Tokyo's Vertical Gardens

Anyone who has spent even one summer day in Tokyo can testify to the reality of the "heat island effect" in the sweltering metropolis. With over 40 million people in the Kanto conurbation that also takes in Kawasaki, Yokohama and other satellite cities, it forms the biggest concentration of humanity on the planet. Factor in rising global temperatures and something of a paucity of parks compared with many other big cities, and it can get hotter in the concrete jungle than it does in the real thing. With space at a premium, it is perhaps no surprise that hectares of cooling greenery are sprouting up on roofs, and both exterior and interior walls, all over the city and beyond.

The rooftop gardens range from simple lawns on private houses, to huge multi-featured landscapes on the roofs of department stores. Advancements in the technology such as lighter artificial soil bases and watering systems, as well as slow-growing grasses, are allowing gardens to be laid down on roofs where their weight would once have been problematic.

A survey by the Ministry of Land, Infrastructure, Transport and Tourism found that in 2008, new roof gardens totaling 33.6 hectares—equivalent to forty-seven football pitches—were laid around the country. The area of new wall gardens was 7.5 hectares, up 50% on the previous year. The study found a total of 242 hectares of roof gardens and 24 hectares of wall gardens—though the actual figure is almost certainly much higher as numerous smaller projects will have been below the radar. More than a third of the hectares of roof garden, and more than half those of the wall gardens, are concentrated in the Tokyo metropolitan area.

There are now more than 400 companies involved in the design, installation and maintenance of roof and wall gardens, including those that make related specialist equipment, according to the survey. Even the world's largest automaker has got in on the act and formed a subsidiary in partnership with a landscaping company, to utilize elements of its technological know-how in developing new products to enhance urban greening. Toyota Roof Garden also designs and installs greenery on roofs and walls for a wide range of buildings and institutions.

"Rooftop gardens have been getting more widespread for a while but recently it's the greening of walls that has really been getting a lot of attention," explains Nagata Nobuhiro from Toyota Roof Garden's Tokyo office. "People don't always get to see a garden on a roof, but on a wall, inside or out, the impact is immediate.

"We've been installing wall greenery in schools, companies, office buildings and all kinds of workplaces. On exterior walls, businesses can also use them to create signs out of the greenery," says Nagata.

The improvements in growing and watering techniques and technology that allow plants to thrive after being planted sideways into wall-mounted units have been a big factor in the recent increase in green walls around Tokyo, according to Kumahara Jun from Toho-Leo, which also installs roof gardens.

"Inquiries increased thirteen-fold after 2005, when we brought these techniques to market," explains Kumahara.

Mori Building, one of Japan's biggest property developers, has long been an advocate of urban greening in its many forms. Its Ark Hills development—completed back in 1986 and in many ways a blueprint for the many multi-use complexes that were to follow—features a prominent roof garden. The Mori Building developments aim to create a Vertical Garden City; increasing roof and wall greenery is an integral part of the overall strategy. The Roppongi Hills complex is an example of that, with gardens punctuating the concrete at various levels. "By efficiently maximizing the use of land by building upwards, we free up space to be turned into green, open, sunlit areas in the city," explains Kishi Michiho from Mori.

So how effective is all this urban greening in mitigating rising temperatures? A study at the Technological Research Institute of Tokyu Construction found that it was five degrees cooler in roof gardens than on a concrete roof—approximately the same number of degrees that the average temperature in Tokyo is estimated to have risen by over the last century. While roof and wall gardens won't actually reverse global warming, they may at least make it easier to bear.