Proc. Amer. Acad. Arts 6: 545. 1865.
Annuals, 30–100 cm. Stems (often reddish) erect, hairy. Leaves mostly cauline; mostly alternate; petioles 0.7–2.5 cm; blades lance-linear to lance-ovate or ovate, 3–15 × 0.5–3 cm, bases cuneate, margins usually entire or shallowly serrate, abaxial faces gland-dotted. Heads 1–7. Peduncles (1–)3–13(–20) cm. Involucres hemispheric, 15–20 mm diam. Phyllaries 12–17, lanceolate, 8–17 × 3–4 mm, apices narrowed gradually, abaxial faces hirsute. Paleae 9.5–10 mm, 3-toothed (middle teeth surpassing discs, apices greenish or yellow-brown, glabrous). Ray florets 10–13; laminae 14–20 mm. Disc florets 50+; corollas 4–6 mm, lobes reddish; anthers reddish purple, appendages usually purplish (style branches reddish or yellow). Cypselae (2.5–)3–3.5(–4) mm, glabrate; pappi of 2 lanceolate scales 1.7–2.7 mm. 2n = 34.
Phenology: Flowering summer–early fall.
Habitat: Gravelly streamsides on serpentine
Elevation: 100–1400 m
Of conservation concern.
Helianthus exilis is very similar morphologically to H. bolanderi and is often included in it; there are discrete differences between the two for fatty acid composition (C. E. Rogers et al. 1982) as well as some molecular markers (L. H. Rieseberg et al. 1988). It is listed as of concern by the California Native Plant Society.