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In natural grasslands, C 4 plant dominance increases with growing season temperatures and reflects distinct differences in plant growth rates and water use efficiencies of C 3 vs. C 4 photosynthetic pathways. However, in lawns, management decisions influence interactions between planted turfgrass and weed species, leading to some uncertainty about the degree of human vs. climatic controls on lawn species distributions. We measured herbaceous plant carbon isotope ratios (δ 13 C, index of C 3 /C 4 relative abundance) and C 4 cover in residential lawns across seven U.S. cities to determine how climate, lawn plant management, or interactions between climate and plant management influenced C 4 lawn cover. We also calculated theoretical C 4 carbon gain predicted by a plant physiological model as an index of expected C 4 cover due to growing season climatic conditions in each city. Contrary to theoretical predictions, plant δ 13 C and C 4 cover in urban lawns were more strongly related to mean annual temperature than to growing season temperature. Wintertime temperatures influenced the distribution of C 4 lawn turf plants, contrary to natural ecosystems where growing season temperatures primarily drive C 4 distributions. C 4 cover in lawns was greatest in the three warmest cities, due to an interaction between climate and homeowner plant management (e.g., planting C 4 turf species) in these cities. The proportion of C 4 lawn species was similar to the proportion of C 4 species in the regional grass flora. However, the majority of C 4 species were nonnative turf grasses, and not of regional origin. While temperature was a strong control on lawn species composition across the United States, cities differed as to whether these patterns were driven by cultivated lawn grasses vs. weedy species. In some cities, biotic interactions with weedy plants appeared to dominate, while in other cities, C 4 plants were predominantly imported and cultivated. Elevated CO 2 and temperature in cities can influence C 3 /C 4 competitive outcomes; however, this study provides evidence that climate and plant management dynamics influence biogeography and ecology of C 3 /C 4 plants in lawns. Their differing water and nutrient use efficiency may have substantial impacts on carbon, water, energy, and nutrient budgets across cities., (© 2019 by the Ecological Society of America.)