The low scrubland of densely packed succulents is in full fall color, a carpet of green fading brilliantly to red and gold. This 2.5-acre oasis, located among a barrens of blacktop roofs that stretches east to Broadway and west to the Hudson River, would be an impressive sight even if it wasn’t sitting atop the U.S. Postal Service’s 1933 landmark Morgan Processing and Distribution facility in midtown Manhattan.

The biggest green roof in New York City and one of the largest in the country, the Morgan facility’s verdant covering was completed in December 2008 and has thrived since. As the inscription above the landmark James Farley Post Office might have it, the roof has been affected by “neither snow nor rain nor heat nor gloom of night,” and has flourished through freezes and thaws, through summer rooftop temperatures that reach 150 degrees, and through weeks of drought and torrential summer storms, despite never being watered, weeded, or fertilized.

The 2.5-acre park on the Morgan Processing and Distribution facility is the biggest green roof in New York. The vegetation is a densely planted assemblage of some 25 hardy, low-growing species that have thrived in their few inches of planting material. The plants’ size and modest requirements, however, belie their substantial biological capacities and environmental benefits. Since the roof has been installed, the building’s storm water runoff into the New York municipal water system has been reduced by as much as 75 percent in summer and 40 percent in winter. The U.S. Postal Service estimates that the plants’ ability to cool the roof in summer and insulate it in winter will reduce the building’s energy costs by $30,000 a year.