Green walls are the current landscape fashion accessory; but what is a green wall and are they here to stay?
In fact, green walls are nothing new. Nature has been installing them for millennia without much credit; look at any oil palm trunk to see a perfect vertical garden complete with support structure, planting pockets and cascading irrigation. Alternatively look at the variety of plants that grow on cliffs, rock formations and derelict buildings with little or no planting medium and limited water source. Left to its own devices, vegetation will naturally colonise most surfaces, vertical or otherwise, particularly in the tropics.
In traditional gardens all around the world the natural tendency for some plants to cling, twine, trail and perch on, up and over structures has been used to decorate walls, pergolas, trellises and hanging baskets for centuries.
So why is there suddenly such a buzz in vertical greenery? And why is there increased excitement in green walls on buildings? One reason is that there has been a positive shift in the attitudes and applications of vertical greening from domestic ‘gardenesque’ applications to commercial urban environments. This shift has necessitated a review of the methodology and technical requirements needed for installing large scale applied vertical greening systems (AVGS) or ‘green walls’, particularly in urban environments and high rise applications.
It is this shift in thinking and not green walls themselves that is creating the buzz and has allowed designers to more fully explore the methodology and possible applications of applied vertical greening.
The impact of climate change on our environment is undeniable (the reasons for climate change are still, however, a hot topic of debate!). The popular media constantly tell us the world is getting hotter; and some places are. Increased urbanization has a huge effect on ambient temperature and micro-climate, as studies on the Island Heat Effect (IHE) in urban areas clearly demonstrate. The disturbing changes in our weather patterns and the perceived negative effects of green house gases have formed a vivid backdrop and driving force for the need to rethink the way we construct, develop and live in our built environment.
Words such as sustainability, energy efficiency and carbon footprint now sit in the same paragraph as design concept, aesthetics and context; and this trend will only continue to grow.
Necessity is another reason for the shift in thinking. Necessity is currently being driven not by economics but by policy and the peer pressure it has encouraged through corporate and social responsibility.
Many countries now require buildings to meet minimum green building standards and many Multinational National Corporations (MNCs) require the building they occupy to be accredited to an internationally recognized green building standard.
Increased urban greening is, therefore, considered a logical solution to improving the quality of urban environments. Opportunities for planting on ground continue to compete in a losing battle for space with the building footprint, so when green coverage is considered a necessity or is a policy requirement, then the next logical step would be to look at other surfaces on the building that can be given over to greenery.
In fact green walls form only one component of a larger movement of ‘Skyrise Greenery’, as Singapore’s Centre for Urban Greenery and Ecology (CUGE) calls it. In the context of Singapore, which is arguably at the forefront of this movement in the greater Asian region, ‘Skyrise Greenery’ is achieved through a combination of four components:
• Vertical Greenery
• Sky Terraces
• Balcony Gardens
• Green Roofs
CUGE has carried out extensive studies and their publication ‘Vertical Greenery For The Tropics’ is a ‘must read’ by anyone interested in the ever-evolving greening of urban spaces, vertical or otherwise.
Green walls can be categorized into two main types;
1.Carrier systems –
where the planting media and plants are ‘carried’ in some form of container that is fixed to the wall.
2.Support systems –
where the plant foliage is supported so that it can climb or trail in a controlled manner.
Carrier systems (sometimes referred to as living walls) tend to be more expensive as the system has to contain the planting, planting substrate and irrigation within a structurally stable frame. Carrier systems generally allow more variety in the types of plants that can be used.
Support systems are generally more economic to install and are easier to design, but the selection of suitable plants is limited and the need for at grade planting or substantial substrate containers for the root stock may be a constraint.
In addition to the obvious considerations of aspect, light levels and intended foliage effect, the following should also be carefully consideration when selecting and specifying a green wall system;
Maintenance |
All AVGS require a long term commitment to maintenance. This is particularly true of carrier systems as the plant selection often uses plants that grow well, but do not naturally regenerate and re-propagate, in a vertical setting. This means that replacement cycles are determined by the natural lifespan of the plant, which can be as short as 18 months. Easy access to the green wall needs to be provided via a maintenance gondola or catwalk so green walls are not a solution to treat inaccessible areas. The system used should allow for easy plant replacement. Hauling plants and planting substrate to elevated situations and replanting in-situ will always pose problems.
Support systems are not without their own maintenance considerations. Lack of maintenance on the rootstock or a pest infestation will affect the entire plant; often with a severe negative visual impact as entire vertical sections of plants defoliate and die. In this situation the foliage cover will be affected until the plant recovers and grows back, which can take months. Such a set-back was experienced at Terminal 3 in Changi Airport which was caused by an irrigation failure.
Irrigation |
All green walls require irrigation. Drying out is one of the biggest dangers to plant success on a green wall. Once the roots have dried out, the plants will usually die, so it is important that a proper automated irrigation system is installed. Most carrier systems are semi-hydroponic and use non soil based planting substrate so a fertigation system is vital to control nutrient release to the plants. Cascading drip irrigation methods are generally more successful than surface applied irrigation as the water distribution is applied in a controlled manner through the substrate. Irrigation run-off should be collected and recycled wherever possible.
Different plants also have different irrigation requirements and one system does not suit all. Irrigation may therefore need to be zoned for different plant groupings.
Plant Selection |
Certain plants grow well on green walls and some do not. In addition, different plants grow better or worse in different systems and orientations; so plant selection and the selection of an appropriate system (and proposed location) go hand in hand. Designers need to be aware of the increasing number of systems on the market and understand their individual constraints and benefits.
Fabrication Material |
As already mentioned, green walls involve a long term commitment and invariably incur a high capital cost. Materials used should therefore be durable and non-corrosive. Systems that are modular and proprietary in design, requiring minimum special framing or site welding, will be easier to maintain in long term. Site welding and mild steel are prone to rusting and maintaining poorly welded or ungalvanised metal in elevated situations is likely to be a long term maintenance problem.
There are many plastic based systems on the market but be aware that all plastics, even when treated with retardants, will eventually break down and weaken when exposed to environmental pollutants and Ultra-violet light and may not therefore be an appropriate long term solutions for large areas.
So are green walls here to stay? I believe yes, they are. Whilst the cost of installing a sustainable green wall is likely to remain high (simply due to the materials necessary for long term success), green walls will continue to be a growing trend. Their application and design is inevitably going to go through a period of refinement and further development, but with continuing environmental pressure coupled with government policy green walls will become an important component of modern urban design.
