Urban habitats are characterized by high levels of disturbance, impervious paving, and heat retention. These factors, acting in concert, alter soil, water, and air conditions in ways that promote the growth of stress-tolerant, early-successional vegetation on abandoned or unmaintained land. In most urban areas, a cosmopolitan array of spontaneous plants provide important ecological services that, in light of projected climate change impacts, are likely to become more significant in the future. Learning how to manage spontaneous urban vegetation to increase its ecological and social values may be a more sustainable strategy than attempting to restore historical ecosystems that flourished before the city existed.