This plant may be started easily from herbaceous cuttings. I currently have a whole flat of them, which I started from my spring prunings. I also started a container full by WS from saved seed. They appear to be just like the parent plant but i won't know for sure until it blooms. I have had this plant for many years & didn't know what it was until recently. It was labeled as an aster when I purchased it.
I didn't realize that this plant was attractive to butterflies. It is a good sturdy addition to the garden and grows rapidly. I had something - a deer I think gnaw off the front of it this spring, or perhaps it was a big rabbit. It does root fairly easily and now that I know it is a butterfly plant I will keep all those dozens of plants I started when I cut my big plant back. It does get to the rough rangy point where you want to discard it however. When we divided it into root segments and planted it in a wilder part of our property it redeemed itself with nice growth.
I thought mine was going to look bad this spring before I cut it back because the stems were so woody. But after a really good pruning it has put on beautiful herbaceous growth & it looks like new. I also usually give it another pruning in early summer to promote an even bushier plant. I don't know if you're supposed to do that or not but I prune it when I prune the mums. And it is always fantastic come fall.
This is now my second spring with this plant. It overwintered great and I moved part of it to another area. Like pamsplantin, I rooted my early spring (just out of dormancy) prunings, and I also recently cut it midway as we've had so much rain I was afraid it would get soft and too tall and by the end of summer. It bloomed well last fall but I can't find a picture at the moment.