Maintaining a vertical forest

vertical greenery bosco verticale
Architect Stefano Boeri is the designer of Bosco Verticale (BV), a residential project consisting 900 trees on the balconies of two towers (Figure 1). It is said that the extensive vegetation produces the same ecological footprint as 10,000 square meters of forest (Click here for details). 
Fuelled by innovative ideas and technological advancements, urban greenery has evolved from ground level landscaping to vertical greenery on building facades, roof gardens and sky terraces.

While the design intention of this project is clear, the maintenance strategy and impact on the microclimate is somewhat ambiguous. The focus of this article is to discuss possible issues with plants and maintainability of this milestone project. 

1. Optimising the benefit of urban greenery

a) Reduction of temperature

Plants can bring about benefits to the microclimate through several physical processes

(Wilmers, 1991; Dimoudi and Nikolopoulou, 2003):

− Shading from plants and trees lower the solar heat gain on the building envelope;
− Terrestrial (long wave) radiation is reduced due to lower surface temperature through shading;
− Evapotranspiration of plants help to lower the dry-bulb temperature; and
− Latent heat of cooling is increased due to the added moisture to the air via evapotranspiration.

In the case of BV, the extensive tree coverage would suggest ample shading from the foliage.  Enveloping the entire tower(s) in greenery will ensure protection against solar heat gain throughout the year.

The sunpath diagram for Milan, Italy shows that the Sun is predominantly from the South. Therefore, plantings should be concentrated on the South facade.

b)      Ecological footprint replacement

There seems to be an assumption that the ecological footprint can be transposed directly between planes, i.e. A tree on a balcony in the city = A tree in the forest. While the physical comparison is somewhat valid, the ecological impact is vastly different. Let us take the life cycle cost of a tree in the city. The embodied energy (pre-growing, transportation, installation, irrigation, maintenance) of the tree in the city vastly outweighs that of a tree in the forest.  

c)      Humidity and oxygen (Figure 4)

As mentioned in point (a), evapotranspiration is the main attribute of plants that provides a reduction in air temperature as well as increasing the humidity of the environment. According to annual weather data, the average humidity in Milan is 70% – 80%. That is really high, and quite possibly contributing to thermal discomfort . Therefore, there is no need to increase the humidity by adding plants. Increasing the humidity may also lead to an increase in energy consumption as the air conditioners will require more energy to remove the moisture in the air.

d)      Protection from radiation and absorbing carbon dioxide

Let us assume that exposure to solar radiation is an issue in Milan, and that daylighting is not a factor in the design process.

Protection from radiation (solar) is quite obvious, and stated as one of the benefits of urban greenery in point (a) (Figure 4). However, solar insolation is not distributed evenly for all building facade (Point 1a). The North-facing facade receives very little sunlight throughout the year. Therefore, with the addition of plants, daylight factor for the North facing rooms might be low.

Absorbing Carbon Dioxide is only half the story; the process of carbon sequestration entails the capture and storage of Carbon Dioxide. Now the storage part is tricky; carbon is stored in the trunk of trees, and in minute amounts (minute compared to Carbon Dioxide produced from anthropogenic processes). In the building, small trees are used due to spatial and logistical restrictions. With a small girth, one can expect the carbon sequestration ability of such greenery to have little impact on the environment. 

2. Plant longevity

a) Tree roots

The functions of roots are to:

  1. Absorb water and inorganic nutrients;
  2. Anchor the plant body to the ground, and support it;
  3. Store food and nutrients; and
  4. Engage in vegetative reproduction.

Most of us tend to underestimate the extent to which roots can spread. Figure 2 shows the roots will naturally extend far beyond the tree canopy drip line. The sectional drawing (Figure 6) shows that the trees are to be planted in planter boxes that are limited in size. In time, the roots of the trees will grow beyond the confines of the planter box and either grow into the concrete or surface at the top layer of the soil. Growing into the concrete might lead to issues with water penetration and building structure integrity. Since trees are used extensively for this design, this can be a real problem.

b)    Irrigation

How much water is needed to irrigate 10 000 m2 of foilage? The allocation of water is largely based on the root-ball size of the tree. Looking at the design, we might be able to assume that the root-ball size is equivalent to the size of the planter boxes used to house the plants. This means that irrigation for all 8 facades will be a massive endeavour.

3. Maintainability

a) Root penetration

As discussed in (2a), root penetration into the concrete can pose structural problems (Spalling concrete, etc).

b) Cost of maintenance

In order to maintain the desired look for the two towers (Figure 1), the plants must be rigorously maintained. During autumn, large amount of fallen leaves from deciduous trees might cause problems for the drainage system, as well as become litter to residents. The maintenance strategy will most probably be very labour-intensive, rather frequent, and might require the use of specialised equipment.  How does one replace a tree at the higher levels (Figure 3)? Must the surround area be cordoned to ensure the safety of pedestrians at ground level?

4.    Biodiversity as nuisance

The notion of providing greenery to increase biodiversity is common practice, and works to a certain extent, the most evident being the sighting of birds. However, they are foraging for food and with the birds, we can expect our fair share of spiders, beetles, aphids and other bugs (Figure 7).

– T


Dimoudi, A., Nikolopoulou, M., 2003, Vegetation in the urban environment: microclimatic analysis and benefits, Energy and buildings 35(1):69-76.

Wilmers, F., 1991, Effects of vegetation on urban climate and buildings, Energy and buildings 15(3):507-514.,Milan,Italy



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