OSAKA -- This summer Osaka is living up to its reputation as Japan's hottest city. Temperatures have not fallen under 25 since July 19.

The heat is all the more oppressive due to a lack of greenery to provide shade and oxygen. A mere 4.4 percent of Osaka's 221 sq. km area is green belt. Tokyo is a jungle by comparison, with 22.3 percent of its 23 wards shrouded in greenery.

New roof gardening technology, however, may help bring more greenery, and in turn, some relief from Osaka's sweltering streets.

A new garden that opened last month on the roof of Hotel New Otani Osaka in Chuo Ward uses synthetic resin mesh mats, on which sedum plants are grown. The plants need no artificial water supply and grow even under the harshest conditions.

New Otani representatives claim it is the first such hotel rooftop garden in Japan.

Technical innovation was needed to build the 1,600 sq. meter garden on the roof of a three-story building that houses the hotel's party rooms. It had to weigh less than 130 tons -- the upper limit that the building could support.

"With this weight limit, regular garden planning was impossible because it requires a huge amount of soil," said Saburo Morishima of Nikken Space Design Ltd., which designed the garden.

"But with the use of the mats and sedum, the garden was possible because the sedum garden weighs 70 kg per sq. meter."

The base portion of this part of the garden is only 6 cm deep. About one-third of the garden is covered with sedum, another one-third with flower gardens that come with a 20-cm-thick base, and the remainder is covered with wood boards.

The roof previously had four tennis courts, but the hotel turned two of them into the garden because it would be better for the environment and could also be used for outdoor parties for hotel guests, said Shuzo Takahashi, director of marketing for the hotel.

So far, the garden has proved popular with guests, some of whom use it for morning walks, Takahashi said. Office workers from surrounding buildings have asked the hotel if they can eat lunch there.

The garden has provided other unexpected benefits. Takahashi said the room temperature of part of a ballroom directly beneath the roof garden is a few degrees lower than the part under the tennis courts.

When used with partitions, the room under the garden requires less air conditioning.

Morishima of Nikken Space Design sees greenery roofs as a new trend. Other building owners in Osaka have visited the hotel to see whether they could introduce a similar system. And major construction companies have started providing similar gardens, including Takenaka Corp., which engineered the garden on New Otani.

In Tokyo, a local ordinance introduced in April made it obligatory for at least 20 percent of the roof space on new buildings of certain sizes to be covered with greenery.

Osaka, however, has no such ordinance. But the municipal government provides subsidies covering half the cost of planting trees or flowers on and around buildings facing public streets. The upper limit of the subsidies for each site is 2 million yen.

In the case of Hotel New Otani, this subsidy was not granted because its roof was not facing a public street.

The city is considering easier requirements for its subsidy program by the end of the current fiscal year. But according to Shigeyuki Ogitani, a city official, Osaka will not introduce mandatory rules on roof greenery as in Tokyo.

Ogitani concedes that unless stronger measures are introduced, it will be almost impossible to reach the city's target of raising the greenery rate to 15 percent.

Takahashi of New Otani Osaka said, "It is not easy to find a space large enough to make a garden. We were lucky that we could secure this much space in the middle of Osaka."