Giant ragweed can reach final heights of more than 8 feet. It contributes to late summer hay fever because it sheds large amounts of airborne pollen. A very competitive plant, giant ragweed is capable of causing dramatic yield loss in crop production and dominating disturbed sites by out- competing other weeds and vegetation. Competitiveness of this weed is due to its vigorous growth during the seedling stage. The stems, which are rough and hairy, do not branch.
Flowers develop in clusters on the tips of branches and stems.
Female and male flowers develop in separate clusters. The true seeds are black and smooth. Giant ragweed is one of the first weeds to emerge in spring, which greatly enhances its competitiveness with later-emerging plants. Plants can emerge from seeds as deep as 6 inches in the soil, but seeds closer to the soil surface result in a greater percentage of plants becoming established. Seeds at the soil surface often do not germinate.