Is this going to be a wall hanging, or a table topper? It looks that size, but I could be way off!! If it's a wall hanging, you could tack them down just making sure that all of them have a tack in them. Or actually just let the fusing be enough and put a note with it that it is to not be laundered. If it's going to need to be laundered, as in a table topper, you could use a decorative stitch and contrasting or harmonizing thread and stitch down the centers like a vein, that would anchor everything down. If you haven't done free motion quilting before, you may want to stitch lines on this piece. Or at least make sure you practice on something similar in weight to it before putting it under the needle. A very modern approach would be to stitch randomly spaced lines across the open areas, and along the farthest out edge of the leaf mass. I hope this helps. I think it's so much easier to show someone than articulate what my mind has for thought.
Luann