Artificial nighttime lighting (light pollution) is increasing worldwide and may have undocumented consequences. In this study, we asked if artificial nighttime lighting affects the performance in monoculture of four grass species: the Eurasian Bothriochloa bladhii (Retz.) S.T. Blake (Poaceae), and Bothriochloa ischaemum (L.) Keng (Poaceae); and the North American Panicum virgatum (L.) (Poaceae), and Sorghastrum nutans (L.) Nash (Poaceae). We conducted a field pot experiment to test for the effects of artificial nighttime lighting and plant density on height, biomass, and leaf number. Height of the tallest individual per population was affected by separate interactions between species and density, light, and time. Final total biomass per individual biomass was increased under nighttime lighting, but more so at low density. Leaf number was increased by artificial nighttime lighting irrespective of species. These results suggest that artificial nighttime lighting may have previously undocumented influences on plant height, biomass, and leaf number within certain species. These findings warrant more in-depth studies into the role that artificial nighttime lighting can have on various plant species.