Common Ragweed, Ambrosia artemisiifolia
Asteraceae or Sunflower Family  

Erect herb, up to 6 feet tall, but usually about 3 feet.  Leaves deeply dissected into many tiny segments.  Flowers tiny, in crowded spikes at the top of the plant, green.  Blooms in late summer

Common native plant of fields, roadsides, gardens and croplands.  Much despised because its pollen, along with that of giant ragweed, is the principal cause of autumn hayfever.

Giant ragweed (Ambrosia trifida) has similar flowers (that also cause hayfever), but it is much taller and its leaves are divideded into three lobes.