Pigweed, in general, covers several different kinds of weeds. A common form of pigweed is prostrate pigweed (Amaranthus blitoides). It is also known as matweed or mat amaranth. This invasive weed has made itself at home in lawns and gardens. Prostrate pigweed grows in a circular form with low-growing stems coming from a central spot so it looks like a spider web. The radial stems are reddish-purple and can grow more than a foot long. The leaves on prostrate pigweed are about a half inch long and are oval shape. The flowers on prostrate pigweed are reddish-green and are not significant. The flowers will produce seeds that look like small black sand grains. The prostrate pigweed spreads through these seeds.
Amaranth grows well in a variety of soil types. Generally speaking those soils suitable for growing lettuce, would make a nice place for growing amaranth. Your soil should be finely prepared in order to achieve the proper seed planting depth. Seeds should be planted no more than 1/4 inch deep.