Climb to a rooftop and scan the horizon in Mexico City, and you're likely to see many nearby rooftops or balconies with vegetable gardens.

Urban gardening is booming in Mexico's capital, sponsored by a city government that sees gardening as a way to alleviate poverty, provide residents with their own healthy food and add some green to one of the world's most populous cities.

In a program that started five years ago, Mexico City's municipal government has given grants to 3,080 families to build gardens on their rooftops. Many more families have attended urban gardening classes and struck out on their own to grow tomatoes, lettuce, chilies, scallions, guavas, passion fruit and other edibles. It is a trend that many other governments are looking at closely as the populations of the world's cities explode.

"There wasn't anything up here before," Sergio Hernandez Rodriguez said from his rooftop, where garden beds now display an array of corn, celery, chilies and herbs. Off to the side, his wife worked inside a small greenhouse. "I'm hoping to grow strawberries in here soon," Estela Lopez said as she showed off a simple watering system created using a pump made for a fish tank.

The couple spends hours each day tending to their rooftop garden, and the project is already paying off — literally. "I sell vegetables to my neighbors," Lopez said. "They know the vegetables are very clean."

Mexico City's small-scale urban gardening project has gained momentum. "We've had growth of about 30 percent a year in the number of projects," said Armando Volterrani, a project manager with the city program.

Residents eager to test their green thumb in Mexico City, a metropolis of over 20 million people, often need help to learn how to grow vegetables. "There are different microclimates all over the city, and rainfall also varies," explained Margarita Garcia, deputy director of the city's sustainable agriculture program.

Scattered about the city are demonstration gardens where volunteers tell visitors about the tax breaks they'll receive for having vegetation on their property and answer questions about how to grow vegetables on a balcony or rooftop.

At one of the demonstration gardens, co-founder Carolina Lukac explained what she tries to convey to those curious about starting a garden. "Among the benefits of harvesting in your own home is that you don't use chemicals and the fruits and vegetables are more delicious. Once you pluck them, an hour later you are eating them. They are also a lot more nutritious," Lukac said.

She dismissed concerns that growing vegetables in the sometimes visibly polluted air of the city could affect one's health. "With a good rinsing, they are fine," Lukac said. A specialist with the city government agreed. "We have a mobile lab that determines whether the vegetables are safe or not," said Vanessa Morales, an engineer who works with the urban farming program. "So far, we have seen nothing that would negatively affect people's health."

Lukac said the vast majority of those who visit her demonstration garden are women, many of them young. "I see this do-it-yourself spirit as stronger among young people," she said.

For those residents who need help getting their garden started and meet certain criteria, the city program offers grants of up to US$2,500 (approximately NT$73,000). It also helps that most buildings in the city have flat rooftops. "Many buildings are made with accessibility to the rooftop," Lukac said. "So those spaces are excellent places to grow food in the city."