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.
It can be invasive in Europe and the US.