Fill your yard with blooms for free! Late-flowering perennials are best divided in spring; wait until fall for early flowerers, but well before the thermometer dips too low. You want the roots to have time to recover before the ground freezes. This method works best on perennials with fibrous or fleshy roots, such as hostas, phlox, asters, and rudbeckia. Plants with tuberous roots, like peonies, or rhizomes, like bearded iris, should be cut apart with a knife.
See Dividing Plants
for the three steps to getting the job done.