I've noticed a huge difference since moving north into a warmer zone. In Utah, zone 5a, I saw very little increase, even after several years. Since moving to zone 7a, I now have clumps of plants that never increased at all. Of course, I didn't plan ahead, since I wasn't expecting so much increase! Now everything is crammed together, thanks to all the growth! I still have lots of single fans that were added last year, and those are taking their time, but anything that was added as a double fan or more has done much better.