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The challenge of rooftop gardens

BEAUTIFUL hanging gardens, walls of greenery, vertical vegetable gardens with rip fruit, and rooftop gardens with streams and shade are among the most delightful aspects of the World Expo 2010.

Living green plants show up in unlikely places, and there are even transparent walls of hydroponically grown plants.

They are not only beautiful and soothing for the eyes, they also help cool, insulate, purify the air, reduce dust and noise. They reduce carbon dioxide while producing oxygen.

But creative gardening is showing up around Shanghai as well. The number of green rooftops has expanded rapidly over 10 years in the city.

Though not as spectacular and creative as the Expo pavilion roof gardens, "three-dimensional gardening" is not new in Shanghai.

Green walls, green spaces around buildings, green elevated road edges, as well as green roofs have improved the city, making it more attractive and healthy.

Roof greenery, which is relatively new, covered around 900,000 square meters of roofs in Shanghai by the end of 2009, according to Li Li, senior horticulturist of the Shanghai Landscaping and City Appearance Bureau.

Rooftops are definitely greener in Jing'an, Yangpu and Minhang districts. Rooftop greenery covers about 120,000 square meters in Jing'an District, 27,565 square meters in Yangpu District and 400,000 square meters in Minhang District.

"The earliest green roof in Shanghai go back to the early 20th century when hotels decorated their roofs to improve the setting," says Li. "But little progress was made for dozens of years."

In the 1980s, a few business areas with garden roofs appeared, but not until 1998 did three-dimensional gardening really expand, with government help.

"Shanghai is a big city with a huge population and limited land," says Li. "Expanding greenery in three dimensions will certainly improve the environment."

In addition to benefits such as pollution and noise reduction, roof gardens reduce indoor temperatures, combat the "heat island" effect, prolong roof life and retain rainwater, reducing runoff.

Research shows that roof with green cover lasts an average of 40 to 50 years, while that of a bare roof exposed to the elements is only 25 years, according to research cited by Li.

Cool off

A study in Minhang District shows that indoor summer temperatures of buildings with green roofs are lower by 3-5 degrees Celsius than those of exposed roofs on days of more than 34 degrees Celsius.

If six percent of the roofs in a city are covered by vegetation, the "heat-island" effect is expected to drop by 1-2 degrees Celsius, contributing to the reduction of 1.5-2.1 trillion tons of greenhouse gas emissions a year.

Green rooftops help during frequent storms in Shanghai's summer. Studies show they can retain up to 75 percent of the rainwater in given storm, gradually releasing it back into the atmosphere through condensation and transpiration, while retaining air pollutants in the soil.

"Roof greenery is definitely suitable for Shanghai, but technical concerns, shortage of funds and low awareness among residents have slowed development for years," says Li, the greenery official.

Roof greenery is technically more difficult to carry out due to safety concerns like load-bearing, and waterproofing. Most new buildings today have good load-bearing construction and water-proofing facilities, but most old houses need reinforcement and water-proofing layers.

Large investments have been made for roof gardens in public areas in some districts, while most residential and private buildings still have not gone.

"Many residents reject green roofs as they think that it only benefits the top-floor residents while adding property fees for everyone," says Li.

To avoid problems with residents and ensure safety, the government is first planting gardens on roofs of public buildings, such as government offices, schools and hospitals.

The district government also gives incentives to encourage private builders to plant green rooftops and advises on suitable plants, trees and flowers.

Individuals can "adopt" trees on public rooftops by making donations.

"We hope that the rooftop demonstration in public areas can attract individuals and persuade them to have green roofs on their own building," says Li.

Expo run-up

More people are aware of the benefits of green space and low-carbon living, especially in preparation for the Expo.

There are three main types of green rooftops in Shanghai, including mini-parks with bushes, flowers and plank walkways; simple lawns with hardy grass; and a combination of lawn in the center, with bushes at the edges.

Planting a rooftop garden is not as expensive as many people think, according to a greenery official in Minhang District.

It only cost 125 yuan (US$18.30) per square meter for rooftop lawns and 370 yuan per square meter for the rooftop gardens the Minhang government created in the past two years.

Li, the city's senior horticulturist, says a top-quality mini-park in the air costs no more than 800 yuan per square meter, and averages about 300-400 yuan per square meter for an ordinary park. It costs only about 6-7 yuan per square meter for maintenance.

The cost is much cheaper for rooftop gardens, which average about 150-180 yuan per square meter for a top-grade lawn, and 80 yuan per square meter for an ordinary lawn. Maintenance is negligible.

Most of the latest rooftop greenery in Yangpu District is the low-cost but effective lawn type.

Lots of roofs

Around 19 million square meters of roof areas can be turned into lawns and gardens, according to a survey conducted in 2008 by the Shanghai Landscaping Bureau. But only 900,000 square meters of roofs were "greened" by the end of 2009, less than 5 percent of the total available space.

Most cities in developed countries have roof coverage of 15-39 percent, while in Germany it reaches 80 percent, says Li.

"A great many spaces can be greened but they are still empty," says Li. "There's a lot to do. We need help from residents, sponsors and city planning departments."

A major problem in rooftop and other greenery development is lack of cooperation among city planning departments and greenery departments, in Li's opinion. Close cooperation is essential to expand the green space in the air, he says.

"The ideal way is to integrate rooftop greenery in new building projects so that the building has appropriate construction and facilities to support the greenery," says Li. And residents are more likely to accept it, when it's already there.

Recommended rooftop plants

To ensure easy maintenance and the safety of the building, trees and plants for rooftops should have shallow, horizontal root systems. They should be hardy, drought-resistant and be able to withstand wind.

Herbaceous plants like sedum linear and stringy stonecrop are recommended, as are honeysuckle and ivies, as well as elecampane bushes and China rose.


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