Human activity, through changing land use and other activities, is the most fundamental source of environmental change on the Earth. Urbanization and the resultant urban heat islands provide a means for evaluating the effect of climate warming on vegetation phenology. Using data from the Moderate Resolution Imaging Spectroradiometer, we analyzed urban-rural differences in vegetation phenological transition dates and land surface temperatures for urban areas larger than 10 km2 in eastern North America. The results show that the effect of urban climates on vegetation phenology decays exponentially with distance from urban areas with substantial influence up to 10 km beyond the edge of urban land cover, and that the ecological footprint of urban climates is about 2.4 times that of urban land use in eastern North America. The net effect is an increase in the growing season by about 15 days in urban areas relative to adjacent unaffected rural areas.