Western ragweed
Ambrosia psilostachya

Flower. Abundant, slender spike-like racemes, 1 to 6 inches long, at the tip of the central stem and side branches. Male (staminate) flowers are tiny, yellow to greenish and bead-like, short stalked, initially densely packed along the cluster but spreading out as the plant matures. Female (pistillate) flowers are solitary and few, typically located at the base of the cluster, along the stem or in the axils, and flanked by large leaf-like bracts.
Leaves and stems. Leaves are thick, oval-lance shaped in outline, up to 5 inches long and 2 inches wide but deeply divided into narrow lobes that are typically further lobed. Leaves are hairy, more densely on the underside and can give the leaves a gray-green cast. The lower stem leaves are oppositely attached, becoming alternate in the upper branches, stalkless or with a short stalk that is often winged. Stems are stiffly erect, may be unbranched or many branched in the upper plant, with long soft hairs when young, becoming rougher textured with age.
Notes. Tolerant of dry sandy sites and an early colonizer of disturbed soil, this native can be common in both high grade prairie remnants and disturbed urban waste places. Very similar to the annual Common Ragweed (Ambrosia artemisiifolia), which tends to be much more heavily branched, less hairy overall, has thinner, more finely divided, fern-like leaves, and is tap-rooted where Western Ragweed has fibrous, creeping rhizomes. Western Ragweed is listed in some references as Ambrosia coronopifolia.