Polygonum arenastrum (Common knotweed) is a summer annual weed that is native to Europe and can be found on most continents as an introduced species and a common noxious weed. Known by many other names, such as 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 throughout all of the United States and Canada.