I've divided blanketflower, catmint, centaurea, tiger lilies and asiatics in fall and all did well. Watch for root heave when the ground first freezes. then after the ground freezes, I put piles of chopped leaves over everything except iris.

Yay! Free plants!

I live in the western mountains of Maine and run a small perennial nursery. I go by the rule of thumb, if it flowers in the spring - divide in the fall, if it flowers in the fall - divide in the spring. It is not always possible to go by this little rule of mine, but if you can then you usually do not loose a season of blooms.

I know many people plant in the fall but I've had bad luck with this from frost heaving the plants the next spring. If I dig it is no later than the end of september so hopefully some roots are set to hold the plants in place. It is a trial and error type of deal. Some people have great luck with this and some(like me) don't.

You may want to try a little extra mulch when planting in the fall. You can use your regular mulch, leaves, some straw (not hay -weed seeds!) or even some branches from some evergreen trees. This may give you enough protection. The only time I have had a problem with heaving is when it is an entirely new bed prepared and planted in the fall and I didn't add extra protection.