I read recently that photosynthesis slows as temperatures exceed 85F. I have always noticed that roses seem to rest through the heat of the summer, then resume growth when the hot weather breaks. In places where humidity is not terribly high, overhead watering will tend to cool the leaves of plants in the heat of the day and this will encourage growth.
The problem comes in places where the humidity is high. One consequence is that evaporative cooling doesn't work. Another is that the moisture can really spur fungal growth. So overhead watering can be an excellent tool for a lot of people in hot, dry places - especially if it gets cool at night. Maybe not so much in places with warm nights and high humidity.
I suppose every garden is a little different, and one just has to try out ideas and see how they work.
