A roof garden that you might call high-concept

Fans of Ken Smith, a landscape architect based in New York, were thrilled to hear that the Museum of Modern Art had commissioned him to design an extensive rooftop garden scheduled to open at the same time as the “Groundswell” exhibition. For the first time since Philip Johnson finished the Modern’s iconic sculpture garden in the 1950s, a designer would have a crack at adding to its outdoor spaces.

But unless they live in one of the apartment towers overlooking the museum, those fans shouldn’t get their hopes up about seeing Smith’s work, which covers about 17,000 square feet. It isn’t open to the public. Even the staff of the museum doesn’t have direct access to it. One New York tabloid dubbed it “MoMA’s secret garden.”

Maintenance crews are, of course, allowed to step out onto the roof where the design has been installed, above the museum’s sixth-floor galleries. But even they don’t have to visit regularly, since it doesn’t require any watering. The garden is made of crushed glass, rubber and plastic, among other materials, arranged into swells and swirls meant to resemble a camouflage pattern. For much of its brief existence, it has been buried under snow.

“We think of it more as an art project than a garden,” said Peter Reed, the curator of “Groundswell.” “We said, ‘Here’s a canvas we can use. Let’s do something with it.’ ”

Though Reed declined to discuss the budget for the garden, funded as part of the museum’s recently completed $425-million expansion, the project is without doubt a testament to the Modern’s unusually deep pockets. How many museums could afford to put a cheekily contemporary garden by a leading landscape architect on its roof just to improve the view for a few neighbors? The answer, at least in this country, is exactly one.

Anybody interested in walking through one of Smith’s designs is better off waiting until later this year, when the skyscraper known as 7 World Trade Center, designed by David Childs of Skidmore, Owings & Merrill, will open near the ground zero site in Lower Manhattan. At its feet will be a new park by Smith, shaped like a slice of pie and planted with boxwoods and azaleas. Yes, real ones.

In the meantime, the plans and some photographs of Smith’s MoMA garden are on display in “Groundswell” until the middle of May.

Reed, the curator, has carefully positioned those images underneath a skylight cut into the ceiling, so visitors to the show can get a sense of exactly where it is they’re not allowed to visit.