Polygonum arenastrum, commonly known as equal-leaved knotgrass,is a summer annual flowering plant in the knotweed family Polygonaceae. It is native to Europe and can be found on other continents as an introduced species and a common noxious weed. Other common names include common knotweed, prostrate knotweed, mat grass, oval-leaf knotweed, stone grass, wiregrass, and door weed, as well as many others, knotweed was first seen in North America in 1809 and is now seen across much of the United States and Canada.