Don't cut perennials too early
Cutting back perennials too early in the fall may result in a quick flash of tender growth during an unpredictable time of the year. Fall temperatures can fluctuate without warning, leaving your plants vulnerable to sudden frost. Allow your perennials to become dormant first, responding to the cooler temperatures and shorter days by naturally slowing their growth. Try waiting until mid-to-late fall (October and November) to start pruning
the top of plants, when growth should come to a halt altogether. You should also take this time to trim any weak, damaged or dead branches on trees, shrubs and other plants before they become snow-covered.