Expansion of shrubs into grasslands is often accompanied by a reduction in understory light and an associated reduction of shade-intolerant species. However, effects of specific canopy architectural characteristics on the light environment under shrub thickets are unknown. Our objective was to determine what characteristics of canopy architecture most influence understory light in monospecific shrub thickets. We quantified understory light and canopy architecture for five shrub species in the eastern United States that have a history of expansion, and we used multiple regression to determine which canopy characteristics best predicted light attenuation and relative contribution of sunflecks. Measurements included leaf angle, leaf azimuth, branch bifurcation ratio, leaf area index (LAI), canopy depth (the vertical distance from the bottommost leaf to the top of the canopy), and leaf area density (LAD) as well as understory photosynthetic photon flux density (PPFD). The best predictor of light attenuation and the occurrence of sunflecks for all species was canopy depth. Multiple leaf and plant-level traits were correlated with canopy depth but not with LAI or LAD. The invasive shrub Elaeagnus umbellata had the lowest understory light levels of the species examined although LAI values for Morella cerifera and Rhododendron maximum were higher. Branch bifurcation ratios for E. umbellata were significantly higher than for other species and this likely contributed to the differences in light attenuation and suppression of sunflecks. The potential of shrubs to intercept light is primarily dependent on vertical distribution of leaves in the canopy which is itself correlated with fine-scale, species-specific variations in leaf display.