My usual disclaimer: I'm a newbie, but I'd like to share my opinion on this topic! I wouldn't feel comfortable throwing any old un-treated stick from outside into my chameleon's cage, though I know that others do and they get along fine. I've heard stories of people taking driftwood from lakes and whatnot, baking them for hours, then putting them into their dart frog enclosures only to find that a colony of ants stayed alive in the center of the wood and attacked their frogs. Plus, I've heard plenty of armchair scientist extolling the dangers of the local bacteria on these outdoor materials that our exotic, captive bred pets have no immunity to. They say the only true way to cleanse materials like these is with an auto-clave.
All that said, I have a live hibiscus that I got from a nursery in my chameleon's cage. I removed all the fertilizers and dirt from the store, washed the plant down heavily, repotted it with fresh fertilizer-free dirt, and THEN covered that top layer of dirt with river stones. I also put him in his outdoor cage as much as I can, and that has a jasmine plant from that same nursery which received the same treatment, right down to covering the dirt with river-stones.
