In situ scanning tunneling microscopy observations of graphene formation on Rh(111) show that the moiré pattern between the lattices of the overlayer and substrate has a decisive influence on the growth. The process is modulated in the large unit cells of the moiré pattern. We distinguish two steps: the addition of a unit cell that introduces one or more new kinks and the addition of further unit cells that merely advance the position of an existing kink. Kink creation is the rate-limiting step, with kink creation at small-angle, concave corners in the graphene overlayer exhibiting the lower barrier.