Amaranthus blitoides

Amaranthus blitoides, commonly called mat amaranth, prostrate pigweed, procumbent pigweed, prostrate amaranth, or matweed, is a glabrous annual plants species. It usually grows up to 0.6 m, though it may grow up to 1 m (3 feet). It flowers in the summer to fall. It is believed to have been a native of the central and possibly eastern United States, but it has naturalized in almost all of temperate North America. It has also naturalized in South America and Eurasia. Some authorities list it as an invasive species.

from Wikipedia