Polygonum aviculare; The knotgrass is a plant that grows mainly on soils disturbed by humans or trampled by larger animals (livestock). The branches grow over the ground stemming from a central point where the root is placed. It is generally regarded as a weed growing in many regions of the world.
It is a member of the dock family (Polygonaceae). The Icelandic name of this species is Blóðarfi