Near the end of the movie when Percy, Annabeth, and Grover are in the Underworld, Persephone is there. Yet, their whole adventure took place around the summer solstice, when Persephone should be with her mother, Demeter, in the real world (this is how Greeks described the change in seasons; the six months of the year where Persephone was with Hades, Demeter, who is the goddess of the plants and fertility is too sad to make anything grow. When she returns in spring and summer, she is happy, so stuff grows.)