Many people ask if they can put a plant in their dragon's cage. Honestly, the environment wouldn't likely support a single plant and your dragon will end up stumbling around on it or just knocking it over. If you're wanting to do something like that, I'd look into bioactive enclosures. There's a few good bioactive groups on facebook that you could seek out. A bioactive setup would be the only type of setup where I'd recommend having live plants.
As far as feeding plants as treats, there are safe options. Some of the most common choices include the following:
The main plants that have been shown to be toxic to dragons and shouldn't be fed at all (its not even worth the potential risk):