I want to try to create something I saw in a*home & garden show but have never done landscaping with boulders before.* I'd like to plant a birch tree, several bushes & perennials all about 3 feet above ground and incorporate 18 in boulders within it.* What material do you use to backfill?* Compost?* Also is there anything used to stabilize a new tree?* I've only planted them below ground (ie compacted soil) and I'm wondering if the compost would be too light to support a tree root ball.