Ambrosia artemisiifolia (common ragweed) is a common annual plant found throughout the lower 48 states. Plants differ greatly in size, from unbranched plants of a few inches tall to large, bushy plants over one meter (and reportedly to over 2 meters) tall. The leaves are usually divided, but the size and degree of the divisions is highly variable. Male flowers are clustered in numerous heads borne in spikes at the top of the plant. Female flowers are few and borne in small clusters in the axils of leaves, below the male spikes. Each female flower produces an achene with several sharp teeth along the upper surface.

A. artemisifolia in Wisconsin is typically a species of disturbed, sunny sites, especially roadsides and various rights-of-way, cultivated and recently abandoned fields, vacant city lots and less often grasslands of low productivity. Pollen is released from early August into mid-September with the highest pollen levels in the Green Bay area usually from about August 20 through the first week of September. This species is a major source of pollen induced hayfever in late summer.

Another ragweed species, Ambrosia psilostachya (perennial ragweed) differs in its perennial growth form with plants spread out along a rhizome. It is most successful on drier and less productive soils, but in areas of heavy disturbance it may rarely be found in heavier soils until the area becomes fully revegetated. The leaves tend to be less divided, but there is overlap of leaf shape with some of the least divided leaves of A. artemissifolia.