From what I can tell, the majority of the trees and all of the ground cover from about 15' above the ground up to the top of the mountain is all fake. I watched them installing the ground cover in big squares of snap together plastic panels. I was super impressed at how realistic it looked. In some places on the pathway around the attraction, you can see the edges of these plastic panels because they weren't all covered completely.
Here's a good article about it from the Orlando Sentinel.
http://www.orlandosentinel.com/trav...disney-roller-coaster-fake-20140528-post.html
As for the artificial grass in the hub, I'm really hoping they only use it on the FastPass+ viewing areas. As someone else mentioned, there's no need to have it anywhere else. I also doubt they'd do that.