An improved anchor device for retaining an erosion mat over a plant growing medium layer in a green roof assembly. The anchor device includes a base member having an elongate rod extending from a planar portion. The base member can be placed in the green roof assembly with the rod projecting above the plant growing layer and the erosion mat. A retainer is secured to the elongate rod and holds down the erosion mat. The retainer includes plant growth openings whereby plants rooted in the plant growing layer may grow therethrough. A central opening in the retainer is defines by flexible engagement members. The rod is received through the central opening and the engagement members engage annular indentations which are formed along the rod between annular protrusions. The engagement members extend at an acute angle relative to the rod.
