hm.. yeah, the DOF mask didn't match the render 100%. For speed reasons, I have removed all the little stones for rendering the DOF mask. Apparently (I should have known that), this has altered the ground level slightly, so the plant were rendered a few millimeters lower.
I fixed it in post as good as I could (see image in first post again), but I will render a more accurate DOF mask later on.
Cheers,
Frank