(from the article `ragweed`) ...rough hairy stems, mostly lobed or divided leaves, and inconspicuous greenish flowers that are borne in small heads, the male in terminal spikes ...
[n] - annual weed with finely divided foliage and spikes of green flowers
Ambrosia artemisiifolia noun annual weed with finely divided foliage and spikes of green flowers; common in North America; introduced elsewhere accidentally