Sintering of a ZnO-MgO powder mixture under argon flow leads to the growth of elongated Mg doped ZnO structures such as stacks of nanoplates and rods with secondary nanowires in a comb-like arrangement. The nanoplates grow perpendicular to a central axis and have a six-fold symmetry. The hierarchical growth is related to the presence of Mg. Cathodoluminescence (CL) in the scanning electron microscope shows a blue-shift of the ZnO exciton peak as well as changes in the deep level bands due to the influence of Mg.