I've never been able to kill the things. I've moved, trimmed, divided just about ever time of year that the ground wasn't frozen solid. I've even done it in the hotest part of August without killing them. You will find that if you divide them now that they will pout for a while. Just keep them well watered and before you know it you will be over-run with more mums.

BTW, when I trim the tops (read butally whack their tops off) I shove the stems back in the ground and by fall I have another blooming mum plant. lol

I was hoping someone would say that they're pretty indestructable. I couldn't wait and did thin out the back portion this afternoon. Now that I've your response, that baby is coming out tomorrow! lol Thanks!