A University of Hong Kong (HKU) experiment to identify the best plant species to insulate the rooftops in new development areas and update old buildings is under way. The experiment, led by Professor Jim Chi-yung from the HKU geography department, began two months ago. It follows a similar experiment in 2006 which found that a 'green' rooftop can lower the surface temperature by as much as 26 degrees Celsius. The 2,000-square-metre rooftop of the university's main library has been turned into an oasis, with 50 plant species being tested and nurtured with compost. Jim wants to know which species is hardiest under the sun's intense infrared and ultra-violet light, is most drought-resistant and can grow with the thinnest layer of soil. 'A thin soil layer is important because many old buildings in Hong Kong were designed with a small loading,' Jim said, meaning they can't carry much weight. He said one square metre of roof area must support a weight of at least 50 kilograms to make greening possible. Despite the fact that green roofs can help to reduce heat and save air-conditioning costs, demand remains low and commercial nurseries in Hong Kong are not keen on growing plants for the purpose. It costs about HK$700 to create one square metre of green roof, comprising a root barrier, drainage, filter and water storage layers and soil. Jim said scores of new private housing developments with tall towers resting on huge podiums would be suitable for greening. 'But the so-called podium gardens you see often have more paved space than plants,' he said. The new campus at HKU will see several roofs covered with greenery. While green roofs are common in Germany, Japan, Southeast Asia and even the mainland, little effort is made by the private sector in Hong Kong to create them. The government has created roof gardens in mere dozens of its buildings.