Patric Blanc, a french eco-artist cretes these amazing green surfaces on urban indoor and outdoor surfaces. His vertical gardens convert any concrete gray environment to green and florishing, creating amazing contrasts and making architecture live and unique. How does this exactly works? Here is what he says on his website -

Do plants really need soil? No, they don't.....The soil is merely nothing more than a mechanic support. Only water and the many minerals dissolved in it are essential to plants, together with light and carbon dioxide to conduct photosynthesis. Plants in the wild are growing on vertical surfaces
Wherever water is available all year long, as in tropical forests or in temperate mountain forests, plants can grow on rocks, tree trunks and soil-less slopes.

The Vertical gardens watering is provided from the top with the tap water being supplemented with nutrients. The process of watering and fertilisation is automated. The whole weight of the ‘Vertical Garden’, including plants and metal frame, is lower than 30 kg per square meter. Thus the Vertical Garden can be implemented on any wall without any size or limitation of height.

read more at http://www.verticalgardenpatrickblanc.com/