Designed in Milan by Stefano Boeri, the Bosco Verticale ("Vertical Forest") turns the facades of two residential towers into a living forest: about 800 trees, 4,500 shrubs, and thousands of plants are grown on the balconies. This is not plant decoration but a structural and ecological system.

The architectural problem is the weight of the trees and the wind. A mature tree, with its soil and planter, weighs many tonnes; and the wind at height applies a large overturning force at the tree's crown. Boeri's team designed the balcony slabs as cantilevered concrete plates, far thicker than usual and reinforced to carry these loads; the trees' roots were fixed to their planters with steel cages so they would not topple in a storm. Which tree grows on which facade, at which height — accounting for sun, wind, and humidity — was planned together with botanists.

The function of the living facade is performative: in summer the foliage cools the building with shade and evaporation, in winter it sheds its leaves to let the sun in, it filters dust and noise, absorbs carbon, and produces a microclimate for the city. The Bosco Verticale thus takes architecture beyond a lifeless shell and combines it with a living system whose maintenance and growth are part of the design; it sets up a prototype for the sustainable tall building.