The thousands of recently planted green and purple shrublike sedum lining the roof of Con Edison’s training center in Long Island City look a bit out of place in the shadow of Manhattan’s skyline. But the tiny absorbent leaves and modest but hardy roots of the sedum — typically found in desert climates — are at the center of a growing effort to reduce greenhouse gases, rainwater runoff and electricity demand in New York.

This month, Gov. David A. Paterson approved tax abatements to developers and building owners who install green roofs, or a layer of vegetation and rock that absorbs rainwater, insulates buildings and extends the lives of roofs. Sedum, which soaks up water quickly and releases it slowly, is an ideal plant for the job.

Europe has had green roofs for decades, and cities like Chicago and Seattle have added many of them in recent years. But there are fewer in New York because of the cost of installing them compared with the benefits, which can be hard to quantify. The new one-year abatements, though, can cut as much as $100,000 a year from a building’s taxes, and are expected to turn what has largely been a hidden luxury into a standard feature of a little-seen part of the city’s landscape.

“This is just the beginning,” said Kari Elwell Katzander, a partner in Mingo Design, a landscape design firm in Manhattan that works on green roofs. “It’s not just about the green roof. This transcends into various ways to make buildings more “green”.”

There are few accurate reckonings of how much of the 944 million square feet of rooftops across New York City — 11.5 percent of the total building area — has gone green, or how much more could be cultivated. But clearly there is plenty of space available. Just in Long Island City, there are 667 acres of empty, flat roofs suitable for vegetation, according to Balmori Associates, an urban design company. That is the equivalent of 80 percent of Central Park.

The best locations for green roofs are buildings with large, flat tops well exposed to the sun. That is why many of the city’s green roofs are in industrial neighborhoods in the Bronx and Queens.

One of the largest installations was completed in 2005 at Silvercup Studios in Long Island City, where parts of the HBO series “The Sopranos” were filmed. About 1,500 plants, in 20 different species of red, yellow and green, cover 35,000 square feet.

Because of high labor and transportation prices in New York, green roofs can cost as much as $30 a square foot to install in the city, up to three times more than in other places. While the environmental benefits of green roofs are real, builders have had a hard time justifying the extra cost when it is unclear how it will affect their bottom line.

Green roofs, for instance, absorb as much as 70 percent of the rain that might otherwise overwhelm the city’s sewage system during heavy downfalls and run directly in the East River, the Hudson River and New York Harbor. By diverting the runoff, the city could prevent millions of gallons of polluted water from reaching waterways.

But while runoff is a big problem for the city, it is a negligible one for individual building owners. Lawmakers and environmental activists, though, say that the new financial credits — worth $4.50 per square foot of vegetation — should prompt building owners to install green roofs that, over time, will help the city grapple with the growing problem of runoff.

“Essentially, cities are going to benefit more than any individuals will benefit because it will save with infrastructure costs,” said Diana Balmori of Balmori Associates, which helped install the green roof at Silvercup Studios. “It’s a modest help, what individuals receive, but it changes the way we think about infrastructure.”

When sunshine hits a blacktop roof, it heats the building beneath it as well as the area nearby. When it hits plants on a roof, in contrast, the plants not only absorb the sunshine, but cool the air when the water in their leaves evaporates.

Temperatures on buildings with green roofs are up to 30 percent lower during the daytime in the summer than they are on those with conventional roofs, which means that tenants on the floors below do not have to run their air-conditioning as much.

The savings can vary, though, depending on how well the windows are insulated and other variables.

And the average life of a typical roof can be doubled when a layer of plants rests on top. Con Ed’s 10,000-square-foot green roof, which was installed in July and is the first at one of its buildings, is more advanced than most projects. The company spent $200,000 to install 1,350 trays filled with 21,000 plants, including 15 varieties of sedum. The plants, which were cultivated at a nursery in Connecticut, sit in a mixture of volcanic rock, sandstone and other light stone capable of absorbing water.

The bottoms of the trays look like egg cartons; they allow a small amount of water to pool beneath the plants. The trays can easily be moved to provide access to the roof if there are leaks that have to be plugged. Con Ed chose sedum not only because it can absorb rainwater quickly, but also because it is not indigenous to New York, making it unlikely to attract potential pests. The plants also require very little maintenance.

Con Ed has teamed up with Columbia University’s Center for Climate Systems Research to evaluate the benefits, using rooftop sensors to measure the temperature, wind and water runoff. Con Edison said it hoped to use the findings to encourage customers to install green roofs themselves.

David Westman, the resource conservation coordinator at Con Edison, said, “It’s not only the right thing to do, but it can make economic sense.”