when should butterfly bushes be cut back--spring or fall?
butterfly bushes
I would say after they go dormant in the fall, and before they start growing again in the spring.
I don't cut mine back at all--I just trim off any dead branches in the spring. It doesn't go completely dormant until January or so, and then not for long. A lot of the new spring growth comes in on old wood.
I don't trim mine either. I don't think mine even went dormant this year.