There are a few "weedy" type Amaranthus commonly called Pigweed, these two in the database have photos:
Prostrate Pigweed (Amaranthus albus)
Redroot Pigweed (Amaranthus retroflexus)
There's a beautiful ornamental Amaranthus, commonly called Summer Poinsettia that is a perennial in warmer zones and acts as an annual in colder climates. Amaranthus tricolor:
http://garden.org/plants/searc...
I don't know if cross pollination could occur between the weedy variety and the ornamental variety of Amaranthus or how long seeds are viable for any Amaranthus; I'd think they'd need warmth for germination.
