Leaves have square ends towards the base (see photos).
Similar species: Upright Bindweed (Calystegia spithamaea) - Large white flowers. Leaves not square at base. Eurasian Black Bindweed (Fallopia convolvulus) - Leaves not square at base. Field Bindweed (Convolvulus arvensis) - Smaller white flowers. Leaves have two pointy ends near the base.