Gardenia, Knock Out Roses
Thank you for your question about pruning and fertilizing these plants. Each one has slightly different needs, so I'll group them.
Gardenias: here's a Clemson Extension article on pruning (and fertilization.)
Knock out roses: here's a U Maryland Extension article link.
Hydrangeas: I'm not sure what variety or cultivar you have, so here's another Clemson Extension article on many of them.
Iris: again, I'm not sure what type of iris you have, so here's an Extension article on their care, including fertilization.
I hope these are helpful to you. Good luck!