Shrubs, 20–150(–200) cm. Leaves 1.5–4 × 0.5–1.5 cm, lobes linear, to 0.5 mm wide. Heads in spiciform arrays (6–)8–15(–35) × (1–)4–5 cm. Involucres 2–3 × 2 mm. Florets 4–8. Cypselae 1.8–2.3 mm. 2. = 18, 36.

Flowering mid summer–late fall. Deep loam soils, usually igneous in origin; 900–1900 m; B.C.; Idaho, Nev., Oreg., Wash., Wyo.

Subspecies tripartita ranges throughout the Snake River and Columbia River basins, extending north through central British Columbia, where average annual precipitation is 375–800 mm. Because much of the range includes fertile agricultural land, much of the habitat has been lost to farming, and populations of subsp. tripartita occur as isolated islands along drainages and at the bases of mountain slopes. It may be one of the parents involved in the presumed hybrid origin of Artemisia arbuscula subsp. thermopola.