I took part in an international conference on green roof systems in Nurtingen, a small town near Stuttgart, Germany, on Sept. 14-15. Some 160 people, including botanists, architects, designers, engineers and government officials from 23 European, Asian, African and American countries gathered for a heated argument on why green roofs are needed now.

``Green roof'' is a system to grow plants on roofs with specially designed new building materials that can be used on the slanted rooftops or flat rooftops of concrete buildings. In Germany, more than 80 percent of roofs on which greenery is planted use this system.

Compared with conventional rooftop gardens, which are more common in Japan and the United States, green roofs are lighter and easier to assemble. That is why they are used extensively not only on the roofs of buildings and homes but also for such facilities as factory warehouses, public toilets in parks, car and bicycle parking lots and garbage collection sites. Local governments also provide subsidies.

Meanwhile, in Japan, because of dense land development and high land prices in major urban areas, it has become extremely difficult to acquire new sites to plant trees. To add greenery to drab urban landscapes, a rapidly growing number of local governments are establishing ordinances that require builders to plant trees on rooftops.

The urban environment is rapidly warming. It is reasonable for local governments to require beneficiaries of industrial and economic activities to plant trees on roofs since it is those activities that advance warming.

In high summer, the temperature on top of buildings can exceed 60 degrees. Buildings that absorb heat during the day continue to discharge it at night. Air conditioners also generate heat. Furthermore, with warming of the environment on a global scale, cities are becoming hotter year by year.

In green-roof systems, it is common to grow succulent plants of the sedum family that can withstand dryness in lightweight soil with good drainage. The plants form a layer of green about 15 centimeters thick, which insulates the rooftop and helps reduce the energy needed for air conditioning inside the building.

As green roofs attract attention, some people say that lawn grass is more effective than sedum while others say the use of insulating materials and special paints can more effectively lower room temperatures than plants.

However, since it is evaporation that causes the temperature to decline, I wish to propose the planting of rice on rooftops. Creating rice paddies on top of buildings is possible given Japan's advanced waterproof technology.

Rooftop rice paddies offer children in the neighborhood a chance to acquaint themselves with rice planting and harvesting. Adult urban residents can also feast their eyes on verdant rice without leaving the city. The important thing is to change the drab cityscape to a greener one.

At the conference in Germany, participants agreed some rooftop greenery that combine technology used to create conventional rooftop gardens and green roofs will become increasingly common in the future.

Traditionally, the Japanese people have lived with nature. Let us advance the greening of roofs in earnest, using plants that not only match the shapes and purposes of buildings but also befit the history and climate of the areas in which they stand.

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The author is a professor of landscape engineering at the faculty of agriculture of Meiji University and the president of the Japanese Society of Revegetation Technology. He contributed this comment to The Asahi Shimbun.(IHT/Asahi: January 12,2005)