In Milan’s Porta Nuova district, two towers push their crowns of trees beyond the edges of their balconies. Trunks vary in size, foliage shifts in density, and each floor loosens the outline of the buildings in a different way. At Bosco Verticale, that variation is part of the architecture.
Turning a three-metre balcony into a place for roots
Stefano Boeri Architetti records about 800 trees, 4,500 shrubs and 15,000 perennial and ground-cover plants across the two towers, representing roughly 100 species. Their roots occupy built-in planters on balconies projecting about three metres, while branches extend into open air high above the street.
These containers are not large pots added after construction. TEKNE describes reinforced-concrete planters that also act as balcony parapets. The building was designed from the start to receive soil, stored water and expanding roots, provide drainage and hold a trunk where wind would meet it.
Before planting, architects and botanists spent about three years studying suitable species and raising trees for the project. Plants were prepared from the nursery stage for restricted root space and a much more exposed climate. The uneven crowns in the photograph show individual growth continuing to reshape the towers.
Species change with orientation and height
The AIPH case study says selection considered root behaviour, cold and wind, sun, water, urban pollution, pests, drought and the stress of transplanting. Trees were filtered by whether they could continue living in a planter under specific conditions, not merely by how attractive they looked together.
Olives and fruit trees were placed on the southern side, while deciduous species on the north allow more daylight into apartments during winter. Humidity and wind also change by floor, so lower and upper levels do not receive the same planting. The diversity of roughly 100 species answers a sequence of small climates stacked through the building.
At height, containing the roots is only part of the problem. TEKNE describes steel cages within the planters and cables used where necessary to secure trunks against strong wind. Behind the movement of foliage, a structural system protects both root balls and trees.
Gardeners and irrigation keep one forest together
Planting is not managed separately by each apartment. According to AIPH, it remains under central care, and residents cannot independently change the trees or their maintenance regime. Treating neighbouring balconies as one living façade requires a shared system unlike the private care of ordinary pots.
Gardeners enter the balconies every three to four months, while specialist arborists descend the exterior by rope once a year to inspect and prune. Drip irrigation supplies water, with moisture sensors and exposure helping to determine demand. The AIPH record says recycled groundwater provides the irrigation supply.
Bosco Verticale did not bring a finished green surface into architecture. It brought in the time of living plants—roots thickening, branches extending, leaves falling, and cycles of pruning and watering. Housing trees high above the ground requires the building to preserve the systems that let them keep living.