This project was created for a contest (that I didn't win :() and my problem that time was the tight deadline and the relatively heavy scene with Ivys. The only possibility I found is to render small parts and assemble them using GIMP. I used also render layers but the result was simply bad. But now, the new possibility with Blender to render using both GPU and CPU let me render more scenes to finally end up with this one.
Here's the progress of the different steps. Thank you