Twenty-something entrepreneur Kate Hofman had a bright idea: why not bring commercial vegetable growing into our cities, close to where most of the consumers live?

Admittedly, she isn’t the first person to attempt such a project, but where she differs from many is in how she plans to grow her produce. Kate is installing a system of farming called aquaponics, in which fish and their waste are used to grow fresh vegetables.

Aquaponics has been slowly attracting the support of sustainability enthusiasts in London and other cities around the world. The opening of a café and urban farm in East London back in 2010 set the ball rolling, showcasing how a tank of fish can be used to cultivate vegetables for salads. The company behind the installation, Aquaponics UK, now reports a steady flow of three or four orders a month for this pioneering system.

Soil-free farming technologies such as aquaponics represent an opportunity not only to return a level of personal or household food production to cities, but also to create a viable commercial urban farming sector. And whilst nobody’s predicting vast swathes of urban space being given over to productive agriculture (there simply isn’t enough room in densely populated cities with high land prices), there is a growing number of examples out there pointing to a growth of urban-based agriculture over the next decade. It just may not be the agriculture we know.

If you want to look for global examples of urban agriculture, you’ll be hard-pressed to find somewhere more in need of it than Singapore. This densely populated Asian state has been desperately trying to increase in size since the 1960s by reclaiming land from the sea. Even with plans to expand its surface area by 7% by 2020, it is still severely land scarce.

With little spare room for agriculture, unsurprisingly Singapore is heavily dependent on imports, which account for more than 90% of its food. Enter aeroponics, which presented the state with an opportunity to grow food in a limited amount of space. Under this farming system, plants are cultivated with their root systems suspended in the air while a nutrient solution, circulated in mist form, maintains a constant film of nutrients and moisture on the roots.

One of the world’s largest aeroponic farms has been running for more than a decade in Singapore, producing cut salads and herbs for local supermarkets – perishable products that are difficult to import.

According to local urban farming expert He Jie, of Nanyang Technological University in Singapore, the ability to grow vegetables without soil makes aeroponics well suited to a dense, urban setting. “While it isn’t possible for arable land to be expanded horizontally, an urban farming system could increase production area through vertical extensions of lightweight troughs,” she explains. “Vertical stacking is constrained by the weight of the troughs, but this could be resolved through the more lightweight aeroponic system.”

While hi-tech solutions like aquaponics and aeroponics are fascinating to document, there is also a larger but perhaps less glamorous urban farming sector based on more traditional techniques. Adrienne Attorp manages the social enterprise Cultivate London, which farms across multiple sites in West London, typically on derelict land on temporary lease from property developers.

Operated by a team of volunteers and experienced gardeners, the three sites are expected to be able to yield 40,000 kilos of fresh salads this year – a minuscule amount in the context of London’s overall food needs, but a successful demonstration of what kind of produce urban farming is likely to expand around. They also produce flowers, herbs and vegetables, but it is the salads that have proved to be the most popular and profitable product.

“I don’t think we can grow all the food London needs, but we could certainly grow all of London’s salads,” says Adrienne. “It wouldn’t require much space and it’s a high-value product that we’re currently importing from Italy and Spain, so it would make sense to grow it here.”

Adrienne says being peripatetic, as well as creative with the spaces available, is critical for aspiring urban farming enterprises, particularly, perhaps, in cities like London where available derelict land is both scarce and expensive.

In New York, space is also at a premium, and this has led to the emergence of a number of innovative urban farming projects, such as Eagle Street Rooftop Farm, a 6,000-square-foot organic vegetable farm on top of a warehouse, with views across to Manhattan Island and the Empire State Building. Opened in 2009, it now supplies local restaurants, as well as selling produce through a weekly farm market. It was designed and built by a company called Goode Green, which has installed a number of other successful rooftop farms, including one on top of a New York hotel.

However, it is the birth of a number of much larger-scale projects that may herald a real step change towards an urban farming sector both in the US and elsewhere. BrightFarms has recently struck a deal to build in Brooklyn what it claims will be the US’s largest rooftop greenhouse, producing enough vegetables to feed 5,000 people. It says the project will help cut out the transport costs of importing produce into New York from the major vegetable-growing regions on the west coast of the USA, as well as producing fresher crops. And in April this year, the world’s largest indoor vertical farm opened in a suburb of Chicago, making use of an abandoned warehouse site to grow fresh basil and salads.

In a unique partnership with a supermarket in London, Food from the Sky presents another example. Here, food produced on the roof with the help of volunteers is sold downstairs in the supermarket the same day it is picked. The fresh salads are usually sold out by early afternoon to a stream of loyal customers. It is very much a shoestring business, with Jack Astbury running the whole project on a part-time basis. Sales of £7,500 enabled him to work one day a week in 2012. He is aiming to push that up to £11,000 this year to cover the farm’s running costs and his wages for two days a week.

By restricting produce to leafy green vegetables, Jack believes urban farming can be profitable. “It is possible [to make urban farming a business] because your customer and buyer are so close and all around you in a city,” he says. “But I think the future is in intensive solutions that can maximise the yield in small spaces, because it’s really hard to make money when food is so cheap and city space is so expensive.”

While profitability may necessitate the emergence of intensive and specialised urban farming models, there is also another, perhaps forgotten, social and educational role that urban farming can play. A report by Kubi Ackerman of Columbia University’s Earth Institute points out other benefits, including utilising neglected space, educating the public on food, and benefiting deprived neighbourhoods through the availability of healthy foods. All of these could yet entice private companies, charities and city governments to support and develop a second, less profit-driven, urban farming sector.

Tom Levitt is a food and farming journalist.