Achillea millifolium, milfoil yarrow

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Intermountain Flora
Perennial, often rhizomatous, loosely sericeous-woolly throughout, sometimes only thinly and inconspicuously so; leaves slender, pinnately dissected, only the lower petioled; heads numerous in a flat or round-topped, short and broad, paniculate-corymbiform inflorescence, the disk 2-4 mm wide; involucre mostly 4-5 mm high; rays mostly 3-5, white or occasionally pink, mostly 2-3 mm long; disk-flowers 10-30; diploids through octoploids based on x = 9.

Cronquist, A., Holmgren, A.H., Holmgren, N.H., Reveal, J.L., and Holmgren, P.K. (1994) "Intermountain Flora" pg.134, vol. 5, The New York Botanical Garden New York.