nodes, weakly erect, un­branched or sparsely branched. Leaves opposite throughout, scarcely smaller distally, equal to or shorter than internodes; sessile or subsessile; blade oblong, 5–13 × 2–6 mm, base rounded. Inflorescences racemes, sparsely flowered. Flowers alternate or oppo­site, subsessile, distylous; floral tube without red spots, obconic, 4–5 × 1 mm; epicalyx segments 2 times length of sepals; petals pale purple to purple, obovate, 2.5–4 × 0.5–1 mm; nectary encircling base of ovary; stamens 6. Capsules septicidal or septifragal. Seeds undescribed. 2n = 20.

Flowering spring–summer. Wet, springy areas; of conservation concern; 0–50 m; Fla.

Lythrum flagellare is relatively rare. A collection from Hernando County is possibly a hybrid of L. alatum and L. flagellare, as suggested by a more robust habit, leaves ultimately crowded and alternate on the stems, and less slender, more cylindrical floral tube than is typical for L. flagellare.