Most perennials decline with age and should be divided every three or four years. New growth can be noticed on the outer edges of clumps. Divide perennials when you notice the signs listed below. The photograph illustrates a Siberian iris that needs to be divided.

Tell Me More contains a list describing the recommended frequency of division for common perennials.

Do not divide perennials when they are in bloom. Divide midsummer and late-summer bloomers in spring (April or early May) when growth is two to three inches high. Divide spring-blooming perennials in late August or September.

		A dead center has formed in the crown area with a ring of growth around it.
		Blooms becoming sparse and smaller.
		Plants look obviously crowded.