Right now people are talking about the wall outside the CaixaForum, on which 15,000 plants from 300 native species grow, including yucca, ferns and begonias. The wall is as long as a city block. It has been there since 2008, though, designed by French botanist Patrick Blanc.
The CaixaForum wall is built using a polyurethane sheet, a plastic mesh, and a non-biodegradable felt blanket (which acts like a sponge to hold water and provide a place for roots to take hold). Water is fed by irrigation hose lines from the top of the wall.
How does greenery improve cities? Besides the shade it provides (keeping the sun from baking the pavement and building walls), it dampens city noise, releases water vapor into the air, increasing humidity, which can be a good thing in places that are not already uncomfortably humid, and adds oxygen to the air. City residents also like the look of the walls, and they provide a focus for tourism (visitors to cities don't often get to see the variety of plants native to the country).
Besides building roof and wall gardens, Madrid is funneling city funds (and offering tax breaks) into expanding parks, converting vacant spaces into gardens and parks (water is provided by re-paving certain areas with rainwater-capturing porous material), installing heat-deflecting windows, and painting roofs to reflect sunlight.
According to an architectural firm consulted by the city, if Madrid covered 10-25% of its roofs or walls in greenery, noise would be cut 10 decibels, particle pollution would go down 20%, and temperatures would go down 18 degrees F.