Herbs, perennial, cespitose, 30--90 cm. Stems compact. Leaves ascending in narrow fans, 6--30 cm; sheaths pinkish, soft, smooth; blade pale green to pale red-brown, narrowly linear, 1--2(--3) mm wide, smooth, margins smooth. Inflorescences: scape sheaths exceeded by leaves; scapes linear, slightly twisted, nearly terete, 0.7--1 mm, smooth, distally striate or 1-ribbed; spikes ellipsoid to narrowly obovoid, 10--16 mm; fertile bracts 4--6 mm, margins entire, aging to erose, apex rounded. Flowers: lateral sepals exserted, linear-curvate, 0.5--1 mm longer than subtending bract, keel scarious, lacero-ciliate, apex not red, thin, lacero-fimbriate; petals unfolding around noon or in afternoon, blade obovate, 3.5 mm; staminodes bearded. Seeds translucent, ellipsoid, 0.4--0.6 mm, distinctly longitudinally ribbed with few, indistinct cross ribs.

Flowering summer--fall. Moist to wet sandy borders of dolines, sandy swales in longleaf pine hills; of conservation concern; 0--200 m; Ala., Fla.

Taxonomically, Xyris longisepala is nearest to X. smalliana, a frequent associate around the dolines. Xyris smalliana is taller, with broader leaves, broader scapes, longer spikes, and longer seeds. The flowering time of day overlaps, but X. longisepala blooms first.