Green Walls from Green Roofs

Green Walls

Green walls are essentially a living, and therefore self-regenerating, cladding system using climbing plants. Whilst climbers have been used traditionally on buildings for centuries, modern methods using high-tensile steel cables have enabled the concept to be used far more adventurously. Contemporary architecture is also more suitable in many ways for facade greening than traditional buildings. With suitable species selection, heights of up to 25m can be attained – additionally if plants can be grown in large irrigated containers at height, then even greater heights can be reached.

The most commonly used species for wall-greening are ivy (Hedera sp.), Russian-vine (Fallopia sp.) and Virginia-creeper (Parthenocissus sp.). Both will climb directly onto wall faces, especially those of brick and stone where the porous surface allows them to attach more easily but the installation of trellises and wires can aid their growth, and to some extent serve to liit the direct contact between wall and plant. All can relatively quickly form a dense evergreen foliage many metres in height, and on many older buildings this is actively managed to ensure that it does not obscure windows and other openings. Such dense foliage provides an effective nesting habitat for a variety of birds, including robin, wren and blackbird, as well as serving to baffle noise. A number of varieties have colourful foliage, that will change with the seasons, often turning red or golden during autumn.

Even small, difficult spaces have potential

Other climbers, such as clematis (Clematis sp.), honeysuckle (Lonicera sp.), wisteria (Wisteria sp.), passion-flower (Passiflora sp.), bougainvillea (Bougainvillea sp.) morning-glory (Ipomoea sp.) and jasmine (Jasminum sp.) can be highly decorative, although will be usually slower-growing and not as bulky. Many will have highly colourful flowers that can serve as nectar sources for insects, as well as providing a pleasant scent.

Although it is often thought that these climbing plants can damage wall surfaces through their tendrils and feathered adventitious rootlets (in the case of ivy), in most cases if the wall is solid and well-built there is no reason why damage should occur. Care should be taken, however, in respect of walls with cavities or with crumbling mortar in which roots could take hold and expand.

Large-scale use of green walls is still a very new concept, however there is a solid research base, which has developed in Germany over the last 20 years. Active research is a major concern for several German academic institutions.

Visual Advantages

Public perception of contemporary architecture is often poor, with it being seen as barren and sterile. However, just as street trees do much to improve public perception of urban environments, so climbing plants can do the same for buildings. Large climbers, up to 25m high, can be extremely dramatic visually, as well as softening what may be seen as hard surfaces.

Climbing plants have traditionally been seen as ‘add-ons’ to buildings. The new approach, being pioneered in Switzerland and Germany, is to see them as integral to the design concept. Clearly this makes sense in both visual terms and in designing in practical features.

In addition to being used on buildings, climbers can be used on freestanding structures such as giant ‘pergolas’ or on poles. see MFO park illustration.

Advantages for Environment and Sustainability

Private benefits

Climbers can dramatically reduce the maximum temperatures of a building by shading walls from the sun, the daily temperature fluctuation being reduced by as much as 50%. The effectiveness of this cooling effect is related primarily to the total area shaded rather than the thickness of the climber (Köhler 1993). Together with the insulation effect, temperature fluctuations at the wall surface can be reduced from between –10º/14ºF to 60ºC/140ºF to between 5ºC/41ºF and 30º/86ºF (Peck et al 1999). 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.

Evergreen climbers provide winter insulation, by not only by maintaining a pillow of air between the plant and the wall, but by reducing wind chill on the wall surface. Reducing wind chill by 75% can reduce heating demand by 25% (Peck et al 1999). Reduction of wind chill is also reduced to some extent by the interwoven stems of deciduous climbers during winter. 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, e.g. there may be a reduction in the dense twiggy growth that forms the most effective insulation.

Climbers on buildings can help protect the surface of the building from damage from very heavy rainfall and hail, and possibly can 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 ultra-violet light, which might be an important consideration for certain modern cladding materials.

Public benefits

Climbers have been shown to be highly effective at trapping dust and at concentrating certain dust-derived pollutants in their tissues, particularly in those tissues that are then discarded. In a study of climbers 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 then falls to the ground (Köhler 1993). The removal of dead leaves and branches and their disposal in sites where the concentrated heavy metals can do minimal environmental damage is thus a key factor in reducing the dangers presented by these elements.

There is strong anecdotal evidence that climbers on buildings help to reduce noise levels.

Reduction of solar heating of the sides of buildings helps to reduce the ‘heat island effect’, i.e. the effect whereby urban areas accentuate high temperatures in summer.

All green plants absorb CO₂ emissions, thus climbers in urban areas help to reduce the contribution made to human-induced global warming.

All plants also absorb and breakdown a variety of pollutants, notably volatile organic compounds, and unburnt hydrocarbons from vehicle exhaust. It is a reasonable assumption that urban climbers could play a role in reducing these in city areas.

The use of climbing plants offers a very elegant solution to the problem of how to contribute to the local and regional biodiversity targets. Because they take up little horizontal space, but make use of vertical space that would otherwise be lifeless, they can add considerably to the area, which is potential habitat without taking up room on the ground.

The green walls also provide an opportunity for wildlife. Any climbing plant will offer habitat to invertebrates, such as insects and spiders, which in turn will be food for insect-eating birds and bats. Any climber will also act as a transit route for wildlife between habitat at ground level and those established on roofs in the form of green roofs.