palmer amaranth
Amaranthus palmeri                                         

Identifying Characteristics:

Seed:  dark reddish brown, lenticular, numerous per spike

Seedling:  hypocotyl red to green, glabrous to softly pubescent; cotyledons green, glabrous

Mature Plant: fibrous roots from well-developed taproot; erect, branched stems; alternate leaves, egg-shaped, veins prominent beneath, long-petioled; male & female flowers on separate plants, terminal spike, few to none lateral spikes; subglobose, splitting along a seam

Method of Propagation: seed

Life Cycle: summer annual herb

References: 
Bryson, Charles T. & Michael S DeFelice. "Weeds of the South."