Cutting back plants & shrubs
It is usually appropriate to prune back butterfly bushes annually since they grow so prolifically. Don't hesitate to cut them back as much as desired. If you wish to maintain a certain shape or size, you can retain some branches to accommodate that plan. Many people simply cut the entire shrub back to 2-3 foot stumps.
Liriope is usually evergreen through most of the winter and yellows in late winter / early spring, at which time it can be cut back to the crown. It is also appropriate to lift and thin out a bed of liriope every two or three years. This, too, is best done in the spring. When the plants become too thick with foliage there is a greater chance for fungal problems on the foliage.