In his late teens and early 20s, Stony Ng Leung-kwai went on many "wild camps": multi-day trips into the Hong Kong wilderness with a focus on survival. For food, his friends hunted wild boar, wild cattle, birds and fish, but Ng "was afraid of killing the living, so I foraged for plants instead".

"I always tried to look for the tastiest ones," he says.

Foraging may seem an unlikely undertaking in Hong Kong - the city is seen more as a concrete jungle than a tropical one, and has one of the highest per capita concentrations of restaurants and cafés in the world - but for Ng, it is a way to appreciate the wild and the beauty of the local flora.

"It feels very close to nature, to eat it. To forage means to have a very in-depth knowledge of plants," he says.

For Wong San, 66, an avid hiker, foraging offers a return to a simpler, more ancestral way of doing things.

"It's not to say that eating Japanese cuisine or ramen isn't good, but we do have to protect traditions and old ways," he says.

For Jun Wu, 36, the chef and owner of American restaurant Amuse, in Shanghai, foraging satisfies his desire to eat a more wholesome diet.

"My definition of healthy is to eat as natural as possible," he says.

Forager gives dining out a wild new meaning

For millennia, our hunter-gather ancestors subsisted on wild animals and foraged plants. It was only with the advent of agriculture and, subsequently, the development of cities, that we became increasingly sedentary creatures and foraging took a back seat.

"Historically, humans have had a lot of knowledge about the types of plants that could be eaten and in what quantity, and also what part of the plant could be eaten, or has specific properties that could cure some disease or heal wounds," says Benoit Guénard, an assistant professor at the University of Hong Kong's School of Biological Sciences.

"As an ecologist, I believe that we have a need for nature and having some contact with nature."

For a few years, Ng rented a house with a couple of friends in Sam A Tsuen, a village nestled deep within the Plover Cove Country Park, in northeast Hong Kong, accessible only by ferry or a 5km trek from the nearest road. They used the house as a base during holidays and weekends, and Ng remembers the foraged ingredients that went into their communal meals: tempura, stir-fried white ginger lilies with mutton and amaranth soup with pork liver.

Thirty years later, the 53-year-old is still foraging. He reckons he's eaten at least 80 per cent of the 60 to 70 wild fruits found in Hong Kong, and some 50 of the several hundred wild vegetables.

"But most aren't that tasty, of course," he adds.

With a wife and two daughters, aged 16 and 24, to take care of, Ng doesn't camp as much these days, but he still leads trips into the wild as a freelance guide for the Eco Green tour company and teaches courses for the likes of the Federation of Trade Unions on wild foraging. On his first day with a new class, he arrives with a bento box full of home-made goodies produced with foraged ingredients, such as pickles, rice-balls with sundried vegetables and home-brewed tea. It's his way of telling his students that you can make a hearty meal out of found ingredients, he says.

"Some of my friends really hate me. They like to watch birds, and think that I'm competing with the birds for their food," Ng jokes. "But I only eat a very small amount."

I meet Ng on a Wednesday morning at Fanling train station. We hop in a taxi and head to Luk Keng, a semi-abandoned farming village that, according to Ng, is one of the best locations in Hong Kong in which to look for food.

"Generally, it's best to forage close to villages. Wild vegetables grow in the cracks on the sides of houses.

"Edible weeds are usually located lower down, close to where humans live, and less so up on the hills," he explains. Just by walking through the village, "you can easily find over 100 types of plant".

Ng is tanned, friendly and chatty, and he soon strikes up a conversation with the taxi driver. He discovers our cabbie is from Sam A Tsuen.

"Then you must be surnamed Tsang?" Ng asks. He is indeed. "Do you know Ah Duen?" It turns out that Ah Duen - whom Ng has known for about 20 years - is the driver's younger brother.

"It's a small world," says the driver, chuckling.

Even before we enter Luk Keng, Ng points out wild edible plants growing beside a couple of old plant pots.

"That's black nightshade," he says, indicating a shrub with wide, ovate leaves. Its berries look like miniature tomatoes. Ng says he would sometimes make a turnip and chicken soup with this weed, but in Chinese medicinal terms, it is known as a "cooling" plant, and eating too much will cause diarrhoea and vomiting.

A few more steps and Ng stops again; he has spotted another edible plant: a hairy burr-marigold, commonly known as black jack or beggarticks. He picks one of its flowers and gently throws it onto my shirt. It sticks. Thanks to the flower's stiff, needle-like bristles, it catches onto clothing and fur - a mechanism that helps the plant disperse its seeds. The flowers are bitter, though.

"One way to deal with the bitterness? Tempura," says Ng. Another way is to boil the bitterness off in salted water. In Taiwan, the flower is commonly used to make herbal tea. "It's very good for relieving the summer heat."

Further down the concrete path cutting through the village - old houses, some of them abandoned and with broken windows, stand to our right, a fish pond lies to our left - Ng squats down next to a low wall outside a house. Growing at its foot are a few shrubs of mock ginseng, also known as panicled fameflower. The small ovate leaves work well in stir-fries, but they're easily overcooked, turning black and "very ugly", Ng says. He digs into the dirt with his fingers and pulls up the plant. The root is a white, fleshy colour, and is shaped like miniature ginseng - hence its name.

"You can peel the root, marinade it and make it into a pickle. Cut it into shreds and serve it like burdock," Ng says. He points to another large shrub nearby. "There's so much here, it's so easy to eat a filling meal."

