Rocks as Foliage - Mannequin

I want to use rocks as foliage - but when using the 3rd person character mannequin, the character runs through the rocks like it’s soft gras.
How can you make sure, the mannequin runs on top of the surface of the rocks?

my engine physics is set to “Use Complex Collision as Simple” - which usually works with any other geometry.
However, when I use a 3d mesh as a foliage… the character runs through the rocks like grass.

what am I missing here?
thx a lot!

Define Foliage.
The grass node in the material does NOT support collision. Foliage does.