Im planning on building a tree in the next few days and the info i gathered was as followed.
to make a tree get weed roots, wash them, dry them, and paitn them.
Assemble several of them together and before painting(mybad)
cover the roots with a fiber cleaning pad(available at industrial cleaning stores)
then spray with glue and cover with the types of leaves(im using herb leaves)
you can buy leaves made for dios or you can by them at the grociery store.
then place thetree in the dio.
Theres a pretty detailed way of how to make a hedge row in a book called Terrain Modeling
by Richard Windrow
its available from osprey.
hope this helps some