Born out of a fear of contaminated foods, the city’s budding interest in high-altitude gardening may help heal an increasingly fractured – and ageing – society.

A butterfly perching on a lettuce leaf is not normally a cause for marvel. But I am standing on the roof the Bank of America Tower, a 39-floor building in the heart of Hong Kong’s busiest district, to see one of its highest farms. The butterfly must have flown across miles of tower blocks to reach this small oasis amidst the concrete desert.

“You just plant it out and nature comes and enjoys it,” says Andrew Tsui. We are joined by Michelle Hong and Pol Fabrega, who together lead Rooftop Republic, a social enterprise that aims to turn the city’s dizzying skyline green.

If it weren’t for the fact that we are 146 metres (482ft) above street-level, this farm would look like any allotment site or garden courtyard – row after row of rectangular crates, some with fresh sprouts poking through the surface, others with established plants almost ready for harvest. The loudest noise I can hear is not the traffic below, but the wind.

Although it is only February when I visit, the sun is so strong I end the morning with a slight tan, and Hong tells me that the local climate offers ideal growing conditions for most of the year, meaning that despite the exposure, they can cultivate a range of plants. “We have things like cherry tomatoes, salad, broccoli – all of that can be grown here,” explains Hong. The greens I see today are as lush as anything you would find growing at sea level. Workers from the offices below tend it day-to-day, and after harvest, the fruits of their labour are sent to a food bank, where they fill lunchboxes for the needy. “We want to share the good products – not just the leftovers,” says Tsui. On other projects, however, the farmers would take the produce for themselves.

Footage courtesy of Matthew Pryor at the Division of Landscape Architecture, University of Hong Kong

Truly fresh, locally grown vegetables are something of a luxury in Hong Kong. To demonstrate why, Tsui points from our rooftop to the rings of mountains that encompass the city. In between are the two bands of dense urban development, straddling the harbour. “Five to six million people are squeezed in these two narrow belts,” he says

Thanks to these crammed conditions, the city imports more than 90% of its food – much of it from mainland China. But after some well-publicised cases of food contamination in China, more and more people are now looking for locally grown goods. And if they can’t grow it on the ground, they have to take it to the sky.

The team try to invite the remaining rural farmers from surrounding regions, to come and give classes to the city workers

Food production is only one of the project’s aims, however. Their greater goal is to revolutionise the city’s fast-paced culture. Like most urban areas, Hong Kong’s society is highly stratified, and its citizens are often isolated to their particular niche, formed of their colleagues and close friends. The Rooftop Republic team hope that the farms can help break through those barriers. “It’s a bit of a social experiment,” says Tsui.

The team try to invite remaining rural farmers from surrounding regions, to come and give classes to the city workers, for instance. “We hope that this rich knowledge of organic farming practice isn’t lost with their generation, so they can pass it on and share it with the community.”

In return, they pay the rural farmers to cultivate seedlings to be planted on the rooftop farms, providing them with a steady source of income that is not subject to the whims of the market. “This part of their income is lower risk and they can pre-empt and manage their time,” says Tsui. It’s a small step, perhaps – but one that helps connect two populations who would never normally interact. The team also work with the hard-of-hearing and people with other disabilities, who can find the contact with nature to be therapeutic.

After we descend from the Bank of America Tower, the team take me to a second project on the Hong Kong Fringe Club – which grows aubergines, tomatoes, oregano, lemongrass, mint and kale for the bar and restaurant below. I will have passed the building a dozen times on my trip so far without realising that it hosted this little haven on its roof.

Here the team tell me about their other goal: education. By running regular workshops, the team hope that Hong Kong’s city-dwellers will become a little more aware of the resources needed to grow the food they are eating. Pointing to a bed of broccoli, for instance, Hong remembers one recent group who had never seen the whole plant. “They didn’t realise that the florets that we eat are actually quite limited,” she says. “And if you look at the quantity we see in the supermarket, you begin to see how much space we would need to grow that,” she says.

We do have a mission, in a way – to make farming cool – Andrew Tsui

Fabrega agrees. He says that when the team give family workshops on farming, the parents often end up learning as much as the kids. “We’re teaching them, even though it’s at a basic level.” By seeing the origins and ecological consequences of their foods, they might just choose to be a little bit less wasteful with their groceries – encouraging greater overall sustainability.

Ultimately, Tsui’s dream is that a restful break on a rooftop farm will become ingrained in everyone’s daily routine. “I use the analogy of coffee,” says Tsui – something that was once a luxury, but which became a lifestyle, through sheer convenience. If he had his way, a trip to the farm would be as essential as a morning caffeine fix. “We do have a mission, in a way – to make farming cool.”

To better understand the broader potential for rooftop farming, I later met up with Matthew Pryor at the Division of Landscape Architecture of the University of Hong Kong (HKU). Originally from the UK, Pryor moved to Hong 25 years ago. “The pace of it, the intensity become addictive – you don’t love it but you can’t do without it.”

I catch Pryor when he is hard at work on his current project, as he attempts to model the city to estimate the total space that could be devoted to rooftop farms. His first estimates are surprisingly vast – 695 hectares in total, nearly five times the size of London’s Hyde Park or twice the size of New York’s Central Park. “Now the total ground level farmable land in Hong Kong was about 420 hectares,” he says. “So there’s more on the roof than there is on the ground.”

They are spending a huge amount of time on the street. Couldn’t they be spending that time on the roof?

As part of this work, Pryor has conducted a series of surveys of the rooftop farms in Hong Kong – finding around 60 in progress so far.  “What attracts me to this is that they are completely unconnected, spontaneous – 60 groups who have had the same idea, at the same time, and they’ve gone out there and done it.”

Like Tsui, he sees the rooftop farms as an efficient form of social welfare – particularly for the elderly. “Life expectancy in Hong Kong is now at 90 – we outlive the rest of the world,” he says. “But there’s nowhere for the elderly to go. They are spending a huge amount of time on the street – particularly the low-income elderly. Couldn’t they be spending that time on the roof?” Light exercise and regular social contact, are, after all, two of the best ways to stave off dementia.

As a bonus, he says, the rooftop farms can also provide thermal insulation and sound insulation – saving the building’s overall energy consumption through air conditioning. “I’d like to persuade the government to recognise rooftop farms as a legitimate use,” he says – so they could be integrated into the city planning.

After chatting in his office, we potter around HKU’s own rooftop farm, which Pryor helped found with students and other staff members. He recalls that when he first got permission to build the farm, he had to lug tonnes of soil and compost up the final flight of stairs – while remaining silent for an exam in progress. Everything I see up here is recycled or reclaimed from construction sites, and they even have a wormery to produce their own compost. He has made sure that that each pot is anchored by the weight of its own soil – a crucial measure come typhoon season. So far, there have been no disasters during the heavy storms.

The biggest challenge is protecting the crops from sulphur-crested cockatoos

Recently, his biggest challenge is protecting the crops from sulphur-crested cockatoos. “They are a real pest – really noisy, aggressive animals.” When I visit this February afternoon, however, the university farm is astonishingly peaceful, with an extraordinary view into the mountains ringing the city. “Most people come in the late afternoon and watch the sunset,” he says.

Could this be a glimpse of the future for city-dwellers everywhere? “Hong Kong offers a kind of laboratory,” Pryor says. “When we’re teaching, we say ‘Don’t take Hong Kong as the norm, take it as the extreme.’” If the rooftop farms can catch on here – and become as popular as coffee – it seems only a matter of time before they become a regular fixture across the world.

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David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter.

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