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The Vertical Forest towers in Milan by Boeri. Phenomenon or archetype?

Place: Milano, Country: Italy

Bosco Verticale Vertical Forest towers Milan Stefano Boeri 01 Inexhibit

The Bosco Verticale / Vertical Forest high-rise complex in Milan, view from the Porta Nuova Gardens looking west. Photo © Riccardo Bianchini / Inexhibit

The Vertical Forest towers in Milan by Boeri Studio. From Phenomenon to an archetype?

In recent times, the “Vertical Forest” (Bosco Verticale in Italian) designed by Boeri Studio (Stefano Boeri, Gianandrea Barreca, and Giovanni La Varra) has been one of the most discussed architectural designs, both in Italy and internationally.
Vertical Forest is a residential complex consisting of two towers, 110 and 76 meters high (160 and 250 feet), completed in 2014 in Milan’s Porta Nuova district. The most interesting aspect of a project which – for its visually striking aspect – is possibly the most famous among the many built in Milan in the last years – is that Vertical Forest is a sort of “pilot project” of a new generation of sustainable buildings.

Many articles, some laudatory some critical, have been written about the “Forest”; yet, only a few of them analyze the key-point of this project; namely, if the idea to “hybridize” a high-rise building and a wood does make sense, from an architectural, urban, ecological and technical point of view. This requires to get rid of apologies and polemics, as well as to ignore opinions based only on aesthetic or superficial socioeconomic discussions. Furthermore, it asks us to be more investigators of possible futures than architectural critics.


The idea
The principles upon which the Vertical Forest project is based are something I totally agree with, admittedly.
After so many years in which form has been the main, if not the only, element to care about in architecture, Boeri focused instead on urban people’s needs, the role of building design within an urban strategy, and tries to push contemporary architecture beyond the ordinary.

“Vertical Forest is a model for a sustainable residential building, a project for metropolitan reforestation contributing to the regeneration of the environment and urban biodiversity without the implication of expanding the city upon the territory. It is a model of vertical densification of nature within the city (…). On flat land, each Vertical Forest equals, in the number of trees, an area of 20,000 square meters of forest. In terms of urban densification, it is the equivalent of an area of a single family dwelling of nearly 75,000 square meters. The vegetal system of the Vertical Forest contributes to the construction of a microclimate, produces humidity, absorbs CO2 and dust particles and produces oxygen.
Vertical Forest increases biodiversity. It helps to set up an urban ecosystem where a different kind of vegetation creates a vertical environment which can also be colonized by birds and insects and therefore becomes both a magnet for and a symbol of the spontaneous re-colonization of the city by vegetation and by animal life. The creation of a number of Vertical Forests in the city can set up a network of environmental corridors which will give life to the main parks in the city, bringing together the green space of avenues and gardens and interweaving various spaces of spontaneous vegetation growth. (…)
Vertical Forest is an anti-sprawl measure which aims to control and reduce urban expansion. If we think of them in terms of urban densification, each tower of the Vertical Forest is equivalent to an area of urban sprawl of family houses and buildings of up to 50,000 square meters.”
From the project description by Boeri Studio

“Vaste Programme” –  General de Gaulle would say – which aims to reshape Le Corbusier’s Ville Radieuse on 21th-century paradigms. Vast, but also plausible? Is Milan’s Vertical Forest really a model? Is it the first individual of a new species or just a sterile branch of architectural evolution?

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East facades of the Vertical Forest towers. Photo © Riccardo Bianchini / Inexhibit

Bosco Verticale Vertical Forest Milan typical floor plan

Typical floor plan of the two residential towers. Image Boeri Studio / Residenze Porta Nuova Agency.


How Vertical Forest is made
From an architectural point of view, the two towers are relatively traditional. They have a reinforced concrete structural frame with 28-centimeter-thick floor slabs. The facades, clad in dark-gray porcelain tiles, are marked by a sequence of large balconies, 3.35 meters / 11 feet deep, which make the buildings’ appearance quite dynamic.
The complex has a total gross floor area of 40.000 square meters / 430.000 square feet divided between the southern tower, called Torre de Castilla, and the western one, the Torre Confalonieri. The apartments there are notoriously expensive and aimed at a wealthy clientele. Two well-designed buildings, in the typical sober style of Milanese architecture, but nothing particularly innovative, per se.

In fact, is their vegetation what makes the two towers really interesting – architecturally, conceptually, and technically – the rest doesn’t matter much.

