By dividing in the fall, the warm soil and steady autumn rains provide a constant supply of water to help promote root growth. The basic rule of thumb though is it is better to divide the early blooming perennials in the fall. But, make sure to give them at least six weeks before the ground freezes. Divide perennials now to ensure a trouble-free spring. However, if you live in a northern climate where hard freezes come early, you may want to consider waiting until spring to divide your plants as I’ve always done.
So how do you know if your plant needs to be divided? The norm is every three to five years, especially if the rate of flowering has dwindled or the growth of the plant is spreading outward, leaving a dead mass in the center. Or if they have overgrown their space. If you’re not sure if the plant can be divided, then check online or in a garden book for the proper way to divide the plant in question.
Here is an abbreviated list of perennials that you can divide in the fall: