The perfect trees for HO or S are tree root. Simply find a tree that has the roots exposed enough to see the finer roots and root hairs. Remove a tree shaped armature of the size you need. Clean it thoroughly and let dry for a few days.
After it has dried for a few days either dip or spray it with sanding sealer. Again, let it dry thoroughly.
Next, dip the branches in thinned white glue sparingly and immediately sprinkle or roll in ground foam of your color choice.
Now you have a tree. Drill a hole in your layout and plant.
LBR