Common ragweed

Ambrosia artemisiifolia

Annual, reproducing only by seed. Seedlings: Cotyledons are thick, dark green, rounded above, narrowed near the base, sometimes with purple spots near the margins; First leaves are alternate; opposite by the fourth node. Stems: Erect; 15- 150 com (6- 60 in.) high; Usually much-branched; Hairless or hairy throughout; Lower leaves opposite (2 per node); becoming alternate (1 per node) higher on the plant; Bright green to slightly yellowish-green on young plants; becoming greyish-green on older plants; Compound and finely divided, the final divisions usually coarsely toothed.