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.
FNA 2006, Wiggins 1964, Kearney and Peebles 1969, Allred and Ivey 2012, Heil et al. 2013
Duration: Annual Nativity: Native Lifeform: Forb/Herb General: Annual (occasionally biennial) herbs, 10-100 cm tall, from a taproot; stems highly branched when mature. Leaves: Alternate, abundant and overlapping along stems; blades 1-10 cm long, pinnately incised to two or three times pinnatifid. Flowers: Flower heads showy, radiate, purple with yellow centers, at the tips of leafy branches; involucres hemispheric, 6-11 mm high, the bracts (phyllaries) in 3-6 graduated series, the outer phyllaries much shorter than the inner; phyllary bases straw-colored and phyllary tips herbaceous, green, long-acuminate, and usually strongly reflexed; ray florets 12-40 per flower head, the laminae (ray petals) 8-20 mm long, blue to purple; disc florets 18-160 per flower head, yellow. Fruits: Achenes 2-4 mm long, hairy; topped with a pappus of bristles. Ecology: Found in dry, open places and along streams, washes, and roadsides, below 8,000 ft (2438 m); flowers March-October. Distribution: Alberta and MT, south to CA, NV, AZ, NM, and TX Notes: This widespread and common annual herb is has the distinctive showy Machaeranthera-type flower with blue-purple rays and a yellow disc, and feathery pinnatifid leaves. Distinguish from its congener, M. tagetina, by its wider, hemispheric flower head and phyllary tips that curl ourward and downward (M. tagetina has broadly turbinate flower heads and phyllaries that can spread a bit but do not curl outward and downward). Other similar species are in the genus Dieteria, which was formerly part of Machaeranthera (D. canescens, D. asteroides). However, Dieteria spp are biennial to perennial, and while their leaves are often pinnately sharp-toothed, they are never feathery and bipinnatifid as in this species. Because of its large showy flowers, M. tanacetifolia is sometimes planted as an ornamental. Ethnobotany: Used medicinally by the Zuni. Etymology: Machaeranthera comes from the Greek machaira, sword and anthera, anther, referring to the shape of the anther-tips; tanacetifolia means leaves similar to Tanacetum, the tansy genus. Synonyms: Aster tanacetifolius, Macaeranthera coronopifolia, M. parthenium Editor: SBuckley 2010, AHazelton 2017