The high specific surface area of carbon nanowalls (CNWs) makes them an attractive catalyst support material or electrode material for energy storage devices (e.g. supercapacitors, Li-ion batteries). Secondary nucleation processes (formation of secondary nanowalls on the surface of pre-grown primary structures) play an important role in CNWs growth. It can significantly increase CNW film surface area, but at the same time be a limiting step for the production of films with an open pore structure. Both of these factors may be important for the applications mentioned above. In this work, we discuss possible mechanisms of secondary nucleation during CNW growth in the plasma of a direct current glow discharge. We also demonstrate a novel multi-step synthesis process with controllable secondary nucleation rates at different stages, which is an effective way to modulate CNW film morphology and produce films with desirable surface area and porosity.