The easiest way to tell Verbena bonariensis from Verbena incompta is to feel the structure the individual flowers are embedded in. In Verbena bonariensis it is definitely sticky. Incompta is not sticky or slightly sticky. The other way is to get a good closeup of a flower. In bonariensis the coloured tube is distinct and much longer than the calyx, which cups the flower. In incompta the tube is hardly noticeable. Your photo is probably bonariensis.