How to make hair particles (grass) bend on z axis I created a grass animation with hair particles, but I cannot get the grass to bend (on the z axis) it just sways from side to side on the x axis.
  screenshots, more screenshots, link to video.

I have created an empty and assigned a texture to it, then linked the particles through the texture and I can control the animation of the grass through the empty but when I move the empty on the Z axis it just pushes the grass to the side without any bending.

Any help on this would be appreciated. cheers. If you need any more info let me know.
I'm not sure I understand the question completely, but I'll start with some possible solutions:

Hair Dynamics:

Using hair dynamics will simulate the strands with cloth physics,

Some examples with a wind force:
With hair dynamics:


Without hair dynamics:


If you really want to simulate it (Note, simulation probably takes longer and you can most likely do it satisfactorily other ways) you can use a Smoke simulation to get the airflow infromation and then use a Smoke Flow force to use it to effect the grass:



(The advantage to this is that you get all the complex little air currents caused by the cube as the air flows around it to effect the grass)

Texture Force field

I'm not sure what you mean by not bending (Is it not bending in the viewport? or only when rendered?)

Texture force with Dynamics:


And without: