Erect to ascending erect, up to 80 cm tall, usually unbranched, white villose-stemmed herb. Leaves linear to linear-lanceolate, 3 – 8 (-10) cm long, up to 2 cm wide, (2-) 3-pinnatisect into filiform-linear or linear-lanceolate, 0.5–1.5 x 0.1–0.3 mm, cartilaginous mucronulate, setaceous, often congested ultimate segments, basal and lower leaves on up to 5 cm long petioles, middle and upper stem leaves sessile and gradually smaller, rachis 0.4 – 0.6 mm wide. Capitula oblong, 40–150 or more, 2.5–3 mm across, on up to c. 3 mm long peduncles, in dense, up to 8 cm broad compound corymbs. Involucre oblong to oblong-cylindric, 3.5–4.5 x 2–3 mm, shortly attenuate and sparse to dense pilose below, phyllaries 3-seriate, oblong-ovate to oblong-lanceolate, outer pale, brownish at the acute tips, inner apically obtuse and scarious. Paleae oblong-lanceolate to lanceolate, whitish-membranous, glabrous to hairy towards the acute apices. Ray-florets 4-5, with inconspicuously 3-lobed, ± orbiculate, 1–1.2 x 1–2 mm, white ligules. Disc-florets 10–20, with yellow, 5-lobed corolla tube. Cypselas oblong-cuneate, 1.8 – 2 mm long, laterally ribbed, light brown.
A. millefolium L. var. setacea (Waldst. & Kit.) W. D. Koch, Syn. Fl. Germ. 373. 1837; A. fililoba Freyn, Fl. 1881: 210. 1881; A. setacea Waldst. & Kit., var. filifolia Boiss., Fl. Or. Suppl. 295. 1888.