I do it in June immediately after the first bloom.

I'm a little late I guess. Thank you for the information though :O)

For cold winter zones the advice is to divide at least 6 weeks before the first suspected frost (for roots to establish). I don't know what that date is in your zone.

I might be OK then. I'll have to wait until the blooms are spent. Then I'll decide. I had heard 6 weeks before frost before, but I didn't know if re bloomers were treated differently. I've never had one before.

She's looking quite unkempt and this is only her second summer. Last year she didn't rebloom so I was happy to see the second blooms this year. Do rebloomers need to be divided more often than the tall bearded?

Thank you so much for your help.

Oh! So once Immortality has stopped blooming, I can chop a portion of it off and I don't have to worry about the roots establishing by the first hard frost?

Yes :-)

Where the rhizomes are broken/cut and fresh tissue is exposed, trowel away some soil so the cuts are exposed to air and sun and can harden off for a couple of days. I also do not water for a week or more. You can sprinkle the cut area with a cleanser containing bleach (Comet ?). I've not used the cleanser and have not had rot. Maybe my soil doesn't contain the pathogens?

Thank you so much! This solves the problem!