I have divided them in May, July and Sept, as long as you give them plenty of water right after you move them I don't think you can kill them. Good luck.

"The bible" (Well-Tended Perennial Garden) says not to plant them in the fall, so I would extrapolate that to include dividing. I've only moved them in spring (with good results.)

I've divided them in both September and in early May. They've lived in both cases, but I've liked the results better if I do it in the fall... they have longer to settle in before bloom season (which for me is early July).

Beth
Z5 Northern Michigan

If it's a tough and vigorous variety, you can probably do whatever you want. Sometimes recommendations of the "best" times to divide/transplant are just that - the best times - while other times still work and you can get it out of the way to free up your spring calendar for other work in the garden.

In any case, fall is a good time to move most perennials, and I think you're still in the window of ideal time to do it. Frost may be coming soon but we're still a ways away from frozen soil, and most of the work that the plants need to do before winter involves rooting which they can do after frost.

You can dig up and divide most of it, and as an insurance policy leave the remaining portion in place where it is already firmly rooted. The half you left in place will still have the breathing room it needs to spread again so either way you solve the problem of overcrowding.

pizzuti, that's an excellent point and plan.

Thanks all!