In detail, the large balconies accommodate an ensemble of plants of over one hundred different species.
800 trees from 3 to 9 meters / 10 to 30 feet high, including holm oaks (Quercus Ilex), downy oaks (Quercus pubescens), golden raintrees (Koelreuteria paniculata), wild pears (Pyrus pyraster), manna ashes (Fraxinus ornus), beaches (Fagus sylvatica), apple trees (Malus x ‘Red Jewel’), Higan cherries (Prunus subhirtella), olive trees (Olea europaea sylvestris), Turkish hazels (Corylus colurna), and Persian ironwood (Parrotia persica), among others.
4.500 shrubs, including hazels, strawberry trees, hawthorns, brooms, Saint John’s Worts, purple osiers, and jasmines.
15.000 ornamental plants

The different plant species were selected and positioned depending on the facade exposition, as well as on the required level of sun-shading of each dwelling unit, along than for aesthetic reasons. The plants were installed into concrete containers with different dimensions, depending on the type of plant. Trees were planted on a 1-meter-thick soil layer, while shrubs on a 50-centimeter-thick one. A water draining and root-resistant system, composed of Polyethylene and geotextile fabric layers, was positioned between the soil and the internal waterproofing membrane of each container.

The first technical problems the architects had to cope with were the static and dynamic loads induced by the trees, and by the effects of the wind on them. This required to simulate such effects in a wind tunnel and to introduce technical elements, such as special steel anchors and security cables which retain the trees and avoid broken branches falling from the towers.

To solve the issues related to plant irrigation and maintenance, the design team developed several rather ingenious solutions. The plants are watered by a complex centralized system divided into sub-sectors and equipped with compensated drippers which filters and re-uses gray-water from the buildings and the groundwater previously utilized by the geothermal heat-pump-based VAC system of the complex. The vegetation maintenance is “facilitated” by two cranes installed on top of the towers, which allow the gardening staff to operate on the plants from the outside. In Milan, the towers’ vertical forest is a shared garden; therefore, its maintenance is managed by the condo administration and not by each resident individually.

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The Vertical Forest towers in Milan, view from south. Photo © Riccardo Bianchini / Inexhibit

Bosco Verticale Vertical Forest Milan facade scheme

The over 100 plant species of the Vertical Forest were installed and positioned depending on their orientation, and on how they look in the different seasons of the year. Image Boeri Studio / Residenze Porta Nuova Agency

Bosco Verticale Vertical Forest Milan scheme 01

Bosco Verticale Vertical Forest Milan scheme 02

Schematic view of the plant positioning and the irrigation system of the western tower; and effects of the vegetation on daylighting and ventilation in the Vertical Forest apartments. Images Boeri Studio / Residenze Porta Nuova Agency

Bosco Verticale Vertical Forest Milan section 01

Cross-section drawings of the towers’ facades and balconies. Images Boeri Studio / Residenze Porta Nuova Agency.


Is it Milan’s Vertical Forest a replicable model?
This is an architecture which is hard to ignore; furthermore, its architects consider it as the first example of a “model” which could potentially change our cities’ shape and our lifestyle forever.
Yet, whether the Vertical Garden of Milan, which is going to be replicated in other cities in Europe and Asia, will really be an archetype or not depends on some conditions.

The vegetation of the Vertical Garden can complement public green space but can’t replace it. This because morphology and costs of installation and maintenance of vertical gardens are arguably very unfavorable if compared to those of “traditional” urban green areas. Consequently, a wise urban strategy could promote vertical foresting on buildings, for example through tax exemption, but only when enough ground-planted public green spaces and community gardens have been created and made available to all citizens.

The trees, that make Vertical Forest different from a building with a traditional green facade or balcony gardens, should be carefully studied, and their growth and long-term conditions further investigated to understand what species better fit a so diverse set of environmental conditions. Does the vegetation be just decorative or it can also feature edible plants and fruit trees? To date, trees are grown full size in a nursery and then moved onto the facades of the towers, how long those trees will live and what their development will be in a so “artificial” environment. It is too early now to draw conclusions on that, so far.

We must better understand the maintenance problem, Who must get it? The condo as a whole? Each resident? The local municipality? This depends also on who is the owner of the “forest”. In Milan, it is all condo as a whole, which assures reliable maintenance procedures and a correct choice of plant species compatible with the environmental conditions; yet, it also implies high maintenance cost for the inhabitants of the complex. Is that the best strategy possible?

Will the construction and maintenance extra costs of Vertical Forest buildings be compensated by the energy consumption reduction and the better impact on the urban environment that they produce? This mostly depends on the actual influence of such buildings on quality of air, pollution reduction, urban heat island effect, on their energy consumption, and on the effect on people’s physical and psychological comfort. Sustainable irrigation strategies, for example through an intensive use of rainwater, are also to be studied thoroughly.

The model of Milan’s Vertical Forest can be applied also to non-luxury residential developments? To date, the exclusive purchase prices and maintenance costs of the Milanese complex make it attractive only to a wealthy clientele. A social housing version of Boeri’s building, currently under development in Eindhoven, will possibly give some answers to that question.

Finally, we have to understand if sustainable urban densification and effective solutions against urban sprawl are better achieved through the model of “vegetalized” high-rise towers or through 5-6-story residential buildings with green facades and roof gardens; or, maybe, with both of them.

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Photos above © Riccardo Bianchini / Inexhibit


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