Fall is not the time to give up and ignore your perennial garden. Proper clean-up and pruning will ensure that the garden is even better next spring. Perennials fall into several general categories regarding their pruning needs:
Shrubby perennials like lavender, Russian sage, and caryopteris (blue spirea) should not be hard-pruned in the fall. While a gentle “tidy up” trim is permissible, leave any hard pruning to the spring, when you can assess any winter-kill, and shape the plant for the coming season. Hard-pruning encourages a plant to flush out with new growth, exactly what is not wanted in the fall., as the new growth will likely be killed in a harsh winter.
Daisies, yarrow, rudbekia, salvia, and coneflowers keep a small low tuft of leaves through the winter. While birds will enjoy the seedheads, the tidy gardener will want to cut down the dead top growth and leave the low basal growth to over-winter.
Heucheras, liriope, pulmonaria, and several other shade perennials require no pruning, and will actually stay fairly attractive over the winter. In the spring, the dead leaves can be pruned out to make room for the new growth.
Peonies, lilies, hostas, coreopsis, and many other perennials are killed to the ground by hard freezes. After the foliage blackens, it can be cut several inches about ground level (leave stubs so you remember where the plant is!). Diseased foliage should never be composted, but discarded in the trash.