rag·weed 
(răgwēd′)
(răgwēd′)n.
1. Any of various weeds of the genus Ambrosia of the composite family, having small, greenish, unisexual flower heads and producing abundant pollen that is one of the chief causes of hay fever.
2. Chiefly British Ragwort.
[From the ragged shape of its leaves.]
ragweedtop: common ragweed
Ambrosia artemisiifolia
bottom:pollen grains of western ragweed
Ambrosia psilostachya
ragweed