I have found that they don't take kindly to disturbance so I wait until The clump is very large before digging it up and splitting it carefully with 2 forks and pulling by hand. I have done this twice in ten years. I have posted photos.

Hi, I had 2 in pots which the frost had got and they looked pretty unsightly, so I thought I would take them out of the pots and throw them away. Could I got those things out of the pots, it was like mission impossible.... In the end I had to cut around the roots and under the roots just to get my hands in enough to pull them out. Well I literally had to yank them out. I thought for sure that I would have killed them as it took me half an hour to just get one out. I was about to throw them in the bin but after my unfair treatment of them decided to put somewhere else in the garden and see what would happen. After a few days new leaves were shooting up!!! Anyway I put them in the wrong place so I moved them again last week and they are still growing so I guess they must be really tough plants. Let me know how you get on, it will be interesting to see if you have the same trouble getting them out that I had.