Fifty shades of green
Patrick Blanc, the pioneer of the “Mur Végétal” (plant wall) introduced in 1988 a method to integrate self sufficient vegetation into the construction of buildings. You can find his walls all over the planet. More and more architects are following his footsteps and integrate vertical green systems into their plannings.
From small dimensions like façades of museums, shopping malls or hotels to giant urban and social projects; vertical gardens are transforming urban landscapes.
Quite recently Movimento 90, a brazilian architecture collective, is turning the Minhocão express way of São paulo into a Green Corridor by covering its blind façades with vertical gardens. To date, seven blind facades of 4,000 square meters are already covered. All in all 20 buildings are planned.
The question that needs to be ask is, how sustainable and affordable are vertical gardening systems in reality?- Is it just a hype or can vertical gardens actually solve urban challenges?
Urban Regions – such as São Paulo, with about 12 million inhabitants – are facing environmental problems. The concentration of the population, raise of energy consumption and industry use are leading to a massive local pollution and environmental degradation as wastewater, solid waste, loss of green and natural spaces, air pollution, the raise of noise level…
On top, urban regions are facing a problem called heat island effect. It refers to the phenomenon of the heat difference between metropolitan areas and the countryside. In consequence of the huge amount of heat sources such as vehicles and industrial production buildings, thermal radiation gets reflected by the surfaces and cumulate through the cities corridors.
According to the EPA (United States Environmental Protection Agency) the heat differences between a city with one million people and its surroundings can increase up to 3°C, and in the evening even 12°C. As a consequence inhabitants are using air conditioning causing air pollution and increasing greenhouse gas emission.
WHAT ARE GREEN WALLS?
Green walls, the integration of plants into the vertical axis of architecture features a range of different modification such as green façades, living walls, vertical gardens, bio-shaders or bio façades. Depending on the scale of integration into the actual construction, it differentiates between façade supported green walls and living walls.
Essentially the main difference is the localisation of the planting medium. Green façades consist of climbing plants based in the ground. Additionally they can be supported by an external construction out of steel, plastic or wood. Living walls on the other hand, are artificial systems which include, plants, a irrigation system and the plant medium in their construction. Therefore the roots of the plants are part of the structure.
HOW DOES IT WORK?
Green walls, such as the corredor verde do Minhocão are based on Patrick Blancs methode including a growing material, irrigation system and, of course, plants.
The plants are inserted between two layers of nonwoven rot-resistant felt. This felt is regularly irrigated with a nutrient solution (water + nutrient salts) which falls by gravity and capillarity in a recovery tank. This solution is pumped again via pump and pipes to the upper part of the layers of felt, flows along and falls back into the tank. It is a watering in a closed circuit. All the watering system (hose, drippers, fittings) can be hidden under the first layer of felt. The felt plays the role of substrate. The roots divide and anchor the water and nutrient salts into the layers of felt. The layers of felts are stapled (stainless steel staples) directly on a sheet of expanded PVC (rigid, waterproof) or on a wooden board sealed by a layer of EPDM. The panels are fixed to the wall via wooden cleats or metal profiles.
Despite the aesthetic tridimensional appearance and their ability to cover unpleasant façades, green walls have a much higher impact on their surroundings.
REDUCING THE URBAN HEAT ISLAND EFFECT
Plants as they are absorbing heat, sheltering buildings from direct sun and increase humidity can help to reduce the Urban Heat Island Effect. They promote natural cooling processes such as photosynthesis and evapotranspiration. In that way they lower both indoor and outdoor temperature. A study by Alexandri & Jones (2006) showed that green walls could reduce the air temperature up to 10 °C for hot and arid climate. Furthermore green walls are able to break the vertical air flow by slowing the air down which cools it off.
