Cut away. By late August, an established perennial has been photosynthesizing for long enough that the plant will be able to recover with no ill effects the following year. Because each year brings its share of extreme weather and at least a few destructive encounters-out-of-bounds soccer balls, squirrel-chasing dogs, home repair workers, errant weed whips-by late summer every garden has something that warrants cutting away before its time. This isn't news to most horticulturists. Visit almost any botanical garden and you'll see spaces where cutbacks have occurred. However, you may miss seeing the gaps unless you're looking for them. Don't be alarmed if a perennial grows back after being cut. This won't set it back or disrupt its next bloom, so enjoy the fresh greenery.
Some plants are almost certain to grow new foliage even after a late-summer clipping-catmint (Nepeta), daylily (Hemerocallis), globe thistle (Echinops), hardy hibiscus (Hibiscus), and perennial geranium (Geranium) come to mind. Others are less predictable. I've seen the likes of astilbe, blue false indigo (Baptisia australis), Japanese painted fern (Athyrium nipponicum 'Pictum'), Joe-Pye weed (Eupatorium purpureum), and queen-of-the-prairie (Filipendula rubra) send up some new shoots, but I've also known them to just lie back and wait for spring. Some perennials such as peony (Paeonia) and old-fashioned bleeding heart (Dicentra spectabilis) are almost sure bets to conserve their energy and hold off on foliage production until the next year.