That is an awesome theme and there is so much you can do with it. Before I make my suggestions, I would like to propose that you watch the following movies and they will give you some great ideas.
Lord of the Rings: Fellowship of the Rings...just fast forward to the two elven forests, totally different but beautiful
A Midsummer Nights Dream...the one with Christian Bale, Calista Flockheart, and Michelle Pfeiffer. Most of the scenes with Michelle Pfieffer are in this enchanted glade.
The Secret Garden...watch the newest two releases.
Decorations
I would definetly go with a mixture of white and green christmas lights, twinkle lights would be really cool. I think trellis work would be awesome, with vines of wisteria (fake or real) going up it. I would put accent your flowers and other things with gold because it's a fall wedding. Just about any flowers would be good, even if they aren't pink or green, because of your theme. There is a company that makes little led butterflies to put outdoors in your garden, but I think making centerpieces out of flowers and putting those in there would be kind of neat. Oh, and going along with the whole Secret Garden theme, a swing is a must. Not like a tire swing, but a tree swing made of rope and wood...One of the Secret Garden movies shows one like I'm talking about. Also, you can have your flower girl and/or bridesmaids wear hair pieces made of flower (real or fake). I'm thinking of a sort of flower halo, like a wood nymph or fairy, and have the flower girl go barefoot. Thats about all i can think of for now, sorry, I see it in my head but its hard to describe