Well-known by allergy sufferers, giant ragweed (Ambrosia trifida, Asteraceae) is an unfortunately widespread plant. As a ruderal species it often takes advantage of disturbed areas, especially along roadways, ditch banks, and the edges of fields. It grows well in full sun or light shade. It prefers moist loamy soil but can survive in sufficiently moist coarse sand or clay. Under ideal conditions it can reach 12 feet in height (the specimen above was about 8 feet tall), with leaves up to a foot long. Even under non-ideal conditions, this plant can still grow well enough to reproduce.
What makes giant ragweed unpleasant for allergy sufferers is that it’s a large, widespread plant that produces a lot of wind-dispersed pollen. While unfortunate for humans, it’s a successful reproductive strategy for the ragweed. Plants have separate male and female flowers. The male flowers grow on conspicuous spikes at the top of the plant. They release pollen, which becomes dispersed in the air by the wind. Some of this pollen can end up reaching the female flowers lower on the plant. These flowers are tiny, without petals, and grow in the leaf axils (where the leaf joins the stem).
Despite general human disdain for giant ragweed, some people find the plant attractive and enjoy it in their gardens (I suspect they don’t suffer from allergies). In addition, at least a few other species are fond of it. Apparently some moth larvae are partial to the foliage.