Herbs perennial, slender, 5–12 dm, green, glabrous. Stems erect, sparsely branched prox­imally, branched distally. Leaves opposite proximally, subop­posite to alternate distally, well separated on stems, not densely crowded distally; sessile; blade lanceolate to narrowly linear, 30–120 × 10–20 mm, narrower distally, base usually attenuate (or rounded in possible hybrids with L. salicaria). Inflorescences racemes at proximal nodes, spikelike distally with flowers in loose, ± whorled clusters. Flowers alternate, or loosely clustered and ± whorled at distal nodes, sessile or subsessile, tri­stylous; floral tube cylindrical, 4–6 × 2 mm, 3+ times longer than wide; epicalyx segments awl-shaped, equal to or shorter than length of sepals; petals rose purple, narrowly spatulate, 5–7 × 1.5–3 mm; nectariferous disc surrounding base of ovary; stamens usually 12, of 2 lengths. Capsules septicidal or septifragal. Seeds 75+, obovoid to fusiform. 2n = 30 (Slovakia).

Flowering summer–fall. Marshes, ditches, wet areas; 100–600 m; introduced; Mass., Mont., N.H., Pa., Va., Wash.; Europe.

Lythrum virgatum is distinguished from L. salicaria in its native Europe by glabrous stems, less robust habit, narrower, more widely spaced leaves with attenuate leaf bases, and less floriferous, more open inflorescences. The species are said to hybridize in their native ranges as well as in North America when they come into contact under cultivation. In the flora area, L. virgatum escapes from cultivation and is apparently naturalized to an unknown extent. The ability to produce fertile hybrids is a major concern because of the invasive nature of L. salicaria. For this reason, the sale of L. virgatum, like that of L. salicaria, is illegal in some states.