Good question, I'm curious, too. Maybe it's too new and nobody knows yet.

Mine is big as well but I want to divide it because it is 3rd year in my garden and I did not get good performance this year while previous years it was magnificent. I am not sure if it is because of very dry summer/I've been watering it but still/ or it just needs to be divided often.

A plant with one central stem cannot be divided. My Rozanne looks to be one of those so I am going to try rooting some cuttings.

One that is easy to dig out side rooted stems is G. endressi. When you lift its growth you easily see portions that have rooted.

I have successfully divided mine by digging up the clump, washing off some of the soil so I can see what I'm doing, and then cutting the root mass in half, or even quarters. I keep an old knife in the garden shed for this kind of work. Cut back the top growth to balance the loss of root mass. I've done this in both the spring and fall.