My wife had some sedum in her garden, so I harvested it at the end of the season a few years back. I didn't do anything special. I just let it dry out, and then sprayed it with "moss green" paint from a rattle can. Then I hand-painted the trunks and branches with gray craft paint.
The sedum trees are the tall, spindly ones with no lower branches. This is the way it grows. I left the original foliage on the branches, and I didn't add anything else. Not being much of a botanist, I have no idea if this is a completely unnatural tree, but it looks reasonable to me. I like the high "canopy" look to these trees, which I supplement with lower ones to get a more natural forest look.
In this picture, the tree behind the engine to the right is another sedum.
I've found these to be kind of fragile, particularly the tops. But, I don't handle them and these have all lasted for years.
Other modelers have augmented the the tops with some leaf flock. That gives a fuller tree, and lets you play with the color a bit, too, but these do the job for me.