The city also plans to offer tax breaks for businesses or individuals who put gardens on top of their offices and apartment buildings.

Left-wing Mayor Marcelo Ebrard has pledged $5.5 billion over five years to reduce greenhouse gases in Mexico City, home to some 20 million people and 4 million cars.

"These are not generic objectives or wishes -- we have a clear goal," Ebrard said at an event to inaugurate the environmental plan.

The aim is to cut carbon emissions by 4.4 million tonnes a year, still a fraction of the 643 million tonnes of gas Mexico produces nationwide each year, ranking it among the world's top polluters.

The mayor has encouraged cycling by providing bicycle paths and some car-free roads on weekends.

His program also aims to capture gas that bubbles up from overflowing landfills and calls for a new subway line and more express bus routes.

Putting plants on roofs soaks up some of the carbon dioxide belched out by cars and factories, one of the main causes of climate change, the US Environmental Protection Agency says.

So far two buildings owned by the public transportation system have layered soil and grass seed over a mesh drainage system on their roofs.

On top of a school for the children of subway workers, gravel paths now wind through patches of grass and a small garden. Bushes grow around air vents and piping.

Maintenance workers say keeping the garden lush is hard under the pounding Mexican sun but worth the effort, even just for the schoolchildren who now climb up there to play.

"Most of the children don't have access to any green places, this is another world for them," said school administrator Juan Rivero as he surveyed his new urban oasis.

The city boasted blue skies and ample greenery as recently as the 1960s. But it has become clogged by traffic and dirtied by fumes as the population exploded.