As green roofs continue to top off buildings in the U.S., other visible but equally verdant structures are sprouting in many Asian cities.

In Hong Kong, Singapore, and Tokyo, for example, vegetated layers are lining the exterior walls of some high-rise buildings. This is the result of beefed-up efforts at vertical planting, also known as green walls. Just think of them as vertical green roofs—and like the ever-popular roofs, green walls boast environmental benefits like filtering out air pollutants and helping to cool down cities.

In many ways, vertical planting is better suited to the Asian cities than horizontal planting. For one thing, the super-dense cities don’t leave much room for greenery. A green façade or sidewall, however, takes no inches of real estate. Meanwhile, the concrete, steel, and glass towers that dominate central business districts in these cities has created a craving for visibly verdant sights. A green roof on an 80-storied skyscraper doesn’t cut it, but an external wall of ferns and mosses can serve as eye candy while also helping the environment.

“Hong Kong is a vertical city. There are lots of opportunities to do vertical planting in Hong Kong,” says C.Y. Jim, professor of geography at the University of Hong Kong (HKU), who has been testing native plants best suited for vertical growth. “It’s a matter of changing people’s mindset.”

Change is certainly coming: In March, the city’s urban renewal officials announced that they would collaborate with developers to foster vertical planting in city-controlled redevelopment areas, contingent on site conditions. This initiative came on the heels of last year’s opening of the city’s largest green wall in a new shopping and residential complex.

The complex, Vision City in the Tsuen Wan district, is aimed to be developer Sino Land Company’s latest major environmentally friendly project. The complex’s signature green element is the 7,500 square foot curved green wall facing the inner courtyard of the shopping mall. The living wall hangs nearly 60 feet above ground to clad the concrete columns of a two-storied parking garage. Nearly the size of a standard championship tennis court, the wall offers a green vista of plants that filter and cool the air—sans electricity—to residents and visitors at the piazza.

Three of the six species chosen to fill the wall, including Boston ferns, are known to trap air pollutants, says Thomas Lau, the landscape architect who supervised the wall’s design. Data is currently being collected to gauge the wall’s cooling effect. Research on vertical planting, done mostly by Japanese building experts, showed that a layer of green can lower ambient temperature by as much as 10 to 20 degrees Celsius, though the cooling varies according to surrounding wind speeds and the amount of shade available.

Though it might seem like a simple structure, green walls can be quite complex. Because of its height, the Vision City wall called for steel frames specially designed to support planters and the wind load, but the most painstaking calibration lay in the irrigation system. Hundreds of trials were run to ensure appropriate water supply to the nearly 40,000 plants that make up the verdant wall. Even now, six months after completion, that is still a big part of the wall’s maintenance, along with fertilization and weeding.

“The challenge is to monitor the water flow,” says Lau.

Hong Kong isn’t the only city building green walls. In fact, Japan has been a pioneer in planting vertically since the 1970s, says Stephen Lam, a HKU academic specializing in sustainable architecture who is under commission by the Hong Kong government to study the environmental benefits of green walls. He also notes that Japanese households can buy modular green wall units and install them at home, and Singapore has been installing green walls on its highways’ structural supports.

Citing earlier research by scientists at Waseda University in Tokyo, Lam says while a green wall’s planting typically is not thick or dense enough to yield noticeable carbon offsets, its reduction to the urban heat island effect is significant. Unlike most rooftops, not all external walls are suitable for planting, depending on their amount of sun exposure. The walls’ aesthetic value and positive psychological effect on city dwellers, albeit less measurable, are also worth noting, Lam says.

Story by Violet Law. This article originally appeared in "Plenty" in April 2008.

Copyright Environ Press 2008