Herbs annual, slender, 0.5–3 dm, green, glabrous or sparsely, minutely glandular-hispid. Stems prostrate to weakly erect, often branched near base, with rela­tively short accessory branches distally. Leaves op­posite prox­imally, subalter­nate distally; sessile; blade oblong to oblan­ceolate, 3–25 × 0.1–0.5 mm, base attenuate. Inflorescences spikelike. Flowers alternate, often crowded on branches, sessile or subsessile, mono­-stylous; floral tube without red spots, narrowly cylindrical, 4–6 × 0.4–0.5 mm; epicalyx segments equal to length of sepals; sepals obtuse, thick; petals lavender, oblong, 1–2(–3) × 0.5 mm; nectary absent; stamens 4–6. Capsules septicidal or septifragal. Seeds ca. 11, subglobose. 2n = 10.

Flowering spring–fall. Seasonally wet areas, drying ponds, vernal pools, ditches; 0–1300 m; introduced; Calif., Idaho, Utah; s Europe.

First collected in Solano County, California, in 1930 (J. T. Howell 5208, UC), Lythrum tribracteatum has subsequently been documented in 12 counties in California and in southwestern Idaho and Utah. In Utah the species may have been introduced through contamination of seed stock of the cultivated dye-plant indigo (Indigofera).