Speaking of gardens in the sky, I once lived in an apartment building that had one on the 25th-floor.

It was just a couple of floors missing from the building and wasn’t really integrated with any other space. It had ponds and palms and even a bit of grass and, on the whole, there was probably more evaporation than transpiration happening.

Transpiration occurs during photosynthesis when the plant’s pores (called stomata) on the underside of leaves open so the plant can intake CO2 and a little bit of water escapes when they do. This escaping water functions to transport nutrients from the roots to the leaves and also to cool the plant in hot weather.

Dubai is hot and its humidity ranges between 40% and 85% with an annual average of around 65%. However, with summer temperatures in the 40s, the air can hold an enormous amount of moisture and still not feel humid. Singapore, by contrast, has consistently lower daily temperatures and consistently higher humidity. Plants don’t need to work as hard there to transpire. This is a good thing since transpiration would only add to the humidity.

In my garden in the sky, the well-watered plants were shifting the moisture from their roots to their leaves and from there into the air. It did feel perceptibly cooler if there was no breeze or a very slight breeze. Air cooled by evapo-transpiration needs to be kept from moving away.

This is the plan of an Egyptian courtyard house from around 3,000BC. 

This arrangement provides drops in air temperature of 10-20°C (18-36 F°) at night. As evening advances, the warm air of the courtyard, which was heated directly by the sun and indirectly by the warm buildings, rises and is gradually replaced by the already cooled night air from above. This cool air accumulates in the courtyard in laminar layers and seeps into the surrounding rooms, cooling them. In the morning, the air of the courtyard, which is shaded by its four walls, and the surrounding rooms heat slowly and remain cool until late in the day when the sun shines directly into the courtyard. In this way, the courtyard serves as a reservoir of coolness. [Natural Energy and Vernacular Architecture: Principles and Examples with Reference to Hot Arid Climates]

This is the town of Meibod, Iran. The courtyards are filling with the cooler night air. Notice how each of those courtyards also has a tree? After sunrise, photosynthesis will kick in again and the (denser) cool air held in the courtyard will be supplemented by air cooled by evapo-transpiration to extend the cooling effect further into the day. The shade provided by the tree will extend that effect further still. It’s a simple arrangement that works.

Less rural Persian gardens utilise the same principles but with more importance on visual beauty than cooling performance. As is the way.

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Like the Persian courtyard house and its tree, certain combinations of plants and buildings work well for certain climates. Planting trees to shade east and west walls in summer is usual for mediterranean climates. In the cool temperate climates, deciduous trees shade south walls in summer yet allow light and warmth in winter. These uses are good but limited to instances where there is unbuilt land for those trees that are part of the climatic strategy. 

High-rise buildings usually occur in places where land is densely developed and trees are scarce. In any case, buildings these days tend to be taller than trees. If we’re going to put trees onto buildings, then we have to think about what it is that trees can do for a building and its occupants that some other type of solution – such as a shading device – can’t. We need to think about this because buildings and enclosing space of any kind is expensive. If we’re going to mess with the building to include them, then plants in general and trees in particular have to have some sort of quantifiable and unique benefits for the building and its occupants to justify their presence. Otherwise, it’s better to just go plant some trees somewhere, preferably nearby. 

Yeang’s Solaris located in Singapore here at approx. 1°N is a case in point. The plants on the high end of the building face south and can’t provide much in the way of shade, the plants on the east and west shade every third level whilst, from the treed terraces on the north side of the building, it’s possible to look out over more trees!  I just don’t think these trees are there for their benefits of shading and evapo-transpiration.

Let’s move to Milan now and check out Bosco Verticale more closely. The architect says

On flat land, each Bosco Verticale equals, in amount of trees, an area equal to 10,000 sqm of forest.
The Bosco Verticale is a system that optimizes, recuperates and produces energy. The Bosco Verticale aids in the creation of a microclimate and in filtering the dust particles contained in the urban environment. The diversity of the plants and their characteristics produce humidity, absorb CO2 and dust particles, producing oxygen and protect from radiation and acoustic pollution, improving the quality of living spaces and saving energy. Plant irrigation will be produced to great extent through the filtering and reuse of the grey waters produced by the building. Additionally Aeolian and photovoltaic energy systems will contribute, together with the aforementioned microclimate to increase the degree of energetic self sufficiency of the two towers. The management and maintenance of the Bosco Verticale’s vegetation will be centralised and entrusted to an agency with an office counter open to the public. architects website

All good. The balconies are generous and, as is the Italian tradition, their construction looks solid and made to last.

On his website, you can see that the architect has thought of the types of plant suitable for each facade orientation. I should hope so.

The system of irrigation has also been thought through. Again, as it should.

I wish the architect and building good luck. It seems a genuine attempt to fuse vegetation a building in a meaningful way. Tall buildings do strange things to wind so there are still unknowns such as how the trees will cope with wind not necessarily coming from the side and rain not necessarily falling downwards but fortunately, Milan is not a very windy city. The trees will tend to grow outwards towards the light so let’s hope overhanging branches aren’t allowed to become dangerously picturesque.

The benefits of all these trees are shown graphically in the diagram below. We shouldn’t be too amazed by any of this. The trees will just be doing what trees have done for the past 400 million years (and, apart from the last 200,000 years of that, for no-one’s particular benefit). Of all the benefits shown, only the noise reduction effect is debatable but even if there is no great tangible reduction in the amount of noise, I believe that at least not being able to see the noise source does go some way towards lessening the awareness of noise. The rustling of leaves may even create a masking white noise. What about birds? Depending upon the type, the time and the amount of noise they make, birds may or may not be welcome. Birds can also be rather messy, but that’s their nature.

The green box in the diagram above suggests that the experience of this building will be akin to that of being lost in a forest with its own microclimate, and isolated from the sights and sounds of the city outside. To a certain extent it will be, or at least it will be more so than most of the other buildings around. But what about evapotranspiration?

Let’s ask weatherspark what the humidity in Milan is like.

The relative humidity typically ranges from 45% (comfortable) to 95% (very humid) over the course of the year, rarely dropping below 27% (dry) and reaching as high as 100% (very humid). The air is driest around March 23, at which time the relative humidity drops below 55% (mildly humid) three days out of four; it is most humid around November 1, exceeding 93% (very humid) three days out of four.

Thanks weatherspark. The blue line is the average daily humidity and seems rather high for transpiration to be considered a benefit. Milan has a Mediterranean-type climate with warm dry summers and cool winters with rain. It’s pleasant. The heat and noise of the city might irritate in summer on a humid day and so an amount of trees equivalent to 10,000 sqm of forest, a bit of evaporative cooling and various other benefits including biophilia in all its forms, is totally welcome. However, keeping it real,

it would be more welcome if the buildings did not stand in a park! 

A lot of money has been spent so that the owners of property within a high-rise building can have one of the attributes of land – the joy of having significantly sized trees outside one’s window. This type of joy does not come cheaply in a city. These plants have various marginal microclimatic benefits but their main appeal is just that. They aren’t exactly a part of the food chain or the waste cycle as they would be in truly integrated ecosystems like those that insects and plants manage to create for themselves. These trees are – I’m afraid to say – “Eco Deco”. There! I’ve coined the word and now offer it to the world.

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The marketing website. It’s always cherry blossom time in renderland.