Fall is clean-up time for perennials, the time when leaves and stalks of herbaceous plants are removed from the garden. It's not pruning in the classic sense, but does involve cutting off the parts of plants that will die back in winter.
The main reason we remove these is for insect and disease control. Fungi, bugs and their eggs overwinter in the debris of last year's foliage, so removing it lowers the population density of these pests. Fallen leaves and flowers too. However, if you don't get it done before winter, the plants will survive.
Foliage is ready for cutting when it begins to yellow or falls over, although on vigorous plants it may be removed earlier, starting now. When cutting, leave an inch or so of the stem, because some species sprout in spring from buds on the stems, while others sprout from new underground shoots. In any case, be careful not to cut or damage any white shoots growing under or near the soil surface, or any small green rosettes of leaves near the stems.
When the big mounds of perennial foliage are gone, spaces to plant bulbs will miraculously appear. Consider early April blooming daffodils and tulips which will flower before the the perennial shoots come up. Then, as their leaves expand, they will camoflague the yellowing bulb foliage - a double benefit.
To liven up the winter landscape, you can leave the dried flower heads of astilbe, sedum, and grasses. Don't mulch until the ground has frozen . Use a light airy mulch which won't mat down and smother the crowns.