I've noticed the same situation. maybe timing will help. Do it just before a forecasted cool rainy stretch in the spring or fall. Or buy new ones on sale - LOL.

When I prune after blooming I usually prune off a lot of that excess that really isn't rooted. I think it makes for an overall better plant in the long run.

Thanks for replying. I'll try to wait until late fall and maybe build up a layer of soil to encourage some root growth.

late fall may not be ideal. September is my favorite month for transplanting just about anything. Still plenty of time for roots to take hold.