Irises do most of their growing in the spring and the fall. They don't truly go dormant in the summer, but they do enter into sort of a quiet phase after spring bloom that lasts until the weather starts cooling off.
Some irises rebloom in the fall, but it's typically a trait that has to be specifically bred for.
Other people will give you completely different answers, but I never fertilize my irises. Don't see any advantage to it really. If you do fertilize, be careful with the nitrogen. Too much nitrogen results lots of leaves and few flowers.
