I took some scraps and developed a prototype leaf, experimenting with different ways to overcast the edges. It’s hard to make out the distinctions in the photos, but I’ll explain.

prototype leaf, closeup
by Tien Chiu
I took some scraps and developed a prototype leaf, experimenting with different ways to overcast the edges. It’s hard to make out the distinctions in the photos, but I’ll explain.
prototype leaf, closeup
And since the look against a green background is irrelevant, here is what it looks like in situ:
And a view from further back:
This is getting closer to what I want. Close up, the leaf has delicate veining which rewards the close-up viewer, but further away, the leaf becomes a single block of color, strong enough to stand on its own against the rest of the garment. I am pleased with this construction and will likely use it for the final leaves (which I will start sewing up tomorrow).
What I have not yet decided is how to handle the edges. The machine zigzagged edge definitely produces a neater edge, but it is also harder-looking, and – to be quite honest – I am prejudiced against machine zigzagged edges. They remind me of Girl Scout badges or sew-on patches commemorating a visit to some touristy location. I just don’t like the look, and I think it’s too “hard” for this application. I’d rather not use it unless I have to.
At the same time, however, I think my hand overcasting needs help, being uneven and somewhat messy-looking. It does provide a softer, more handmade look, which I like, but it also looks amateurish. If I could get neat, even stitches, I would totally go for it; as it is, I’m ambivalent. I think I will probably use it, in the end, if only to get the specter of Girl Scout badges out of my head. But I will try to practice some more before doing the “real” leaves.
The only thing I haven’t tried yet is adding fine wire to the edges to make the leaves more three-dimensional. I will play with that in the morning.
In a weird way I feel like the top fabric of the leaf should be folded over the edge of the bottom fabric and appliqued in place on the back, so that there’s no edge stitching showing at all on the front.
I think you should consider a blanket stitch for your appliquéd piece. Google blanket stitch edge for images.