Vertical Gardening

Green walls are found most often in urban environments where the plants reduce overall temperatures of the building. "The primary cause of heat build-up in cities is insolation, the absorption of solar radiation by roads and buildings in the city and the storage of this heat in the building material and its subsequent re-radiation. Plant surfaces however, as a result of transpiration, do not rise more than 4–5 °C above the ambient and are sometimes cooler."

Pic Courtesy 

Living walls may also be a means for water reuse. The plants may purify slightly polluted water (such as greywater) by absorbing the dissolved nutrients. Bacteria mineralize the organic components to make them available to the plants.

Living walls are particularly suitable for cities, as they allow good use of available vertical surface areas. They are also suitable in arid areas, as the circulating water on a vertical wall is less likely to evaporate than in horizontal gardens.

The living wall could also function for urban agriculture, urban gardening, or for its beauty as art. It is sometimes built indoors to help alleviatesick building syndrome.

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Hydrophonics

So... exactly what is it? It is growing most any kind of plant without soil. Soil is mixed with plant nutrients or mineral salts put there by constant erosion and climactic action.
Fortunately mineral salt nutrients mix well with water so when plants take in water they are also taking in these nutrients.

Take away the soil leaving the mineral salts and you have what is termed 'a hydroponics growing system'.
This is probably as good a time as any to provide a bit of clarification. Throughout this site and this page I will be referring to 'nutrient solution'. This is merely water mixed with a small amount of hydroponics fertilizer that is composed of the required mineral salts.
For more information click Here

Urban gardening

“In an accessible rooftop garden, space becomes available for localized small-scale urban agriculture, a source of local food production. An urban garden can supplement the diets of the community it feeds with fresh produce and provide a tangible tie to food production.” At Trent University, there is currently a working rooftop garden which provides food to the student café and local citizens.

Available gardening areas in cities are often seriously lacking, which is likely the key impetus for many roof gardens. The garden may be on the roof of an autonomous building which takes care of its own water and waste. Hydroponics and other alternative methods can expand the possibilities of roof top gardening by reducing, for example, the need for soil or its tremendous weight. Plantings in containers are used extensively in roof top gardens. Planting in containers prevents added stress to the roof's waterproofing. One high-profile example of a building with a roof garden is Chicago City Hall.

For those who live in small apartments with little space, square foot gardening, or (when even less space is available) living walls (vertical gardening) can be a solution. These use much less space than traditional gardening (square foot gardening uses 20% of the space of conventional rows; ten times more produce can be generated from vertical gardens). These also encourage environmentally responsible practices, eliminating tilling, reducing or eliminating pesticides, and weeding, and encouraging the recycling of wastes through composting.