Components necessary to consider in maintaining a green wall include: location, structural inspections, irrigation, drainage and plant maintenance. 

Location: physical setting of the wall
Access: how can it be seen/reached
Structural: mechanical—waterproofing, penetration methods
Irrigation: drip irrigation system
Drainage: closed/recycled or open system
Plants: in the green wall have the same requirements as when planted in any horizontal plane.  The green wall, however, does provide the physical advantage of an ergonomically advantageous of working in the vertical plane. 

Green walls as visual attractants, artistic expressions, marketing campaigns and one of a kind pieces of art.   Although most will find the green wall itself a work of art, you have to look beyond the initial beauty to see that there is also environmental and economic function.
Most green walls are simply vertical planting systems for the interior and exterior of a building. The basic green wall system is created by providing a planting substrate into a modular wall system. The components of a typical vertical planting system include:

• Plants that have been selected to meet the particular design intent. Interior green walls use tropical plants—mostly vining, climbing, and fern varieties. Exterior green walls use sedums and other hardy vines and climbers.
• A modular panel system to contain the roots and the growing medium.
• An integrated drip irrigation system and controls.
• A catch basin to control water runoff.
• A structural support system.

Plants stay in place because they're rooted into the growing media, which is held in place by the modular wall system and attached to the wall using a structural support system.
Source: http://www.buildings.com/Magazine/ArticleDetails/tabid/3413/ArticleID/6458/Default.aspx