Thanks for writing. Most rhododendrons don't take well to heavy pruning. But you can do light pruning and the best time to do that is right after they finish flowering for the season.
If you have average or rich soil, you probably don't need to fertilize your rhododendron. But if you have lean soil, then any general-purpose product should work. Or, you can also look for the fertilizers designed for use on rhododendrons, conifers, and other acid-loving plants. They should be readily available at your local garden center.
Apply the fertilizer in late spring or early summer.
---Justin, Senior Garden Editor, BHG.com"