MILAN — The International Highrise Award enjoys great prestige because, unlike other similar competitions, it is granted neither for a building's height nor form. Instead, the prize — bestowed by the German Architecture Museum, the city of Frankfurt and Deka Bank — honors criteria such as sustainability and innovation. And this year's winner, Milan's Bosco Verticale (Vertical Woods), has that in spades.

The 2014 jury, led by architect Christoph Ingenhoven, cited Italian architect Stefano Boeri's work for its environmental ambitions in difficult circumstances. Piazza Gae Aulenti, in an area of new construction around Porta Garibaldi, is Milan's ugliest square. The huge urban space on the northern edge of the historic city center is virtually empty of people at night. With its skyscrapers and buildings of international companies that are present in every large city in the world, it looks like something between Berlin's Potsdamer Platz and the Dubai Mall.

Only a closer look reveals something unusual behind Italy's tallest building, the Torre Unicredit by César Pelli: two black towers of differing heights with 100 white balconies that look like pulled-out drawers.

The towers, which were completed a little over a month ago, are somewhat reminiscent of the women's opened drawers in Salvador Dalí"s The Burning Giraffe. But just how right it is that the buildings have been baptized the Bosco Verticale is only perceptible from up close.

Unlike the imagery in the project's prospectus, the vegetation on the balconies is still far from jungle-like. But vegetation there is, and it is so impressive that hordes of tourists come here to take pictures of the towers.

"The Bosco Verticale is a worldwide unique architectural experiment, a model for future inner cities," says its architect, 57-year-old Stefano Boeri. He hails from an influential local family, was an urban planner for many years, and ran for mayor of Milan two years ago.

It's only on the 25th floor of the higher of the two towers, which are 110 and 76 meters high respectively, that the project's full uniqueness can be properly appreciated. From the living room of an apartment with a floor-to-ceiling window, you look south onto the tops of trees up to nine meters high that are in planters 1.3 meters deep on the balcony below. Looking to the north through shrubbery, tree trunks and branches, there is the silhouette of the snow-covered Alps in the distance.

It is one of those rare sunny November days in one of Europe's most densely populated large cities where air pollution is so rampant that the authorities are forbidding driving and closing schools.

The equivalent of 2.5 acres of green

The Bosco Verticale could be described as an attempt to bring the forestland Milan so urgently needs back to the built-up city. The 20,000 shrubs and 800 trees on the balconies, covering a total surface of 8,900 square meters, incidentally serve aesthetic purposes, but they are primarily meant to ensure a better micro-climate in the apartments, filtering dust particles from the air and producing oxygen.

As natural green insulation material, the balcony gardens also protect against heat, cold and noise, and help save energy. The architectural landscaping concept was developed by agronomist Laura Gatti working together with the University of Milan and scientists at Karlsruhe's Institute of Technology.

It was a real fight to convince the building owners, American project developers Hines, of their plant concept, Gatti says as she leads visitors through two unoccupied apartments.

The developers simply couldn't believe that the oaks, hazelnut trees, beechwoods, plum trees and Persian ironwood trees could withstand local winds of up to 160 kilometers an hour. The trees had been prepped for two years at a tree nursery for their future life in the highrise.

"We were pretty sure that anchoring the trees in the 1.3-meter-deep balcony planters and concrete ceilings was enough, but the developers wanted to make absolutely sure and stuck the entire plant package into a wind canal in Miami and subjected it to winds at speeds of up to 190 kilometers an hour," Gatti says. "That was how we got the final proof that our calculations were correct."

But the residents, whose apartments cost between one and 10 million euros each, won't be out there with clippers, shovels and watering cans. The plants are watered automatically with the help of an electronically controlled system that pumps non-potable water from the ground water via hoses into the balcony planters.

A permanently installed crane on the roof makes controlling the growth of the trees possible. Three gardeners have been hired to maintain the vertical woods that cover the equivalent of one hectare (2.5 acres). They are charged with pruning, fertilizing, and generally caring for the plants year-round.

For reasons of aesthetics and safety, residents are not allowed to make individual additions to the plantings, so rose breeders will have to find somewhere else to pursue their hobby. "But most of the residents will be thankful that plant maintenance is taken off their hands," Gatti says.

Of course, that luxury has it price. Residents have to pay some seven euros per square meter of living space annually for the care of their woods.

Standing in the huge, minimally furnished entrance area of the larger tower, Boeri, the architect, thinks it's a pity that so few can afford to live in Bosco Verticale.

But the buildings are also a prototype, he says, and to develop prototypes more money is always required in the beginning. The costs would pay for themselves if the model is replicated. Thanks to the international prize, the chances of that happening are quite good.