The idea in itself is not new. In cold, windy regions, it has long been common practice to cultivate hedges and rows of trees in front of walls as a means of protection, while in the Mediterranean, planted facades have formed part of a natural system of climate control for buildings since antiquity. Training vines and fruit up facades is also an ancient practice, since the heat stored in the wall to the rear is conducive to plant growth.
Structures covered with vegetation are a symbol of building in harmony with nature and of architectural design subordinating itself to the natural environment. The planted facades created by the garden-city movement at the end of the 19th century may be seen as one of the first ecological reactions to industrialization.What is new about the modern use of planted walls is the context. Today, not only private family houses are covered with vegetation, but the facades of museums, insurance buildings, luxury hotels, restaurants and boutiques. Greenery is not only fashionable; it stimulates sales among environmentally conscious customers. The international spread of this practice is new, though. Planted facades are particularly popular in large American, Asian and Australian cities, where the concentration of heat poses a greater problem than in Europe.
The direct planting of a facade may lead to building damage, however. Roots can break through waterproof seals and penetrate the smallest crevices of brickwork. Tendrils can force their way behind cover strips and window abutments, while leaves and twigs may block rainwater pipes and gutters. Plantings can also impose an additional load on an outer wall and have a negative effect on the structural balance. But if one follows the guidelines drawn up for the various forms of wall planting, green facades can be implemented without serious problems.
Plants that grow up from the ground have a limited height. To create a planted facade beyond this range, containers can be distributed over the face of a wall, although many years may pass before a homogeneous coverage is achieved (ills. 1, 2). To restrict damage and obtain a continuous skin of vegetation with a range of different plants, rear-ventilated system facades are available with names like Vertical Garden, Living Wall, Plantwall and Wonderwall – all of which are examples of planting beds laid out vertically. In other words, the flowers do not grow upwards, but horizontally out of the face of the wall. In this respect, one should distinguish between two different principles of cultivation (ill. 3):
• modular container systems in which the plants draw nourishment from the earth;
• hydroponic systems in which the nutrient is dissolved in water and fed directly to the roots, thereby obviating the need for a substrate of soil altogether.
Traditional methods of covering the entire surface of a wall with plants have a number of disadvantages. Although earth soaked with water as a substrate layer functions well on the ground or in plant containers, its great weight and the constructional depth that is necessary make it less suitable for suspension from walls. Patrick Blanc observed that plants do not need soil for their growth if the water that reaches their roots is adequately enriched with nutrients. From his studies in tropical rainforests, he developed the so-called “vertical garden” based on hydroponic cultivation; i.e. without soil. Where pot plants are involved, the principle is known as “hydroculture”. With vertical hydroponic forms of construction, the demands made of the substrate are much more complex. Even with computer programming, it may be difficult to ensure the correct proportions for watering the plants if the weather conditions are unpredictable. Vertical gardens can be created on both indoor and outdoor walls.
Substrate can be defined as the layer that provides the roots of plants with mechanical support, that ensures the supply of nutrients and that stores and drains off moisture. All vertical plantings have to be artificially irrigated. The liquid, containing additional nutrient solutions, is fed in horizontally at regular intervals and sprinkles the substrate by means of a droplet system. Important aspects of this are an even distribution of moisture over the entire area of the facade, exploiting the force of gravity; scope for moisture retention, particularly on hot days; resistance to UV radiation; rot resistance and adequate strength for fixing the plants; and not least, an attractive appearance during the growing phase – as long as the substrate is still visible between the plants.
Various felts and foam-like substances in a series of layers have proved suitable as substrate material. Trials with rock wool have been largely abortive, since the material becomes unevenly compacted, and some areas then absorb too much moisture.
Patrick Blanc regards himself as an artist and botanist, not a technical planner. His design sketches resemble the organic patterns of carpets and comply with the “séquence végétal” in the distribution of the colours and textures of the chosen species. He leaves the execution of the work to specialist planners and firms. The standard form of construction of a “mur végétal” (ill. 12) consists of:
• the outer wall, with a waterproof seal;
• a non-corrosive supporting structure (e.g. stainless-steel) to allow for ventilation;
• a 10 mm waterproof expanded-PVC base board;
• two 3 mm polyamide-felt substrate layers;
• the plants themselves in felt pockets.
The roots spread radially within the felt, and after a number of years they can support growth up to six metres long.
The plants are watered and fed with nutrient (0.1 gr./l) from horizontal polypropylene tubes along the outer layer of felt at the top edge of the facade. In winter, the system is operated 2–3 times a minute; in summer, 5–6 times. If the temperature drops below 1 °C, the supply of liquid is stopped electronically.
The worldwide success of the “mur végétal” has led many architects, landscape architects and firms to develop similar systems. One of the most convincing projects using a comparable form of construction is the Sport Plaza Mercator in Amsterdam by Venhoeven CS-Architects (see Detail 7–8/2007), even if the steel supporting structure seems too heavy for the thin layer of plants (ills. 14). That a planted facade with felt substrate can be a convincing low-tech form of construction was demonstrated in 2007 by the temporary Black Box Pavilion at the University of Technology, Delft (ills. 5–7, 10), by Philip Mannaerts and Martijn de Geus, who were students at the time. The timber construction is light in weight, and the irrigation installation, connected to a rainwater tank, is a closed-circuit system (ill. 5).
Heinz W. Twaroch, an engineer from Graz, has developed a substrate containing fast-growing sedum shoots. Applied to a wall in the manner of sprayed rendering, it has been used for autobahn embankments, the Glacis inn in Vienna and a Swarowski development.
With modular “living walls” made up of cassette elements, the planting texture usually has a chequerboard appearance. Used repetitively, the individual units can create an extensive planted surface, as in the “Bio-Lung” at the 2005 exposition in Aichi, Japan, where a 12-metre-high and 150-metre-long grid-like planted facade was created and irrigated with a vapour-moisturizing installation – the largest air-conditioning system consisting of plants. The modular form of construction facilitates greater flexibility and mobility. Living walls thrive only as long as they are properly cultivated, though, and slow-growing plants reach their proper density only after some months.
Green, planted walls are used in internal situations for the positive effect they have on people psychologically and physiologically. As well as improving the indoor air quality to a measurable degree, they can help to reduce stress. If a great use of resources is necessary for their production, however, or if long periods of artificial lighting are necessary for the upkeep of the plants, the arguments in favour of green walls as an energy-saving measure no longer apply. The demands of building clients and the potential of ecological engineering increasingly point in the direction of using vertical planted surfaces for air conditioning and indoor climate control.
