When they get way big (bigger than yours are now - maybe twice that size) they need to be dug up, and split in half. Replant one half, put the other half somewhere else.

Like with most perenials, if you don't divide them, the center of the plant will die back. You do it about every four to six years in the early spring (just as they are starting to come up). You can also do it in the fall, but I've not had good luck with that.Directions