I have a butterfly bush that is not even 2-years old, but it's the size of a medium tree. How severely can I cut the bush back without killing it?
You can cut it back nearly to the ground, but if I were you, I'd wait until spring to do so. I can't remember the reason, but cutting anything back before winter makes it more likely to be killed by cold weather. I imagine if you live in a northern climate, it would be especially important to wait.
I cut mine back to about 2' tall each spring, and they always grow back nicely.
Thank You Sherry.
If this is a factor, don't shovel excess snow onto/near your BB, or water it while dormant. Totally agree about the pruning advice.