Greenhouse and growth-chamber experiments were conducted to determine the influence of the ratio of red to far-red (R:FR) radiation and of neutral filtration of plant-available radiation on the vegetative and reproductive growth of selected nightshade species. Exposure of eastern black nightshade and black nightshade to far-red radiation resulted in greater partitioning of resources to stem tissue, resulting in taller plants. Number of flowers, timing of flowering, branching, and biomass production of black nightshade and eastern black nightshade were not influenced by the ratio of red to far-red radiation. Eastern black nightshade shoot and berry dry weight decreased as neutral density shading increased from 0 to 71%. Shoot dry weight decrease was associated with lower stem weight and production of fewer berries per plant. Neutral shading did not reduce leaf weight or leaf area per plant but increased the specific leaf area of eastern black nightshade. Internode elongation of nightshade species into a soybean canopy should primarily be associated with exposure to low R:FR irradiance ratios, whereas the thinner leaves of eastern black nightshade growing under shade should be associated with lower irradiance levels. Both responses are common adaptations of shade-avoiding plants.