Trying to steal Vray's thunder.
I'm gonna wait for Steam version
Karen Stanley uploaded a great tutorial, which covers the basics of modeling and texturing a foliage with Maya, Zbrush and Substance Designer.
It’s a very quick read, which covers only 19 pages. There’s a lot of illustrations and some pretty cool advice. The tutorial starts at references, touches on the creation of the base mesh and work with high poly. Karen also shows how to work with the color map and baking. Finally, she exports everything in Unreal Engine and you’ve got yourself a beautiful piece of vegetation.
Here’s a little example.