(California Academy of Sciences's grassy rooftop)

When Renzo Piano's instantly lauded California Academy of Sciences opened in San Francisco last week, its stunning, two-and-a-half acre roof, where circular windows pop out into a rolling landscape blanketed with 1.7 million plants -- made everyone sit up.

Atop an airy, classical pavilion, here was "green architecture" right in front of your eyes: Piano's floating exaggeration of the landscape of Golden Gate Park where the new $488 million museum is located.

But wait a minute. Haven't we seen a hill of grass filled with circular lights on the top of a building here in Manhattan recently?

(HWKN's "Mini Rooftop")

Indeed we have. An illuminated, grassy knoll was the central feature of HWKN's "Mini Rooftop": a day-and-night party space paid for by the German car company that ran for 10 fun-filled days during fashion week.

"Building a hill in such an urban setting made people reconnect with nature," said Marc Kushner, a partner in the New York firm.  Pursuing what it refers to as Econic Design, HWKN aims to put nature's forms -- streams, hills, even trees -- back into architectural design.

Although on a tiny scale compared to Piano's spectacular raised landscape -- his grass-laden roof further acts to warm the museum, reducing its heating bill -- it's a stance that we are likely see more of. The days when "green design" meant installing solar panels, self-circulating water, and other eco-friendly systems are over. Now nature has to be not only a construction material but a formal element that makes you want to embrace the building itself. --DAVID HAY

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