Vertical garden or Green wall

Green Wall at Queen's University,
 Kingston, Ontario, Canada

A green wall is a wall, either free-standing or part of a building, that is partially or completely covered with vegetation and, in some cases, soil or an inorganic growing medium. The vegetation for a green façade is always attached on outside walls; with living walls this is also usually the case, although some living walls can also be green walls for interior use. For living walls there are many methods including attaching to the air return of the building to help with air filtration. They are also referred to as living walls, biowalls, or vertical gardens.

Types

There are two main categories of green walls: green façades and living walls. Green façades are made up of climbing plants either growing directly on a wall or, more recently, specially designed supporting structures. The plant shoot system grows up the side of the building while being rooted to the ground. In a living wall the modular panels are often made of stainless steel containers, geotextiles, irrigation systems, a growing medium and vegetation.

There are three types of Growth Mediums used in living walls, loose mediums, mat mediums and structural mediums.

Green wall of bitter melon shading a school in the summer.
Primary school in Itabashi, Tokyo

Loose medium walls tend to be "soil-on-a-shelf" or "soil-in-a-bag" type systems. Loose medium systems have their soil packed into a shelf or bag and then are installed onto the wall. These systems require their media to be replaced at least once a year on exteriors and approximately every two years on interiors. Loose soil systems are not well suited for areas with any seismic activity. Reparations are only achieved by re-stuffing soil into the holes on the wall, which is both difficult and messy. Loose-soil systems should not be used in areas where there will be a lot of public interaction as they are quite messy and lose their soil little by little over time as their mediums wet and dry failing out onto the floor in front if not properly designed for. Most importantly, because these systems can easily have their medium blown away by wind-driven rain or heavy winds, these should not be used in high applications over 8 feet high. Loose-soil systems are best suited for the home gardener where occasional replanting is desired from season to season or year to year.

Indoor green wall

Mat type systems tend to be either coir fibre or felt mats. Mat mediums are quite thin, even in multiple layers, and as such cannot support vibrant root systems of mature plants for more than three to five years before the roots overtake the mat and water is not able to adequately wick through the mats. The method of reparation of these systems is to replace large sections of the system at a time which compromises the root structures of the neighboring plants on the wall. These systems are best used on the interior of a building and are a good choice in areas with seismic activity. It is important to note that mat systems are particularly water inefficient and often require constant irrigation due to the thin nature of the medium and it's inability to hold water and provide a buffer for the plant roots. This inefficiency requires that these systems have a water re-circulation system put into place at additional cost. Mat mediums are better suited for small installations no more than eight feet in height where reparations are easily achieved.

Structural mediums are growth medium "blocks" that are not loose, nor mats, but incorporate the best features of both into a block that can be manufactured into various sizes, shapes and thicknesses. These mediums have the advantage that they do not break down for 10 to 15 years, can be made to have a higher or lower water holding capacity depending on the plant selection for the wall, can have their pH and EC's customized to suit the plants, and are easily handled for maintenance and replacements. They are the most robust option for a living wall for both exterior applications and for interior applications. They are the only choice in areas where high-winds, seismic activity or heights need to be addressed in the design. Structural mediums are superior to the other mediums for their longevity and high-level of performance in a variety of circumstances. Depending on the installation, they do tend to be more expensive to install, but lower cost to maintain.

There is some discussion also around "active" living walls. An active living wall actively pulls or forces air through the plants leaves, roots and growth medium of the wall and then into the buildings HVAC system to be recirculated throughout the building. A problem with these systems is that building code still requires that all the standard air filtration equipment that would have to be installed anyway, be installed despite the living walls installation. This means that active living walls do not improve air quality to the point that the installation of other air quality filtration systems can be removed to provide a cost-savings. Therefore, the added cost of design, planning and implementation of an active living wall is still in question. With further research and UL standards to support the air quality data from the living wall, building code may one day allow for our buildings to have their air filtered by plants.

The area of air quality and living walls has been and is continuing to be heavily researched. Much of this begins with Dr. Darlington and his work at the University of Guelph. (Indoor Air |Darlington, A.; Chan, M.; Malloch, D.; Dixon, M. A. Indoor Air 2000, 10, 39-46) Other research has shown the effect the plants have on the health of office workers. "The Effect of Indoor Foliage Plants on Health and Discomfort Symptoms among Office Workers". Retrieved 2010--12--23. This all has stemmed from the Ontario Centre of Excellence. "Ontario Centres of Excellence". Retrieved 2010--12--23.

Function

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."

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 alleviate sick building syndrome.

Green facades

Some popular plants (for temperate climates) include:

Wall of living plants near Atocha station Madrid

• Actinidia
• Akebia quinata/trifoliata
• Aristolochia
• Campsis
• Celastrus
• Clematis
• Cotoneaster
• Euonymus fortunei
• Hedera
• Heuchera
• Humulus lupulus
• Hydrangea petiolaris
• Lonicera
• Nephrolepis
• Parthenocissus tricuspidata/quinquefolia/inserta
• Vitis berlandieri / riparia
• Polygonum auberti
• Pyracantha
• Selaginella
• Wisteria