Perennials should be divided when they are growing strongly and look healthy. Dividing a perennial when it is not going well will result in poor establishment. Also some perennials will start to die out in the center of the mass. This is a sure sign it is time to divide. This is very common among grasses, irises and sedum. Most perennials can be divided in fall or spring. Mid September is a wonderful time to divide. The soil is still warm enough for the roots to start re growing, thus giving the plant a great start in spring. Some plants such as Bleeding Hearts, Primroses, Anemone and Monkshood prefer to be divided in spring.