Some people take their principles to their grave. And for some baby boomers, that means planning for their funerals, or their parents' funerals, in an eco-friendly way.

Shedden Farley's mother, Linda Farley, was passionate about a lot of causes, including the environment. When she died of cancer in 2009 at age 80, the family knew they wanted a natural burial that would have a minimal ecological impact, said Farley, a general contractor in Fort Collins.

Many traditional cultures practice natural burials that don't interfere with the organic process of decomposition. In modern North America, however, it has become common to place an embalmed body in a wood or metal casket and inter it in a concrete vault. Cemeteries are often flat, with landscaped lawns and rows of headstones.

Because there were no "green" cemeteries near Linda Farley's home in rural Wisconsin, the family got permission to bury her on their property.

"Not only did we dig the grave, but we put her in the grave and we filled the grave - and it was an amazingly cathartic thing to do," said Shedden Farley, one of five sons. "It was a way of being intimate with our mother right up until we would never see her again."

The experience spurred his father, Gene Farley, to suggest that the family set aside 25 acres, which has become the Natural Path Sanctuary, a nature preserve burial ground near Madison, Wis.

The environmental burial movement is mostly a recent phenomenon. Ramsey Creek preserve opened as the country's first "green" cemetery in Westminster, S.C., in 1998. It would take another seven years before the Green Burial Council, a California-based advocacy group, formed.

Today there are more than 40 certified green cemeteries in North America, according to Joe Sehee, the council's founder.

"When baby boomers start to die, I think you'll see a lot more of it. They have a keen interest in environmentalism," said Sehee, 53, who lives in Melbourne, Australia.

Natural Path Sanctuary, which has not sought council certification, opened in 2011 and has about 30 people buried there, including Gene Farley, who died last year at 86.

About 50 others have made pre-arrangements.