Ambrosia psilostachya, western ragweed

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Intermountain Flora
Colonial perennial herb from deep-seated creeping roots, 2-10 dm tall, rough-strigose or hispidulous and often also atomiferous-glandular; leaves all cauline, opposite below, alternate above, thick and firm, shallowly to fairly deeply pinnatifid with broad midstrip and few, entire to few-toothed or sublobate segments, mostly 2-15 cm long overall and 1-8 cm wide, on the average appearing narrower, firmer, less dissected, and often less petiolate than in A. artemisiifolia; staminate heads nodding, the involucre somewhat oblique, cupulate, 2-3 mm high, sparsely to moderately hispidulous, only shallowly and inconspicuously (or scarcely) few-lobed; pistillate involucres 1-flowered, at maturity 4-6 mm long, with a single cycle of short tubercles above the middle, or the tubercles obsolete; 2n = 36, 72, 108, 144.

Cronquist, A., Holmgren, A.H., Holmgren, N.H., Reveal, J.L., and Holmgren, P.K. (1994) "Intermountain Flora" pg.64, vol. 5, The New York Botanical Garden New York.