House exterior WIP

Hi!
This is my first post here. I’m making an exterior scene, and I am close to the finnish as well. Unfortunately I got some problems with hair rendering. For some reason my grass is invisible. I am using the newest version of blender (2.67) and the cycles render engine. I also watched a bunch of tutorials on hair rendering with cycles, but none of them helped.
Please leave a comment below if you have some tips! Thank you!