Farm Direct is a far bigger, more established venture. Set up in 2013, it operates a 100,000 sq ft site in Kam Tin and a 20,000 sq ft set-up in Fanling, where vegetables grow under plastic-and-netting shelters in raised containers fed with a flowing nutrient solution. The network is connected to a recycling system which filters the water before pumping it back into the containers.

Between them the two facilities produce 700kg of lettuces daily (seven varieties, ranging from romaine to green butterhead and red lollo) that supply restaurants and supermarkets as well as Farm Direct's own sales points at MTR stations. Co-founder Michael Ng Wai-hung regards his form of hydroponics as a protective method.

"Unlike indoor hydroponics, we make use of natural sunlight. The plastic canopy and netting protect the plants from rain, birds and butterflies. Our summers are wet, which brings lots of insects. But the pests tend to group at ground level. As the racks are high above ground, bug tapes are enough [to control pests]," Ng says. "A nursery supplying potted plants occupied the site before we came, so the land was already concreted over."

The hydroponics boom may be due in part to the promotional efforts of the Agriculture, Fisheries and Conservation Department.

It set up the Controlled Environment Hydroponic Research and Development Centre (dubbed iVeggie) in 2013 to provide technical support to people interested in setting up such ventures. The HK$6.5 million research facility occupies a 2,500 sq ft space within the premises of the Vegetable Marketing Organisation in Cheung Sha Wan.

With just 300 hectares of land devoted to vegetable cultivation in Hong Kong, agriculture officer Chan Siu-lun says hydroponics can greatly boost vegetable production by making better use of vacant industrial buildings and abandoned sites.

"It can be done in warehouses, which addresses the problem of land shortage in Hong Kong," says Chan, who serves as a consultant at iVeggie. "Our controlled environment is bacteria-free as workers have to wear special clothing. Not even a fly can get into the facility, so we don't need to use pesticides."

However, the hydroponics boom has drawn flak from Liber Research Community, a green group which recently released a 60-page report on the trend.

Liber spokesman Johnny Lau Hoi-lung says hydroponic farming has become a worrying fad because indoor facilities use a lot of water and electricity," he says.

Citing iVeggie, whose daily electricity bill is HK$1,000, Lau says "The centre's carbon footprint is 13 times that of a traditional farm. It is not environmentally sustainable."

Moreover, many hydroponic farms are being built on old farm sites. "They are agricultural land that has been concreted over," Lau says. "On one of our visits, we saw sand being dumped at a wetland site in Kam Tin to construct a hydroponic farm. The original aim of hydroponics is to make good use of contaminated land for farming. Like in Japan, where land contaminated with radiation is converted into hydroponics facilities. But in Hong Kong, good fertile farmland is being used for hydroponics."

Roy Ng Hei-man, assistant campaign manager of the Conservancy Association, shares similar concerns.

"We have received reports that good farm land in Fanling and Kam Tin is being concreted for hydroponics ventures. While they may make economic sense, it's not a form of sustainable agriculture."

Although his IPC Foodlab venture in Fanling also engages in indoor growing, CEO Louis-Antoine Giroud does not favour hydroponics. Giroud, who also serves as executive chef at IPC's restaurant, says most people in his native France wouldn't embrace hydroponics.

"It's only found in cities where people want short-cuts; the texture of hydroponic vegetables is odd. They taste watery," he says. "It's natural for plants to grow in soil."

IPC Foodlab, which primarily cultivates mushrooms in its converted factory building in Fanling, also grows some herbs, such as basil, indoors for its restaurant.

"There's a controlled environment with set lighting, temperature and fans. But we still use soil," Giroud says.

"With plants grown on our rooftop beds, worms and ants are necessary [to improve soil quality]. There are butterflies and [pests] eat holes in our plants. But that is natural. We can't live in a non-bacteria world, we would die. If there were no bees, no worms and no ants, no food would grow naturally in the world."

There is generally little difference between regular and hydroponic greens in nutritional value, says Vicki Fong Lai-ying, a senior lecturer in food and nutritional science at the Hong Kong Institute of Vocational Education.

Water forms the biggest component in vegetables, with edible fibre and minerals making up 20 per cent. Because hydroponically grown plants have a higher water content, diners need to eat a bit more of them to compensate, Fong says.

Still, the range of vegetables suited for hydroponics is limited.

"Plants grown in soil develop sturdier roots because they must extend further to seek water and nutrients. So root and stem vegetables such as carrots and eggplants cannot be grown using hydroponics," Fong explains. The technique is more suitable for vegetables such as salad leaves, which have more delicate structures.

The hydroponics sector is likely to see even greater expansion under the government's plan to create an 80-hectare agricultural park to boost vegetable production. The proposal also calls for the setting up of a fund to provide matching grants to ventures adopting more sophisticated farm methods, including hydroponics.

However, environmentalist Lau argues the government should not promote advanced techniques such as hydroponics without first conducting extensive research.

Setting up a hydroponics farm requires an investment of at least HK$3 million; so the matching grants will benefit bigger companies while neglecting small individual farmers, he says.

"We met farmers overseas who told us that produce from hydroponic farms have lower fibre content. There's a lack of local research on the nutritional value of such produce compared to those grown with traditional and organic methods. The government should do more research before pushing this type of farming."

Chan insists the government is not promoting hi-tech agriculture at the expense of conventional methods.

"We are just introducing another technique to the public, not trying to replace traditional types of farming," he says.

And as restaurateur Yuen sees it, recent criticism about the sustainability of hydroponic farms is unfair.

"If we did not grow the salad vegetables ourselves, we would have to import them from overseas, which would leave a much bigger carbon footprint because the greens would be flown in."

Moreover, much of the agricultural land in Hong Kong has poor soil, says Farm Direct's Michael Ng. "We also use a close-loop irrigation system, which recycles 95 per cent of water. In a traditional farm, much of the water runs off the land and goes to waste.

"Growing in soil [fertilisers are] leached out of the soil. But in a hydroponic farm, plants absorb all the nutrients in the solution."

Farm Direct, which invested HK$20 million in the hydroponic venture, has been doing a brisk business since setting up its own retail outlets, which offer competitive prices. Its salad leaves sell for HK$175 per kg, compared to about HK$250 for imports.

The venture has already broken even and Ng expects to see profits soon. "The market for salad vegetables in Hong Kong is far from saturated. There's still a lot of room for entrepreneurs like us."