Just watched the youtube video. I know its a bit late now, but the problem you guys had with the plant not blocking your nav mesh is due to a bug in the editor. When manually adding collision to a mesh (like you did with the plant), you then need to close and restart the editor before the nav mesh will recognise the collision. Long story short, I reported this a couple of days ago and Flint was able to reproduce it, and has logged an internal bug report about it. Very annoying bug as you guys found out.