Green roofs have once again hit the headlines when the government announced on Wednesday that no one will be prosecuted for the City University of Hong Kong (CityU) roof collapse last year. Safety was the key concern but this incident also prompted questions about the benefit of green roofs.

The collapsed CityU roof was essentially a carpet of vegetation that very few people had a chance to visit and enjoy. Contrast that with farming organic vegetables on planters installed on roof tops, and you have a very different story.

It has been reported that roughly 30 percent of Hong Kong people would like to buy locally grown organic produce but the supply comprises just 2 percent of sales. Clearly there is unmet demand.

In recent years we have seen people giving up well-paid jobs to move into organic farming, and children of retired farmers reviving their family business. Modern-day urban farmers are highly skilled entrepreneurs. They adopt new technologies and learn to farm through internationally and professionally accredited courses. Many of them embrace permaculture principles which simulate the patterns and relationships existing in nature, allowing for diversity, stability and resilience in the ecosystem.

Gone are the days when the image of farming involved wrinkled men in straw hats soaking their feet in flooded soil, accompanied by an ox-driven cart in the far-flung countryside. Trend-setting magazine Vogue ran a feature article on urban farming in its August issue last year, in case you need proof urban farming is indeed in fashion.

Hong Kong offers the perfect setting for making farming palatable to city dwellers - by turning under-utilized rooftops on urban high-rises into organic farms. And if safety is a concern, Wildroots Organic founder Fai Hui said any proper rooftop farming projects are preceded by investigations by a structural engineer on factors such as weight loading. Unlike the ill-fated green roof carpet at CityU, urban rooftop farming can offer a myriad benefits to many people.

The farms can be centrally located so they are close to clusters of people and the end users, offering a ready supply of potential farmers and customers. Carbon emissions from transport are eliminated. Transaction costs decrease. Some of these rooftop farms are located in the central business districts which means a young farmer doesn't necessarily have to be banished from the heart of the city. Needless to say, rooftop farming is a source of livelihood to this new generation of farmers.

The rooftops of many commercial or even residential buildings in Hong Kong are left unused. Turning suitable sites into rooftop farms means there is no need to set aside land, and makes productive use of idle resources.

Apart from improving urban aesthetics, green rooftops can lower surface temperatures. Studies have shown rooftop gardens reduce ambient temperature as much as 4 degrees Celsius.

Owners of commercial buildings who let their rooftops be converted into farms win an extra brownie point when it comes to their corporate social responsibility, often at relatively little installation and maintenance cost. They can even collaborate with restaurants within their buildings to provide direct farm-to-table meals.

In schools, rooftop farms provide food education at students' doorsteps. Hui told me about a local school project in which students who learned to grow their own vegetables were generously sharing their surplus with school workers and security guards. The development of a sense of community, co-ownership and sharing could be an antidote to increasingly polarized Hong Kong.

The most direct benefit for Hong Kong, of course, is increased local supply of fresh organic produce, a chance of meeting the 28 percent unmet demand mentioned above and increasing Hong Kong's resilience, not to mention promoting healthier eating habits.

And the downside? I can't really think of any.

The government defines sustainable development as balancing "social, economic, environmental and resource needs, both for present and future generations, simultaneously achieving a vibrant economy, social progress and a high quality environment, locally, nationally and internationally".

Organic rooftop farms fit quite nicely into that definition. But why aren't we seeing them on more buildings? Urban farmers complain of inertia and lack of incentives. Most landlords aren't going to take the initiative to convert their rooftops into farms just because it's a great thing to do. Clearly more public education is needed here, preferably with the government taking the lead with a pragmatic and pleasing approach to entice young people eager to launch their own startups.

Rooftop farming has proven to be an unqualified success in Singapore, which has even less land space and fewer skyscrapers than Hong Kong. Taking a leaf from Singapore and the United States, we need to offer attractive enough incentives to kick-start this new industry, such as by offering tax deductions or financial subsidies.

In Hong Kong, the Small and Medium Enterprise Funding Scheme, the Sustainable Development Fund and Social Innovation and Entrepreneurship Development Fund are already existing options through which public money may be channeled for the cause. A genuine and respectful dialogue between the government, funders, landlords, technical experts and those already practicing urban farming in Hong Kong will help steer effective allocation of resources to the benefit of all.

(HK Edition 06/06/2017 page8)