1. Machaeranthera tanacetifolia (Kunth) Nees, Gen. Sp. Aster. 225. 1832.
Tansy-aster, tansyleaf aster , tahoka-daisy
Aster tanacetifolius Kunth in A. von Humboldt et al., Nov. Gen. Sp. 4(fol.): 74. 1818; 4(qto.): 95. 1820
Annuals or biennials, 5–100 cm. Leaves 8–110 × 4–40 mm. Involucres hemispheric, 6–11 mm. Phyllaries (24–)40–80(–100) in 3–6 series, linear-lanceolate to linear, 4–11 × 0.5–1.2 mm, apices usually spreading to reflexed, sometimes appressed, long-acuminate. Receptacles flat to convex, 3.5–8(–10) mm diam. Ray florets 12–40(–50); laminae 8–20 × 1–3.5 mm. Disc florets (18–)40–160; corollas 4–7 mm, glabrous or glabrate; lobes 0.3–0.7(–1) mm, glabrous. Cypselae 2–3.5(–4) mm; pappi 2–8 mm. 2n = 8.
Flowering Mar–Oct. Grasslands, Larrea-dominated desert scrub, pine-oak and pinyon-juniper woodlands, streambeds, roadsides, disturbed areas; 800–2500 m; Alta.; Ariz., Calif., Colo., Kans., Mont., Nebr., Nev., N.Mex., Okla., S.Dak., Tex., Utah, Wyo.; Mexico (Aguascalientes, Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Zacatecas).
Machaeranthera tanacetifolia is present in the southern Rocky Mountains, western Great Plains, and southwestern desert regions. It has also been reported from Illinois and New York, but in both cases is most likely introduced. Because of its large showy heads, M. tanacetifolia is sometimes planted as an ornamental.