Literally Green Facades
Species and cultivars of wisteria, particularly Wisteria sinensis, are among the most popular really large climbers, and there are few sights more magnificent than a huge plant, which may be more than a century old, smothered in its smoky blue-violet flowers in early summer.
Support for Climbers
Yet, supporting frameworks for large climbers like this are rarely seen that make the best use of the plant; usually they are simply too small, and there are a lot of stray new shoots visible, flailing wildly at thin air, trying to find somewhere to latch onto. All too often these shoots return to wind themselves around the parent plant, resulting in large tangled masses of growth, unattractive and increasingly structurally unsound with age.
New support materials and their increasing availability through an international network of manufacturers and suppliers are making facade greening a much more practical option for construction professionals.
Owing to occasional incidents of damage, some designers look unfavorably on having climbers on buildings. However, with new technologies and with many contemporary buildings offering extensive surfaces suitable for coverage, and fewer sites for damage to occur, the combination of climbers and architecture deserves reevaluation.
Vines (Vitis species) in particular have often been used on the sides of buildings for summer cooling in some warm-summer climate zones or over extensive pergola type constructions designed to give shade below. Climbers have also long been used for screening on the sides of utilitarian domestic buildings such as sheds and garages.
Despite their potential for screening extensive areas of blank wall on commercial or industrial premises, climbers have been used little for this purpose. Given that so little structural work is needed to make it possible to grow climbers up to several stories high, this use does seem to have enormous potential.
Effects on Building Temperature
The cooling effect of a green wall is related primarily to the total area shaded rather than the thickness of the climber. Together with the insulation effect, temperature fluctuations at the wall surface can be reduced from between 14 and 140 degrees Fahrenheit (between 10 and 60 degrees Centigrade) to between 41 and 86 degrees F. (5 and 30 degrees C.).
Buildings are more effectively insulated against high summer temperatures by shading than by building insulation into the structure, for the simple reason that shading stops the heat from entering in the first place. Climbers are quite effective in achieving this.
It has been calculated that a 10 degree F. (5.5 degree C.) reduction in the temperature immediately outside a building can reduce the amount of energy needed for air conditioning by 50 to 70 percent. The use of climbers to reduce solar heating is most effective if they are used on the wall that faces the sun, together with the west wall, which experiences afternoon heating.
Windows may also be shaded by climbers on a seasonal basis, which has a particularly dramatic effect on reducing summer heating because their foliage will stop solar energy entering the building. Solar energy heating the side of a building will generate more powerful convection currents than it will on a horizontal surface, which climbers can minimize through their cooling effect and the creation of complex air flows.
There is thus a contribution to the reduction of the heat island effect and dust generation. In climates with cold winters, it makes sense to have deciduous climbers on walls that receive sunlight, because this enables them to absorb more solar radiation when it's desirable. The use of evergreen climbers on walls that do not receive sunlight, on the other hand, helps reduce heat loss in winter.
Evergreen climbers provide winter insulation, not only by maintaining a pillow of air between the plant and the wall but by reducing wind chill on the wall surface. One third of a house's demand for winter heating is generated by wind chill, either through drafts or the cooling of the walls, at least in climates where cold-season winds are a regular feature. Reducing wind chill by 75 percent reduces heating demand by 25 percent.
The effectiveness of winter insulation is related to the thickness of growth, which is generally related to the age of the plant. In some cases, however, growth patterns change as the plant ages. For example, older vines may have less twiggy growth that forms the most effective insulation.
Fallopia baldschuanica, for instance, becomes less effective after ten years. German research results show Hedera helix with a thickness of 8 to 16 inches (20 to 40 centimeters) is the most effective insulator.
Climbers and urban trees are highly effective at trapping dust and concentrating certain dust-derived pollutants in their tissues, particularly in those tissues that are then discarded. In a study of Parthenocissus tricuspidata, lead and cadmium concentrations were shown to be highest in dead leaves and dead wood. These heavy metals are thus taken out of the atmosphere and rain and concentrated in a form that falls to the ground.
The removal of dead leaves and branches and their disposal in sites where the concentrated heavy metals can do minimal environmental damage could thus be a key factor in reducing the dangers presented by these elements. Trapping of dust is proportional to the amount of leaf surface to wall area.
Climbers on buildings can help protect the surface of the building from damage from very heavy rainfall and hail and may play some role in intercepting and temporarily holding water during rainstorms, in the way that green roofs do. They also help to shield the surface from ultraviolet light, which might be an important consideration for certain traditional as well as modern cladding materials.
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Nigel Dunnett is a senior lecturer in the Department of Landscape at the University of Sheffield, where he has developed innovative research programs into naturalistic and ecologically informed planting for gardens and public landscapes. NoŽl Kingsbury is a writer on plants and gardens and has played a major role in popularizing a more naturalistic and sustainable planting style.
This article is excerpted from Planting Green Roofs and Living Walls, by Nigel Dunnett and NoŽl Kingsbury, copyright © 2004, available from Timber Press and at Amazon.com.