GIANT RAGWEED, Ambrosia trifida L. 1, portion of stem showing leaves and flowers; 2, seed. Annual, reproducing by seed. Stems coarse, rough, reaching a height of 12 to 18 feet (3.6 to 5.4 m) on fertile, moist soils; 4 to 8 feet (1.2 to 2.4 m) in less fertile, drier areas. Leaves distributed in pairs at nodes; large, slightly hairy, entire, with 3 or, occasionally, 5 lobes. Flowers separated, with male flowers abundant in clusters on tips of branches and stems. Female flowers sparse, without petals, in axils of upper leaves. Seeds 1/4 to 1/2 inch (0.6 to 1.3 cm) long, with a woody hull bearing blunt ridges that end in several short, thick spines at the tip. Found mostly on fertile, moist soils, especially bottomlands. May be serious in corn, soybeans, and other cultivated crops. The abundant pollen is a hazard to hay fever sufferers. Also known as horseweed.