A roof grows in Brooklyn.

One of the nation's biggest green roofs on a sports arena is taking shape atop Brooklyn's Barclays Center, where cranes are hoisting tons of steel beams and decking that will provide the base for an urban prairie in the sky.

Some sections of green are already sprouting. And when the project is complete by the end of the summer, the 3-acre curving rooftop is expected to be a hardy, verdant dome of grasses, flowers and plants.

Planners say the roof will not only make the hulking arena look nicer, it will also dampen noise from loud concerts and basketball games, and act as a sponge to capture about 1 million gallons of storm water to keep it from draining into city streets and waterways.

"We're very excited about the green roof because it will add additional texture and color and send an extraordinary message about urban areas as innovative, sustainable and attractive," said MaryAnne Gilmartin, president and CEO of Forest City Ratner Cos., which built the arena.

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There's one caveat: The public won't have access to the 130,000-square-foot rooftop. Steep angles and curves make strolling through the grass too dangerous.

Linda Chiarelli, the project's head of construction, said the design is high-tech, using 35,000 "trays" of 4-inch-deep growing media, a lightweight mix of synthetic and organic materials, and weather-resistant grasses that do not require watering unless there's a severe drought.

Although a green roof was included in the original design for Barclays Center, financial difficulties prevented it from becoming part of the final plan before the $1 billion, 18,000-seat arena opened its doors in 2012 as the home to the NBA's Brooklyn Nets and major entertainment events.

Designed by SHoP Architects, the Barclay's roof is a joint project between Forest City Ratner and Greenland USA. Developers declined to say how much it will cost.

Another major sports arena with a massive green roof is the 113,000 square feet of grasses atop the Target Center in Minneapolis, home to the Minnesota Timberwolves.