Flower:

Slender spike-like racemes, sometimes branched, from 1 to 6 inches long at the tip of the central stem and branches. Male (staminate) flowers are yellow to greenish, tiny, hanging down on short stalks, petal-less and bead-like. Female flowers are indistinct, tucked away in clusters with short leafy bracts at the base of the cluster, along the stem or in the axil. Clusters are initially densely packed, spreading out as the plant matures.

Leaves and stem:

Leaves are thin, fern-like, deeply lobed with many of the side lobes deeply lobed again (1-2 pinnate), generally egg-shaped in outline, up to 6 inches long and 4 inches wide near the base, hairy but may become smooth with age. The lower leaves are oppositely attached, the upper are alternate. The stem is typically heavily branched and variably covered with soft, erect, white hairs and often has a purple tint.

Notes:

Common Ragweed is an early pioneer species of disturbed soils, has been introduced worldwide and is now a common weed in both agricultural and urban sites. With high population densities and prolific production of highly allergenic pollen, it is a major contributor to the agony of hayfever suffers around the globe. It can become quite bushy and grow in clumps or colonies. Most similar is the perennial Western Ragweed (Ambrosia psilostachya), which is less heavily branched, has leaves less finely divided, and has fibrous rhizomes where Common Ragweed is taprooted.