Nonpolar m-plane AlGaN offers the advantage of polarization-free multiple quantum wells (MQWs) for ultraviolet (UV) emission and can be achieved on the sidewalls of selective area grown GaN nanowires. We reveal that the growth of AlGaN on GaN nanowires by metal organic chemical vapor deposition (MOCVD) is driven by vapor-phase diffusion, and consequently puts a limit on the pitch of nanowire array due to shadowing effect. An insight into the difficulty of achieving metal-polar AlGaN nanowire by selective area growth (SAG) in MOCVD is also provided and can be attributed to the strong tendency to form pyramidal structure due to a very small growth rate of { $10\overline{1}1$} semipolar planes compared to (0001) c-plane. The nonpolar m-plane sidewalls of GaN nanowires obtained via SAG provides an excellent platform for growth of nonpolar AlGaN MQWs. UV emission from m-plane Al_xGa_1−xN/Al_yGa_1−yN MQWs grown on sidewalls of dislocation-free GaN nanowire is demonstrated in the wavelength range of 318–343 nm.