We had to use several tricks to optimize the scene, split the scene in references, work with instances. Our biggest problem was to work efficiently in the viewport. Viewport 2.0 was our biggest problem throughout this scene production. If it was selected when opening the scene we could wait 15 minutes just to open the maya file, and at some point we were not able to open the scene at all with viewport 2.0. Since we were working on other scenes that relied on viewport 2.0 feedback (glyph projections), it has been a nightmare to work with this scene only because of viewport issues.
My vegetation was on a display layer that had a bounding box display override that I could toggle on and off when I needed to place things, and the scene was modelled in a half-cylinder manner that I instanced at the end. Because we were working with references for all the parts of the scene, we could unload and load at will, freeing ressources on the computer when working on a particular task..
Here is wip of one section of the shot. The end result was not as good as I was hoping but this kind of shot with tv deadlines can get crazy lol..