So I siliconed in the trees and a couple pots, then did the bulk of the foaming, I'm electing not to foam the entire back since we have other ways of adding character.
here you can see detail of the bottom half, I added a half pot near the base of the left tree under the Branch dish as well as a pot in the joint of the branch to the tree.
Here is detail of the top, the two pots atop each tree are evident as well as a third I added against my girlfriend's design wishes. You can also see the shallow dish I am molding out of foam for the water to flow into, pool and free fall out of. the glass pebbles illustrate the intended flow path of the water along the wall, we are are thinking of spreading the pebbles around the rest of the background with moss growing between them. brings me to my question for Bill.
Would I be able to securely silicone these glass jewels over the titebond method you describe? or should I attach them straight to the back wall then paint between them? and is the Titebond grow-able for mosses?