Did you check out the "How To" thread about making tree's? I am experimenting with different garden vegetation that when dried out looks like a tree......my wife called it something but ,well, I forgot(I already know what Sedum is(Nov. 2009 MRR). Can I ask what you are making or what kind of trees are you making?
MacDaddy has made some terrific trees, aionta. Finding the sedum is the problem! *L* I've tried to grow it and the plant is still there, but must be a sunshine lover. My backyard is so shady, I have tomato plants over 5 feet tall that are just now finally starting to flower!
As an option along with the discussions above, I made some trees using clumps of pre-packaged lichen (of various shades of green) along with some carefully selected and pruned real winterberry branches. I used hot glue to build up clusters of branch groupings. I don't think they turned out too bad.