Thanks for writing. You can cut as many of the suckers, or shoots that come up from the roots, back as you like to help the plant from growing out of bounds. If it's large and dense, you can also thin out entire shoots from the center of the plant. Just don't remove more than one fourth of its total branching structure at one time to avoid shocking it.
The best time to prune it is spring, just after it finishes blooming. That way the stems you leave in place will have time to develop flowers for next year.
---Justin, Senior Garden Editor, BHG.com