Cyperus difformis (Variable Flatsedge) is native to the Old World, and is widespread in East Asia and also in Africa. It is now widely scattered in temperate North America including the southeast United States and California. Its present range includes southern Europe, Asia, Central America, Mexico, South America, Australia, and the Pacific Islands. It is known as a weed in 46 countries, primarily affecting rice fields, but also occurs in fresh and brackish marshes. Its first North American collection was in Maryland before 1753, and has probably had multiple introductions to North America. It was first collected in California in 1915, and in Virginia marshes in 1934. It now occurs in 21 states, It is established in marshes in the San Francisco Bay Delta, where it was collected in 1998, and probably earlier. Likely vectors include agriculture and bird dispersal.