Herbs perennial, slender, 5–10 dm, green, glabrous. Stems erect, much-branched distally. Leaves opposite proximally, alternate distally, branch leaves abruptly and conspicuously smaller than those on main stem; sessile or subsessile; blade (on main stem) broadly to narrowly lanceolate or oblong, 20–75 × 5–17 mm, (on branches) oblong to narrowly oblong, 3–15 × 1.5–3 mm, base narrowly attenuate. Inflorescences racemes. Flowers alternate, subsessile or pedicellate, pedicel slender, 1–1.5 mm, distylous; floral tube obconic, 3–6 × 1 mm; epicalyx segments about equal to length of sepals; petals deep to pale purple with dark central vein, oblanceolate or oblong, 1.5–2 × 0.5–1 mm; nectary encircling base of ovary; stamens 6. Capsules septicidal or septifragal. Seeds ca. 20, narrowly obovoid to fusiform. 2n = 20.

Flowering summer–fall. Wet or moist shady wood­lands, streamsides; of conservation concern; 0–100 m; Fla., Ga.

The shady habitat of Lythrum curtissii, a rare, delicate-flowered species, is unlike that of other native species of Lythrum, which tend toward more open, sunny areas. Putative hybrids of L. alatum var. lanceolatum and L. curtissii have been noted in the vicinity of Leary, Calhoun County, Georgia.