You can cut them back anytime now through fall--if the foliage still looks nice I'd leave it a few more weeks, if it's looking rough cut it back whenever is convenient. You might want to leave a few inches more than the herbaceous in your zone to allow the buds on the stems to grow in the spring. I'm zone 4b, those buds above ground don't make it, so I trim back to an inch or 2 above ground.