Imagine a farm. What do you see?

Rolling hills? Lots of open land where crops can grow? Wide open spaces?

That's a pretty normal picture. After all, that's how farming is usually done: on large plots of lands in rural areas.

But in a country like Singapore, which is facing serious land scarcity, that's not really an option.

Enter vertical farming. A company called Sky Greens Farms has just built the world's first commercial vertical farm in Singapore.

So what is vertical farming? It works like this: instead of planting vegetables in the ground, you grow them in planters, installed into vertical towers made out of metal.

The technology allows farming to take place in urban areas, and also reduces the amount of land needed to grow produce.

This is especially important in Singapore. The entire country is only 710 square kilometres, and there's very little land left over for farming. The country currently grows only seven per cent of its vegetables locally.

Right now, the Sky Greens Farms facility produces 0.5 tonnes of vegetables each day. They're hoping to attract investors so that they can upgrade and add enough towers to produce a full two tonnes a day.

The farm grows three types of vegetables, and sells them through local supermarkets.

They're a little more expensive than vegetables from other sources (10 to 20 cents more each). But apparently customers are enthusiastic about the products: supermarkets are struggling to keep the vegetables in stock.

The idea of vertical farming has been around for a while - futurists, environmentalists and sustainable development experts often talk about it - but until now, no commercial vertical farming facilities had gone into production.

Defenders of the technology say it's a great way to grow food where people are actually living - some studies predict that 70 per cent of the world's population will live in urban areas by the year 2050.

Farming within those urban areas could cut down on transportation costs and carbon emissions, as well as providing better access to fresh, healthy food.

And the hydroponic technology used to grow crops in vertical farming consumes as little as 10 per cent of the water used in traditional farming, according to Dickson Despommier, a professor of public and environmental health at Columbia University.

But not everyone is convinced that the idea can work. Back in 2010, 'The Economist' questioned the amount of lighting needed to grow food in a vertical farm.

The prohibitive cost of that much energy might be worthwhile in a place like the Antarctic, where importing food is so expensive. But the author said keeping a farm going year-round in other places might not be feasible.

And land in cities is very expensive. Critics of vertical farming question whether the economics make sense.

"I think it's going to be really hrd to make the numbers work out," said Ryan Avent, a U.S. blogger with an interest in urban land use and a contributing editor to 'The Economist.'

Steel-and-concrete buildings, without fancy finishing, cost $300 per square foot or more in large cities according to Avent. That's about $13 million per acre.

Farmland in Indiana costs $3,000 per acre.

For now, the Sky Greens Farms facility is up and running in Singapore.

It will be interesting to see how the company and its commercial vertical farm does does. If it succeeds, maybe other countries that are facing land shortages will experiment with vertical farming.

If nothing else, it could lead to some fascinating new city skylines - featuring a healthy serving of vegetables.

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