Are you sleeping with her? If so I'm thinking body paint is the way to start. Tell her you've always wanted to be a forest, you have tried to do it yourself but it is awkward.. have a lot of greens and browns and yellow body paints and brushes on hand. Use 'em up a bit so it looks like you did attempt this. Wouldn't hurt to get some under your fingernails too.
"I like the idea of moss.. on my lower stomach, and vines and tendrils.. [blush here] Do you think you could help me?"
But if she's not a nature person you could tell her you want to exist as an H. R. Giger creation. This would go well with your jewellery actually. You may have to body shave for it. Have some plans sketched out. A private installation. Or otherwise.