I once heard a landscape professor say the best time to prune is when the pruners are in your hand. Following this rule will certainly get the job done, but there are a couple of things to consider to really get it right.
We all know that timing is important when it comes to the pruning and shaping of flowering shrubs. The general rule is to prune just after flowering is complete so that new growth will have time to grow and initiate flower buds for next seasons bloom–following the flower.
Secondly, most pruning of trees and shrubs should be avoided from late August through the second or third hard frost of the fall season. Late season pruning combined with a warm fall often promotes new growth that will not be hardened off before the onset of cold weather. This can result in damage and death to the growing tips.
Concentrate now on cleaning and cutting herbaceous perennials and leave tree and shrub renovation for the dormant season of the winter months.