I'm in Florida and I've found this to be one of the best methods of keeping spider mites away from my Brugmansias and Hibiscus.
I do always spray in the mornings, or as early in the day as I think of it so that the leaves have a chance to dry off before nightfall. We also have every fungus known to man floating around in the air here, so wet leaves and warm nights are an engraved invitation.