With longblooming perennials, like 4 months or more, the spent flowers need to be cut off.   Why?  If you don't prune, it will usually stop blooming after a couple of months and can get floppy, and look messy.  Why does it do that?   The plant makes flowers that then fade and turn into seeds and then it's done its job, so stops blooming.   SO... we clip the spent flowers off (where the seeds are) and that will trigger the plant to make more flowers so it can make seeds.   Therefore it's important to give them a summer haircut to keep them flowering for many months.   

Generally, there are 3 different ways to prune.   1.  Prune just under the spent flower, 2.  Shear the plant to half its height, or 3.  cut flower stalks to the ground.   And some perennials won't flower again, so no need to prune.   BUT, how do you know which way to do it for which plant and when?   Check out this video I did for you that shows you 12+ different perennials, how to prune them, and how to figure out from just looking at the plant how you should cut it.