The Green Prison movement

"It's nice to be out in the elements," said Knowles, 42, stirring dark, rich compost that will amend the soil at the small farm where he and fellow inmates of the Cedar Creek Corrections Center grew 8,000 pounds of organic vegetables this year.

Inmates of the minimum-security facility, 25 miles from Olympia, the state capital, raise bees, grow organic tomatoes and lettuce, compost 100 percent of food waste and even recycle shoe scraps that are made into playground turf.

"It reduces cost, reduces our damaging impact on the environment, engages inmates as students," said Eldon Vail, secretary of the Washington Department of Corrections, which oversees 15 prisons and 18,000 offenders. "It's good security."

As around-the-clock operations, prisons are voracious resource hogs, and administrators are under increasing pressure to reduce waste and conserve energy and water.

In 2007, states spent more than $49 billion to feed, house, clothe, treat and supervise 2.3 million offenders, the Pew Center on the States reported this year.

As the prison population has grown this decade, up 76 percent from 1.3 million in 2000, the number of prisons and jails has risen with it. The latest U.S. Bureau of Justice data show 1,821 facilities in 2005, up from 1,668 in 2000.