REDUCING AIR POLLUTION
The Environment center of lancaster (2012) examined the impact of green walls in urban high traffic streets called street canyons. As a result, they found out that vertical green systems are able to reduce the street-level concentrations of nitrogen dioxide (NO2) and coarse particulate matter (PM10), the most harmful urban air pollutants by as much as 40% (NO2 ) and 60% (PM10) respectively in those canyons. Furthermore vertical green systems remove nearly 10 times as much NO2 and nearly 12 times as much PM10 from street-canyon air as horizontally grown rooftop vegetation.
REDUCING NOISE LEVEL
Green Walls work as a passive acoustic insulation system for buildings. This Research from 2014 showed the potential of the green wall as a sound insulation tool for buildings. The green wall showed a similar or better acoustic absorption than other common building materials especially in the absorption of low frequencies.
In spite of the economic and ecological efficiency of green walls, vertical systems have a proven impact on the health and wellbeing of inhabitants. For example in office buildings it can increase productivity including work engagement, satisfaction and focusing of the people by 15%.
Green walls aren’t necessary expansive. They are easy to implement and it is even possible to self-build them. More problematic is the maintenance. Green walls need to stay moisturized constantly, because the felt has a low retention. That means the watering systems needs to irrigate three times a day, for four minutes. The felt dries quickly which leads to a quick mortality of plants. Above this, the quality of the nutrient solution needs to be controlled on a frequently base. If the pH value goes beyond 5.5 – 6.5 plants don’t absorb enough salts and die.
Comprehensively green walls are in general a good thing. They have a proven environmental and social impact on the urban milieu, the building itself and their inhabitants. Nevertheless we need to think of ways how to include green walls into the urban landscape which, at the same time, secure a longtime maintenance without raising costs and finally without endangering the existing indigenous vegetation.
In case of the vertical gardens of São paulo, the costs of about R$ 850 (ca. 260 Euro) per square meter, are financed by a construction company, which will also be responsible for the maintenance for six month. The question that comes up is, why does a private company invest in a mammut project like this? There is a simple answer: environmental compensation-
In 2009, a construction company deforested 10.000m2 of indigenous trees in the center of São Paulo, in total 837 trees were put down. Before vertical gardens were trendy, they agreed with the district municipality to make an environmental compensation of building four parks in the city center.When the law changed, in 2015, the company renegotiated the agreement and settled to build eight vertical gardens in private buildings. After building a couple of gardens in private building, this year with a new mayor in town, the deal changed again and a vertical garden will transform one of São Paulo’s major avenue into a green corridor.
Even though the use of vertical gardens in cities is praised by many experts, it is still considered inefficient when used as compensation for indigenous and exotic trees. “A green wall is like a patient in Intensive Care. It costs a lot, needs constant maintenance and doesn’t absorb as many carbon as trees. We estimate that the environmental service of 2 trees corresponds to 1.500 m2 of green walls.” (free translation) says biologist Marcos Buckeridge, researcher at Institute of Biosciences and Department of Botanics of the University of São Paulo(USP). Making the math, it would take 642.000m2 of green walls to replace those 837 trees.
The environmentalist Nik Sabey, from the collective Novas Árvores Por Aí, affirms that green walls are often used as a greenwashing tool. “A tree can never be replaced by a vertical garden. A tree lasts hundreds of years and lives without maintenance. A vertical garden in the other hand, as beautiful as they might be, if not irrigated, they die. Cutting a tree today is losing environmental patrimony and needs to be avoided to the maximum. Even so, if necessary, it needs to be compensated in a serious and lasting way.”(free translation)
Environmentalist Claudia Visoni stresses out that greenwashing is not efficient because vertical gardens do not give back to the environment the same as the trees that were put down. “There are benefits of trees that are not compensated by green walls such as wildlife shelter and others(…) This exchange can not become a trend, because we are running the risk of the city becoming even more deserted. When we exchange trees on the ground and compensate for walls, we will have a hotter, more polluted, drier city”, said the specialist. (free translation).
In the end, more studies have to be made to see which long term benefits green walls can actually offer. Surely, a synthetic green wall can’t replace indigenous vegetation but why shouldn’t there be a future where one can complement the other?
Coming Up: How to build your own vertical garden with low cost.