Common knotweed (Polygonum aviculare), also known as prostrate knotweed, is a a prostrate summer annual with small, elliptic leaves that is primarily found in compacted areas of turfgrass such as pathways or sports fields. Prostrate knotweed is found throughout the United States. Identifying characteristics of common knotweed include prostrate-growing plants with small lanceolate leaves that are primarily found in hard compacted areas of turfgrass and landscapes. Some of the spurges like Spotted Spurge (Euphorbia maculata) may be confused with prostrate knotweed, however the spurges do not have an ocrea and emit a milky sap when cut unlike prostrate knotweed.
Source: Virginia Tech Weed ID Guide
To learn more about ways to control common knotweed, watch this video from AgPhD.