One night in Singapore was all it took to convince Margaret Chen that she needed more greenery in her life.
"I stayed with a friend who lives next to an area with big canopy trees that's basically like a jungle, and they keep their windows open all the time," she said.
"I have pretty bad allergies, but after one night of sleeping there, my sinuses were all clear."
That experience inspired Chen to launch SquAir Spaces, a not-for-profit enterprise that designs, installs and maintains vertical gardens.
In a city with bad air pollution and cramped apartments, Chen says bringing greenery indoors is an easy path to better air quality and peace of mind. "It's beneficial for you in terms of the air you breathe and it's also good for your mental well-being because it's so relaxing," she said.
Most attention-grabbing vertical gardens are massive commercial installations like French botanist Patrick Blanc's 2,500 sq ft green wall in the lobby of Hotel Icon in Tsim Sha Tsui, but a new crop of local companies offer more modest green walls perfect for Hong Kong's small flats.
"Indoor wall gardens are usually all-season with very low maintenance and very low light requirements," said Jay Yang, a product designer for Ecotropolis, which makes and installs green walls. "It is also very appealing visually and you will definitely feel a difference in the air."
That's not an overstatement - even the most common houseplants can do a lot to clear the air.
In 1989, research by the US space agency Nasa on the International Space Station found that common indoor plants eliminate toxins such as benzene and formaldehyde, which are commonly found in vehicle exhaust.
"Even most air purifiers don't take away this kind of chemical pollution," said Chen.
The Nasa study concluded that a 900 sq ft apartment needs just eight large houseplants to improve the quality of its atmosphere significantly.
Golden pothos, peace lilies and chrysanthemum are some of the most effective air scrubbers.
Green walls take many forms - Blanc's multimillion-dollar installations are renowned for being entirely soil-free - but most residential walls consist of permeable pouches filled with soil. "The product design itself needs to be focused on user-friendliness and low cost," said Yang. Both his pouches and those offered by SquAir Spaces are made from recycled plastic bottles, which creates a breathable felt that allows air to circulate through the soil and roots.
A waterproof moisture barrier keeps water from dripping on the floor. The pouches can be mounted directly on the wall, on a trellis or suspended by wires - SquAir Spaces uses a wire system normally employed by art galleries, and it also sells a series of elaborately framed pouches that Chen calls "living art", for HK$3,800 each.
Individual tabletop pouches cost HK$880 from SquAir Spaces while a large wall installation costs around HK$25,000, including biweekly maintenance visits for three months and a year's warranty. Ecotropolis sells its pouches for HK$265 each, including warranty.
Another company, HK Green Walls, offers even more elaborate vertical gardens with automatic irrigation, aeration and lighting systems, which range in price from HK$3,000 to HK$50,000.
"Hong Kong people are very busy so this is good for people who can't always be around to water their plants," said director Christy Li. While that cuts down on maintenance, it doesn't eliminate it completely: "You still need to trim the plants and check on them," she added.
Chen doesn't offer automatic irrigation as a matter of principle. "We have an educational mission," she said. "People in Hong Kong have very little plant knowledge. All of our walls are human-scaled so people can touch the plants and feel when they need to be watered.
"A lot of people say, 'Oh, I have a black thumb, my plants always die,' but it's really about knowledge and confidence."
SquAir Spaces, Ecotropolis and HK Green Walls all employ landscape architects and plant specialists to evaluate a space and determine the best placement for a wall and the best plants to populate it.
"If someone doesn't have a green thumb, we'll give them plants that need less maintenance," like succulents and cacti, says SquAir Spaces partner Serinna Chau. "We pick the right plants for the right conditions."
Maintaining the green wall is fairly straightforward. Chen said it is important to keep your flat well-ventilated to avoid mould.
She also said it is essential to test the soil before watering. "Most people water their plants too often," which can waterlog the soil and cause root rot, which could eventually kill the plant.
SquAir Spaces officially began business in February and it has so far installed green walls for three residential clients.
HK Green Walls has done a half-dozen residential projects since it launched two years ago. "It's most popular with foreigners, because they miss their gardens," said Li.
But building a local market is more challenging. "Hong Kong people might love the idea but they think they don't have enough time to take care of it."
That's something Chen hopes to change by soon installing green walls in schools in the city.