Being able to stick the bow into a tree for storage is a neat little ability, but I wouldn't even raise the price of it for doing so. Even though the weapon would have this (questionably) useful ability, it doesn't seem any more useful that a sword sheding light. Maybe even less. And light doesn't cost anything extra.
Basically have it be able to do it, but it's just a cute flavor thing, IMO.
The tree shape ability itself should be granted to the wielder.