A much-branched annual, often growing by the thousands in bottomlands. Flowers lack petals and sepals and are grouped into drooping clusters that are arranged in spikes. Male flowerheads quite small, green, in loose, terminal racemes; female flowerheads form below male heads, nearly hidden in leaf axils. Blooms July–September. Leaves opposite, palmately 3- to 5-divided (or at times undivided), the lobes pointed with fine teeth, rough, hairy (scabrous).
Similar species: There are 6 species of Ambrosia in Missouri. The most common are common ragweed (A. artemisiifolia), with ornate, 2–3 times pinnately lobed leaves; lanceleaf ragweed (A. bidentata), with alternate, lanceolate, mostly unlobed leaves; and western ragweed (A. psilostachya), with 1-time pinnately lobed leaves.