PORTLAND, Ore. — Urban gardening used to seem subversive. People planted tomatoes in public parks, strung their hops to rooftops to make homebrew and reclaimed empty lots as community farms, never mind the property owner.

Yet here in one of the more thoroughly tilled cities in America, subversive has come full circle: the federal government plans to plant its own bold garden directly above a downtown plaza. As part of a $133 million renovation, the General Services Administration is planning to cultivate “vegetated fins” that will grow more than 200 feet high on the western facade of the main federal building here, a vertical garden that changes with the seasons and nurtures plants that yield energy savings.

“They will bloom in the spring and summer when you want the shade, and then they will go away in the winter when you want to let the light in,” said Bob Peck, commissioner of public buildings for the G.S.A. “Don’t ask me how you get them irrigated.”

Rainwater, captured on the roof, and perhaps even “gray water” recycled from the interior plumbing are both possibilities, the architects say. But they concede that they are still figuring out some of the finer points of renovating the Edith Green-Wendell Wyatt Federal Building, which was completed in 1975 and is currently 18 stories of concrete, glass and minimal inspiration.