Hitbox to Material Grass

I have a Landscape Material and I use it to spawn automatic Grass and Big Stones, I enabled the collision in the mesh and it works if I place it by hand but I can walk through it when its spawned automatic in the material, My question is how to give it in the material a collision.

There is not really a solution to give a material a collision. Collision are bound to meshes or for an actor. There is an option to set and disable collision for meshes in blueprint. So depending on how you currently spawn the meshes, verify that collision is enabled when you spawn the mesh.