However, soil systems may not necessarily be the cultural method of choice for use in living wall venues. Compared to plants grown in native soils, plants on a wall are much more intensively managed. Being of a substantial shallower profile than field soils, soil on a wall has to be watered more frequently than in the field. Even with careful engineering, repeated watering can break down the aggregates that give the soil its structure, and with the loss of structure, the channels that allow air to be delivered to roots are also lost. Anaerobic conditions are created and literally cause the roots to suffocate from the lack of oxygen. Anaerobic conditions also encourage a number of root pathogens which can also stress the roots. It is interesting to note that different plants have different tolerances to the anaerobic conditions associated with the flooding of the inter-particle spaces in the soil. We typically think of this as the tolerance of the plant to overwatering but it is actually the lack of oxygen in the soil, not the excess water, that damages the plants. When properly aerated, there is no such thing as too much water to the roots of almost all plants.

Another issue that arises from soil being a collection of particles is that these particles will eventually succumb to gravity and therefore cannot be applied to the vertical surface without some sort of containment system. Soil will only work if it is contained in some sort of “pot.” Containment systems that are arranged so that their openings are not horizontal must also contend with the soil slouching out of the container. The soil frequently succumbs to gravity and sloths off the wall. This is also true for the entire plant; with the torque forces that the plant applies to the soil, the entire soil ball can be leveraged out of the container. Both of these issues are frequently addressed by covering the soil with a mesh, leaving only a small amount of soil around the base of the plant uncovered.

Soil-based vertical walls of plants have the issue of containing and keeping the soil in place over time, in addition to concerns about proper irrigation of the plants. Illustration courtesy of Nedlaw Living Walls, Inc.

Containers used in living wall systems range in size from less than a quarter of a pint (100 ml) to many quarts (liters). But irrespective of their size, the containments offer a physical barrier to the expansion of the plants' root systems which in the long run will reduce the viability of the plants—i.e., they become pot bound. This is much more of an issue with small containers than large ones and will influence plant selection and longevity of the system. The containment of the soil also creates barriers to the flow of water through the system. Water cannot disperse freely horizontally because of the sides of the container.

Correcting the water flow is sometimes addressed by watering each container separately to avoid the requirement that water flow between vertical containers. This has some advantages but it is difficult to balance water delivery particularly to the entire wall. For example, the upper sections of the wall receive their water from the water supply system while the lower sections of the wall receive water from the water delivery system and drained from the container above. Frequently under these conditions, it is the bottom of the wall that appears to be “overwatered” and exhibits flood injury while the top appears to be under drought stress.

The last issue related to the use of soil-based plant walls is the longevity of the soil media. One way for a synthetic or engineered soil to withstand the intensive maintenance of the plant wall is the inclusion of organic material such as peat, fibers, or wood chips. This will help with the soil structure but being organic the material will decompose and the soil media will need to be replaced typically within a few years. This can be a very costly process.

Hydroponic-Based Living Plant Wall Systems

As mentioned previously, an alternative approach to soil-based planting is the use of hydroponic-based living plant walls. Hydroponics is increasingly used in agriculture for high-value crops where the reliability of production, and the quality and consistency of the end product are paramount. Hydroponics can be defined as a cultural method where the plants are grown in an inert rooting material. The hydroponic rooting material typically offers little water holding capacity and is simply a means to support the plant (keeping it upright). The hydroponic root substrate is also inert in terms of the plant's nutrition. All of the plant's essential nutrients must be supplied from the irrigation water.

Agricultural applications of hydroponic plants have focused on horizontal beds of plants placed in growing media with water pumped from a reservoir with the needed nutrients. Images courtesy of Nedlaw Living Walls, Inc. and Agriculture Canada (public domain)

Hydroponics has made great headway in the traditional greenhouse production systems for a number of reasons. Agricultural hydroponics are much more sophisticated production systems than traditional soil systems which means that they can be easily integrated in an automated control system, giving better control and monitoring of the plant's growth environment. While soil mixes tend to be inconsistent because of variability of the various components, the simpler (in terms of its composition) fabricated hydroponic media also streamlines the production of plants. The simple synthetic inert media used in hydroponics, such as rockwool or horticultural foam, gives the provider direct and immediate control over what is happening in the plant's root environment (the “rhizosphere”). Lastly, hydroponic designs facilitate the efficient use of resources. Although not impossible with soil culture, hydroponic irrigation systems lend themselves to be designed as a closed loop such that the water and its contained nutrients can be collected after use and circulated back through the system. This substantially reduces the amount of water and nutrients used by the system, fitting it better into sustainable design. All these benefits are directly applicable to the living wall venue.

Based on the above, it is clear that two critical components of a hydroponic system are the rooting or growing media and the irrigation system, both of which we will look at in more depth.