Cities are known to be several degrees hotter than the suburbs, especially in the summer. Asphalt, concrete, tall buildings, cars (subways, trains, other transport), and dark rooftops all absorb heat. By contrast, green urban spaces (where there are parks, trees, and various plants) cool down cities via evapotranspiration. Outdoor vertical garden planters therefore cool down heat that is trapped, while also making efficient use of valuable urban land.