To Heat a Greenhouse, Add Some Chickens

Cerridwen Farm at Vermont’s Green Mountain College usually has around 60 chickens, but right now it’s more like 40. (Foxes have been an issue.) The birds are primarily laying hens, and in the summer, they have the range of the place. But they also have a chicken coop, built by students on the college’s farm crew and in an ecological design class to serve a dual purpose. When the chickens gather in their coop, they’re also heating the greenhouse.

As Northeasterners know, eating local produce in the winter time can be a dour exercise. But winter farmers’ markets are becoming more popular, and for many farmers, that means growing vegetables in greenhouses so that they can offer more than potatoes, squash, and onions. Heating those greenhouses, though, can suck up energy, and farms dedicated to sustainability need ways to keep their plants happy without running generators.

Integrating the chicken coop into the greenhouse helped the Cerridwen Farm solve that problem. The chickens' body heat contributes to keeping the greenhouse’s temperature up. They also help the plants grow by exhaling carbon dioxide. In a tightly closed space like a greenhouse, CO2 can run low as plants suck it up.

The birds alone can’t heat the greenhouse, though. “The chickens are just one aspect of it,” says Kenneth Mulder. “The whole system is designed for passive heating.” The back walls are made from recycled concrete, and water tanks absorb and store heat. “We haven’t burned any propane in three years,” Mulder says.

It was Mulder who proposed building an integrated system, but he wasn’t the first to come up with the idea, which was “blatantly stolen from Anna Edey,” he says.

Edey, who built a solar house on Martha’s Vineyard in the early 1980s, was living a green lifestyle long before it was hip. In 1983, she began work on a solar-powered greenhouse, where she would grow a variety of lettuces to sell as salad mix to high-end restaurants on the Vineyard and in Boston. She had about twice the number of chickens as Cerridwen Farm, “125 at most,” she says. “Plus about 30 rabbits.”

Edey also created heat for the greenhouse by composting the droppings the chickens left in their coop. Compost gives off heat as it decomposes, and Edey says that the temperature of hers could rise to 90 degrees Fahrenheit. “That made it very cozy for chickens and was a tremendous heat contribution to the greenhouse,” she said.

Mulder read about the idea in Edey's book about her experiences with her house and greenhouse. Edey, in turn, had lifted the idea from another horticulturist, an Oregon man who was heating his greenhouse with hundreds of rabbits that he’d later sell as meat. Edey found that she liked chickens better, though.

“Those cute little bunnies that seem so peaceful can turn into screaming, dangerous, fast animals. They have sharp teeth and sharp claws, and they can scream,” she says. She didn’t like that the males would fight and that the females had a tendency to eat their own young. “I got very disillusioned with rabbits,” she says. “Chickens have no tendencies like that at all. They purr like happy cats. I never had any problems with my chickens.”

Photo via Green Mountain College