As we continue, making our way now towards freshwater marshes, Ng points out many more plants. Next to an old shack, overgrown with vines and weeds, he picks some skunk vine ("You can shred it and stir fry it with eggs."). Close by, hanging from a vine, he spots a creeping fig, green and slightly larger than a golf ball. He picks it off, places it on a concrete slab and stomps down hard with one foot, cracking it open.

"I wouldn't usually eat it. The texture is hard to work with," he says. "But, if it's ripe, you can juice the fruit and use it to make grass jelly."

Growing in the marsh are several taro plants with large, fan-like leaves. The root of this plant is perfectly edible, Ng says. The catch: just five metres away is a very similar, very poisonous wild taro: the Alocasia macrorrhiza.

A friend of his - a teacher of survival techniques - tried this plant, also known as elephant ear taro, boiling the tuber, changing the water and boiling it again, "but it wasn't enough to do away with the poison," says Ng. The friend cut a small cube of the taro and popped it into his mouth to taste it. "His mouth became so swollen that he couldn't open it to take the cube back out. He had to go to the emergency room."

Therein lies the danger of foraging: the risk of eating something toxic, with possibly fatal consequences.

"There's a lot of trial and error," says Guénard. Both animals and humans have to deal with this risk when foraging, but some creatures have the advantage of being able to detect the chemical cues plants give off to warn that they are poisonous.

"Insects evolve in a different kind of world, which is more chemical than visual, whereas us humans" rely more on visual cues, explains Guénard. And many of us in the modern world have never learned those cues.

A week after foraging with Ng, I meet Wong in Tsuen Wan. We take a minibus up to the Shing Mun Reservoir, getting off at Pineapple Dam - an appropriate start to our foraging trip.

The dam gets its name from the pineapples that Hakka villagers used to grow nearby, and Wong says he used to pick the fruit here in the 1970s.

However, "once the village was abandoned, there were no more pineapples", he says.

Wong, known as Mountain Wong to his hiker friends, is semi-retired. He runs an outdoor equipment store and occasionally organises walking and hiking tours. He grew up at the foot of Lion Rock, and remembers heading up the hill to forage for wild fruit as a child.

We start to hike a short section of stage seven of the 100km MacLehose Trail. As we climb stone steps, Wong points to a thick tree root, a metre or so of which is growing above ground. The white spots of fungi growing on the root reflect light in the dark and are helpful for navigation at night, he says. The fact the root is growing above ground and has fungi on its surface means that it is in a windy spot.

"It tells you that this place is awesome," says Wong. As if on cue, a breeze blows - something we haven't felt elsewhere on the trail. "Feng shui might sound like a superstition, but it's really just the dynamics and interactions of nature."

We walk on and come across a trench that dates from the second world war. It is part of a network of fortifications built as a defence against a Japanese invasion. Next to a tunnel opening, Wong sees a spotted ardisia, with slender, dark green leaves and a red berry about to burst. He picks off a leaf; no white sap comes out. He turns the leaf around and feels its underside; it has no hairs.

"That means it's edible," he says.

The day is overcast but, like every other summer day in Hong Kong, it is hot and humid. Wong pulls out a bottle of chilled, sweet, home-made wild chrysanthemum tea and offers it to me. He tells me he picked the chrysanthemums while travelling in Yunnan province last year, and dried them in the sun. After that, all he has to do is boil some in water and add a little sugar.

Wild dandelions are everywhere and, it turns out, the plant is very useful. Its receptacles can be chopped, ground and applied to snake and insect bites to reduce pain and itchiness, Wong explains.

"People like to drink a lot of coffee these days, don't they?" he says. "I tell them to pick these flowers and make tea with it. It gives off a very strong coffee aroma."

Despite the abundance of edible plants in Hong Kong, there isn't much of a foraging community here. Within country parks, the government prohibits the cutting, picking or uprooting of any plant without a permit, and those who want to forage in private areas or in villages such as Luk Keng need to first ask for permission from residents.

Wu says people look at him as if he's weird whenever he goes foraging around the Shing Mun Reservoir. He did so regularly until recently, when he moved to Shanghai to open his restaurant, in which he uses some foraged ingredients as well as home-grown herbs.

Born in Guangzhou, Wu managed his own hedge fund in Los Angeles before moving in 2012 to Hong Kong, where he and his wife, Priscilla, lived in Kwai Chung, ran a food blog (homemadeinhk.com) and operated a private kitchen.

"I try to use foraged ingredients as much as possible, especially when it comes to herbs that are growing in the area, such as rosemary, Thai basil and lemongrass," which works wonderfully in a marinade for pork chops and chicken wings. The basil and lemongrass do not grow naturally in Hong Kong, but often foragers will stumble across exotic plants that have sprouted from seeds dispersed from home gardens and elsewhere.

"I usually just walked around the neighbourhood, where we have lots of trails and hills," says Wu. "There are lots of herbs and dandelions. I picked leaves, brought them home and made a salad."

The first time he came across a wild passion fruit, though, Wu didn't recognise it.

It was "very small, cherry sized, bright orange", he says. "I peeled it, and the fragrance just came rushing out." He took it home and made a crème brûlée with it. "Totally unexpected," he says. "One of my great creations."

Wu has foraged for wild bananas as well, and uses the leaves for cooking and steaming, as well as for plate decorations.

Having left Luk Keng, Ng and I take a minibus back to the train station. As we are about to part, he points to a green shrub growing in a roadside barrier by the taxi stand.

"That's edible, too," he says. "It's related to asparagus."

A free-growing treat is never far away from the master forager, it seems, even as the traffic lights ping and trains rumble past.

"We are city people, but want to return to nature," says Ng, as we go our separate ways.