The butterfly bush I know as a shrub, probably won't last long in a tropical tank, but it will become a part of the substrate, if it doesn't.
I'm a non-sterilizer, because I like the different things that crop up from introduced, simply washed off, wood. I don't think you have a concern with any intestinal parasites such as coccidia, that would be a problem with darts from collected wood of any source. That's just not the way these parasites are transmitted. Darts may come with their own coccidia parasite load. Unless they are stressed or immunocompromised in some way, they are in balance with this. Some people do regular parasite checks and even treat prophylactically on a regular basis, but that's another topic and a different argument. Josh is more careful with sanitizing. We each do what works for us and what we feel comfortable with.
On the other hand, the butterfly bush will break down fast in a humid vivarium, so essentially will be like putting in leaf mold in the long run. No harm either way.
For something more permanent, you could try Osage orange, Manzanita, cypress root (swamp cypress), Mountain mahogany, depending upon where you live and can collect. Never use pine or resinous branches from any of the conifers. They contain turpentines, but you can try any of the hardwoods. Cork bark, which you will have to purchase, is great.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus