September is the best month to divide and transplant perennials, but you can still be successful in early October. This is a good time to see the plants that are not healthy and to assess your garden design.
Some signs that a perennial should be divided include dead centers with the growth on the outer edges, not blooming as well as it used to and blooms that are smaller than normal. Some perennials are aggressive spreaders and need to be divided to prevent them from taking over. You also may simply want more plants to give away or to fill holes in your garden.
There are some general rules for dividing perennials. It is best to do this work early enough in fall so the plants have approximately four weeks to re-establish roots before there is a hard frost (which usually occurs here in late October or early November). Tough plants like hostas can be moved most anytime with good results; I have divided hostas in late October and even left some clumps above ground for the winter before planting with success.
Even though this is a good season to divide, most perennials can be divided in either spring or fall. It is best to avoid dividing or transplanting perennials during their bloom time or when they are about to bloom. Instead, wait a few weeks after they bloom.
Another strategy is to divide the plants based on their bloom time. Late summer and fall blooming perennials like coneflowers, rudbeckia, sedum and asters get divided in spring while spring and early summer flowering perennials like astilbe, peonies and iris get divided in fall. The perennials that have begun to show signs of going dormant with browning foliage can also be moved or divided in early fall.
Dividing and transplanting perennials is easiest to do with a flat spade. A sharp spade will make it much easier to cut through the roots to make your divisions. Select the healthiest sections to replant. You may need to cut back the foliage of tall perennials — cutting back the foliage by half will not hurt the plants.
After you have transplanted, it is very important to mulch and water the newly divided perennials throughout fall. This gives them the best chance to get established. Newly purchased and planted perennials also need to be mulched. Mulch up to the crowns of the plants but do not bury them.