From what I've seen, Princess Tiana's tiara is more organic than the usual Disney princess tiara: http://www.google.com/products/catal...ed=0CG4Q8gIwAQ
The decorative detail seems to be made from leaves in a soft pastel green. I would almost suggest cutting, shaping and heat sealing grosgrain ribbon into leaves and then gluing them to a stiff head band.
Although it may be hard to find grosgrain ribbon that's wide enough. I suppose you could try using fabric but even if you could stiffen it I don't know if you could heat seal the edges.