There are two very distinctive categories of plant walls. The first being a plant façade where the plants are rooted at the base of the wall and with the aid of a mechanical system, the plant “climbs” the vertical surface. The second group of walls is where the plants are planted into rooting material that is attached to the vertical surface or “wall” and not at the base as seen in plant façades. While green roofs frequently use prairie grasslands or savannas as a natural analogy for their design, the non-façade type green walls are frequently described as cliff type ecosystems.

Two relatively distinctive approaches are used for the culture of planted vertical surfaces (non-façade) and are commonly available. Both are based upon traditional agricultural systems adapted to the vertical plane. The first is simply a modification of conventional potted plant culture. In these systems, plants are rooted into separate pots that may be arranged anywhere from having the pots parallel to the ground and opening outward from the wall to the pots aligned parallel to the wall and opening towards the pot above it.

Living plant walls occur naturally outdoors in areas where water and nutrients combine to support them. People enjoy the benefits of these walls whether they are located indoors or outdoors.

The second alternative planting approach is to modify traditional hydroponics to the vertical condition. Hydroponics is a method of growing plants that does not use conventional soil. It is commonly thought that plants need soil to survive but this is entirely not true. Plants need water, air, light, nutrients, and support. Soil facilitates many of these requirements but is not in itself required. Hydroponics is a cultural method where the role of the “soil” or planting media has been reduced to little more than a support substrate such that this media does little more than keep the plants from falling over.

Living Plant Walls and Biofiltration

The increasing interest in living plant walls is well founded for a number of reasons; these plant walls can greatly improve the built environment through connecting the occupants to the natural world while occupying only a minimum footprint in the building. The aesthetic appeal of the plant walls is exceptional which has also been demonstrated to improve occupants' emotional well-being. Further, indoor air biofilters (a special subgroup of living plant walls) have been clearly able to demonstrate improvements in the physical qualities of indoor environmental quality (IEQ). In fact recent studies conducted by the University of Guelph in Canada have demonstrated that indoor living wall biofilters reduce common indoor air pollutants by 30 percent. This completely biological (i.e. natural) method of maintaining the quality of indoor air has become recognized as an exceptionally functional and very aesthetic system that can truly enhance indoor environments in many ways.

A particularly effective means of creating this indoor biofiltration is to use hydroponically grown plants in the system. By utilizing many of the benefits of hydroponic growing techniques, they are able to integrate engineering technologies to create an interior plantscape that effectively removes common indoor contaminants and improves the living environment.

At its heart, the hydroponic plant wall is an indoor vertical wall of green plants. However, the plant wall is most effective when it is actually an integrated part of the air handling system for the building to form a biofilter. Ambient air is actively forced through the wall of plants and as the dirty air from the space comes in contact with the growing (rooting) media, contaminants are moved into the water phase where they are broken down by beneficial microbes in the root zone. Highly specialized biological components on the hydroponic media and roots of the plants actively degrade pollutants such as formaldehyde and benzene in the air into their benign constituents of water and carbon dioxide. The clean air is then dispersed throughout the space by a fan system that may be built into the system or may be remote. In essence, the indoor air biofilter is a part of the air handling system for the building with plants integrated right into it as a living air filter.

From an internal processing standpoint, the biofilter is an adaptation of two separate processes. First is biofiltration, which is described as the passing of a contaminated air stream through a biologically active substrate where beneficial microbes use the pollutants (such as VOCs) as a food source. The second process is phytoremediation, which uses green plants to help the growth of these beneficial microbes.