“Buildings should function like trees and cities like forests.” This phrase by the American architect William McDonough defines well what is (or should be) to come. Vertical gardens are one of the solutions recognized by international organizations to mitigate climate change in cities. It is one of the measures included in the 2030 Agenda to achieve more sustainable cities.
They beautify spaces, but they are much more. These hanging botanical gardens, of which there are more and more examples around the world, have the ability to counteract the devastating effects of rising temperatures in large world cities, responsible for 70% of greenhouse gas emissions. Placing gardens on the walls is one of the ways humanity has to reduce deaths from air pollution. According to the World Health Organization, each year more than four million people on the planet die from this problem.
The advantages of exterior green walls seem unquestionable. “In addition to being one of the most beautiful tapestries that can be placed on a facade, they purify the air, filter pollution and generate energy savings of up to 30%, since the plants maintain a more constant temperature than the facade and produce a air cushion between the two ”, says Rafael Moreno, biologist member of the General Council of Official Associations of Biologists. Different studies calculate that a square meter of plant façade extracts 2.3 kilos of CO₂ per year from the air and produces 1.7 kilos of oxygen.
The Spanish biologist Ignacio Solano gives an example of the properties of a garden of about 60 square meters on a wall: “It is capable of producing oxygen for about 60 people a year, trapping about 15 kilos of heavy metals and neutralizing about 30 kilos of dust per year ”. It is also a powerful insulator, reducing noise pollution by eight decibels and a building temperature by another eight degrees. It improves the heat island effect and even “in the areas surrounding the building, one or two meters away, the temperature is reduced by five degrees”, adds Solano.
There are numerous examples of hanging forests and they all compete to be the most spectacular in the world: Santalaia building (Bogotá), Gaia B3 hotel (Bogotá), Quai Branly Museum (Paris), Parkroyal on Pickering hotel (Singapore), Bosco Verticale (Milan )… They bear prestigious signatures such as that of Stefano Boeri, Italian architect and urban planner, the French botanist Patrick Blanc or the Spanish Ignacio Solano.
The latter is in charge of the Alicante studio Paisajismo Urbano, which has worked on the largest vertical garden in the world, that of the Santalaia residential building in Bogotá. It has more than 3,100 square meters of vegetation, 115,000 plants of ten species and five different families.
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Solano has built more than 800,000 square meters of indoor and outdoor vertical gardens around the world since he founded the company in 2018. He has different projects on his hands, including what will be the largest indoor garden in the world, he says. “It will be in Valencia and will add 700 square meters.”
Its vertical ecosystems use criteria of biology. “Plants alone do not work. My job has been to add fungi and bacteria to the roots of the plants in an artificial substrate so that they grow stronger ”, he points out. And 30% faster than a soil crop, which means “30% more potential for filtering harmful gases.” It has patented its technique that it exports to more than 12 countries. In addition, it combines plant varieties in such a way that some act as a natural repellent for other pests or even promote their growth. “This was a huge step for these megaprojects, because they hardly need fertilization and never phytosanitary products,” he adds.
It is important that these giant green walls become sanctuaries for birds and insects with biological interest “using plants that attract certain pollinators or certain birds,” says Solano. It is what the Boeri study defines as the habitable coexistence between humans and other species such as trees, birds and insects. Biologist Rafael Moreno explains that they act as a reservoir for arthropods and pollinators. Furthermore, “its performance as an insect hotel allows pollination by animals (zoocoria) to prevail over pollination by wind (anemophilous), notably reducing sick leave due to allergies”.
Three meter tall plants
In these macro-projects, which do not weigh more than 30 kilos per square meter, plants of more than three meters in height can grow. Its maintenance requires pruning, annual irrigation reviews and replacement of systems every two months. “They are gardens that must be pampered because they grow a lot and can weigh too much,” says Solano. Regarding water consumption, they spend an average of two liters per square meter per day, 10% of what a traditional landscaped area requires.
For all this, the forests that hang from the walls are postulated as one of the answers to contain the alarming pollution in the most densely populated cities, where the horizontal surface is scarce and expensive. Not so the vertical, which is not used and, moreover, is free. The negative part —and the great challenge— of these hanging ecosystems is their high cost, which is between 250 and 500 euros per square meter, including structures and automatic irrigation, calculates Moreno. “They are somewhat more expensive than a traditional garden because they have more technology,” says Solano, who places the price band between 250 and 300 euros.
This is the reason why these green carpets are more frequent in hotels or company headquarters, for which it is part of corporate social responsibility. The few examples that exist in residential buildings are of high standing. Although things seem to start to change. Stefano Boeri’s studio has made the first vertical garden in a social housing building: Trudo Tower, in Eindhoven (Netherlands), reaches a height of 75 meters and houses up to 125 trees of various species on its four facades, which about 5,200 shrubs and smaller plants are added. Each terrace, measuring more than four square meters, has a tree and 20 bushes. Although his most recognized project is Milan’s Bosco Verticale, two skyscrapers on the edge of the Isla neighborhood, in the center of the city, which house 800 trees, 4,500 shrubs and 15,000 plants distributed along the facades.
The advantages for the environment and people’s health are the same when it comes to interior gardens, with the exceptions that they are smaller projects, use artificial light and tropical understory plants.