Helianthus uniflorus Nuttall, J. Acad. Nat. Sci. Philadelphia 7: 37. 1834; Helianthella uniflora var. douglasii (Torrey & A. Gray) W. A. Weber

Plants 40–120 cm. Cauline leaves: largest at mid stems; blades usually 3-nerved, usually lanceolate to elliptic, rarely ovate, 12–25 cm, faces puberulent, hirsute, or scabrous. Heads usually borne singly, sometimes 2–3+, erect. Involucres turbinate to hemispheric or broader, 15–30 mm diam. Phyllaries lanceolate to lance-linear (subequal or outer larger, margins sometimes ciliate, faces ± cinereous-pubescent to sparsely puberulent). Paleae chartaceous, relatively firm. Ray florets 11–13(–21); laminae 15–30(–45) mm. Disc corollas yellow. Cypselae strongly compressed, narrowly obovate (margins ciliate, faces strigose); pappi of 2 ± subulate scales plus 2–4 shorter scales. 2n = 30.

Flowering May–Jul(–Aug). Grasslands, meadows, sagebrush scrublands, swales, aspen forests, rocky slopes, and spruce-fir forests; 300–3400 m; B.C.; Ariz., Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.

Two infraspecific taxa within Helianthella uniflora may be distinguished; they have been named at varietal rank.

Variety douglasii has stems hirsute; involucres (15–)20–25(–30) mm diam.; outer phyllaries rarely elongated, margins ciliate, abaxial faces sparsely puberulent; ray laminae 30–40 mm; and 2n = 30. It grows in grasslands in the northern Rocky Mountains and on the east side of the Cascade Range (B.C.; Idaho, Oreg., Wash.) at 300–2500 m where it flowers May–Jul.

Variety uniflora has stems sparingly puberulent to scabrous; involucres 15–20(–25) mm diam.; outer phyllaries frequently elongated, margins not ciliate, abaxial faces uniformly cinereous-pubescent; ray laminae 20–30 mm; and 2n = 30. It grows in meadows, sagebrush scrublands, swales, aspen forests, rocky slopes, and spruce-fir forests of the Rocky Mountains, intermountain plateaus, and Great Basin ranges (Ariz., Colo., Idaho, Mont., Nev., Oreg., Utah, Wyo.) at 1500–3400 m, where it flowers (May–)Jun–Jul(–Aug).