in N. L. Britton et al., N. Amer. Fl. 25: 86. 1907.
Herbs, (5–)15–30(–45) cm, glabrous or glabrate; branches from distal nodes, alternate, strongly virgate (proximal unbranched main axis usually long in comparison to branched distal portion). Leaves alternate; stipular glands present; blade linear, (5–)15–25 × 1–1.5(–2) mm, base flat, not clasping, margins eglandular or glands minute. Inflorescences: cymes monochasial, dense, sparingly branched, internodes sometimes all condensed or long proximally and condensed distally, flowers condensed at apices; bract margins eglandular. Pedicels 0.5–2(–5) mm, scarcely longer in fruit, ascending, not bent at apex. Flowers: sepals erect, ± reflexed at tip, lanceolate, 3–4 mm, equal, margins minutely glandular, surfaces hairy, hairs intertwined or matted; petals ± reflexed, pink to rose, oblanceolate, (3–)6–7(–8) mm, apex obtuse, sometimes slightly notched; cup white, rim with petal attachments set in deep notches between filaments; stamens exserted; filaments 4–5 mm; anthers rose to purple, dehisced anthers 1.5–2 mm; ovary chambers 6; styles 3, white, (3–)4–4.5(–5) mm, exserted. 2n = 36.
Phenology: Flowering Apr–Aug.
Habitat: Grasslands on serpentine soils.
Elevation: 0–200 m.
Hesperolinon congestum is known from a narrow band within the Outer Coast Ranges from Marin County south to San Mateo County. It can be distinguished from all other species in the genus by its hairy sepals.