Most perennials grow by sending up new stems from their underground parts. They spread and form clumps that eventually become crowded and exhausted. In some cases, the centre dies back, producing a “doughnut” effect. Crowded plants may produce smaller blooms or become less vigorous. To revitalize such plants, all you have to do is divide them, taking care to keep only the healthy parts.
Perennials that bloom after mid-June are usually divided in early spring (late April or early May), and those that bloom in early spring, in early September. In fact, spring-blooming bulbs can be divided immediately after they have finished flowering, in early June. There are some exceptions to this rule, though. Bearded irises should be divided in July, and peonies and poppies, in late August.
It is best to work in the shade or on a cloudy day when dividing plants, to protect the roots from the sun. Use a spade to dig up the entire plant, removing a ball of earth with it. Perennials that grow from rhizomes or tubers, like irises, peonies and poppies, should be cut into sections, keeping three or four buds per section. Those with fibrous roots can be severed, keeping a few buds per section. Plant the healthy sections, taking care not to bury the collar (the point where the roots meet the stem) in the ground. Firm the soil around the roots and water thoroughly.