The effects of various annealing temperatures (350-550 C) of Au nanoparticles (NPs) on the surface-plasmon enhanced p-Si/n-ZnO nanorods (NRs) heterojunction photodetectors (HPDs) have been investigated. The photoresponse of the surface-plasmon-mediated HPDs was found to be determined by the extinction band of the Au NPs, the defects of ZnO NRs, and the Schottky-barrier height (SBH) between the Au and ZnO interface. The higher annealing temperature (550 C) causes more defects in ZnO NRs and lowers the ultraviolet (UV) response of the fabricated p-Si/n-ZnO NRs HPDs. The higher annealing temperature also renders a rougher surface in the Au NPs, thereby leading to destructive interference and hence the narrowest extinction band. In contrast, the modest temperature (450 C) results in fewer defects in ZnO NRs, the widest extinction band in Au NPs, and the lowest SBH at the Au/ZnO interface. Such a result enhances the UV-to-visible rejection ratio from 439.6 to 6447 as compared to the HPDs without Au NPs. A band diagram considering the above investigations is illustrated to elucidate the surface plasmon resonance effects on enhancing the UV response. V