PORTLAND, Maine - The vegetable-growing season used to end with the first hard frost in Maine.

Jeff Tarbox picks Tuscan kale in a greenhouse at Sunset Farm Organics in Lyman on Thursday. The farm sells more than 300 pounds of vegetables weekly through the winter to Portland restaurants, at a farmers market in Brunswick and through a community-supported agriculture program. 9Gregory Rec/Staff Photographer) Not anymore.

An increasing number of farmers are pushing the growing season into the winter to take advantage of the surging demand for locally grown food. As a result, more farmers are operating greenhouses, branching out into cool-weather crops and creating new markets for their produce.

"Basically, people have gotten into it because their infrastructure is already there," said Mark Hutton, vegetable specialist and assistant professor of vegetable crops with the University of Maine Cooperative Extension.

Winter farming was pioneered in the 1990s by organic farmer and writer Eliot Coleman and his wife, Barbara Damrosch, at their Four Season Farm in Harborside. The two took a trip to Europe in 1996, following the 44th parallel through France and Italy - the same latitude as Maine - when the idea of winter farming hit Coleman.

"The whole time, we had seen gardens in January with Brussels sprouts and leeks, and the minute we got above the snow line there was nothing," said Coleman.

Coleman said he realized there was plenty of sunlight in Maine during the winter to grow vegetables - he just had to modify the temperature. So he came up with the idea of layered greenhouse structures that require minimal or no heating.

While there are no recent statistics on how many Maine farmers are venturing into winter gardening, agricultural experts say the number of new winter farmers markets and winter community-supported agricultural ventures reflects the increase.

There are about 18 community-supported agricultural operations selling winter shares of organic crops raised on Maine farms, according to the Maine Organic Farmers and Gardeners Association. The organization has seen its list of winter farmers markets more than double in the past year to more than a dozen across the state.

Other farmers markets are extending their seasons, including the Portland Farmers Market in Monument Square, which is staying open a month later than in past years.

Just why winter farming was not widely practiced before is a bit of a mystery.

Coleman said it could be that people simply assumed vegetables wouldn't grow when there is snow on the ground.

Hutton attributes the practice's growth to the advent of the locally grown movement in reaction to the rise of global corporate marketing, creating a demand that farmers are now rushing to fill.

Paul Lorrain, who raises lettuce and other vegetables in the winter at Sunset Farm Organics in Lyman, said it probably was just that vegetable farmers burned out in the summer and needed the winter to recuperate.

A landscaper in the summer, Lorrain has been steadily expanding his operation since he started in 2000. His first greenhouse was unheated. But after a two-week stretch of cloudy, minus-20-degree weather destroyed his crop, he started heating his greenhouses with propane to 37 degrees.

Today he operates eight greenhouses between Oct. 1 and the end of April, harvesting about 300 pounds of produce a week. He sells it to local restaurants, at a winter farmers market in Brunswick and through a new community-supported agricultural operation based at Wolf Pine Farm in Alfred.

"We have gone from not being able to give it away to not being able to grow enough," Lorrain said.

Tom Harms, who runs Wolf Pine Farm with his wife, Amy Sprague, left his job as a computer programmer this year to manage the winter community- supported agricultural venture at the farm, which until last winter sold shares of the harvest only in the summer.

"We are not just extending the season, we are making the winter our whole business," Harms said.

If all goes well, next year the farm will grow vegetables just for distribution in the winter, he said.

Harms has sold 350 shares this winter, signing up summer customers as well as new customers at agricultural fairs. He hopes to sell 50 more shares. It is possibly the largest winter share operation in the state, delivering as far away as Portsmouth, N.H., to the south and Falmouth to the north.

"People figure there is just a lot of turnips and kale, but we have worked really hard to bring diversity," Harms said.

Every three weeks, customers receive a box of produce - enough for three weeks - from 10 Maine organic farms.

The contents vary and may include dried beans, flour and other grains, fruits and berries, vegetables and eggs. Shares cost $500 if the customer picks up the produce; it is $600 for door-to-door service.

Scott Jillson of Jillson's Farm and Sugarhouse in Sabattus is venturing into winter farming this year for the first time.

Using techniques developed by Coleman, Jillson is growing lettuce and radishes in a hoop-style greenhouse - a series of hoops covered with a thick, taut layer of greenhouse plastic - to sell at new winter farmers markets that have opened in Falmouth and Cumberland.

Jillson said the winter markets give his family another way to sell the vegetable crops it raises from the 30 acres under cultivation. The family also sells vegetables at a year-round farm stand, through its own community-supported agriculture venture, and at summer farmers markets.

Jillson said that in the past, with fewer sales venues, the family often ended the traditional growing season with a vegetable surplus.

"Sometimes we would have to feed them to the animals," he said.

Winter farming doesn't work in all northern regions of the United States. Hutton said some areas, such as New York state and parts of Pennsylvania, are too overcast.

Most of the winter farming is being done on small existing farms because of the high cost of starting large-scale ventures, such as Backyard Farms LLC. That company opened a 24-acre, year-round tomato-growing operation in Madison in 2007, and this year it added another 18 acres of greenhouse capacity.

For many farmers, Hutton said, winter farming generates enough cash to allow them to retain some of their summer help and keep themselves on the farm rather than drive a snowplow or take on other temporary winter work.

Kathy Shaw of Valley View Farm in Auburn said the new winter farmers market in Falmouth keeps her busy on Wednesdays selling the meat and produce that she raises on her farm.

"In the past, I would have rested," she said.

Last week she had cauliflower, beets, turnips, Brussels sprouts, potatoes, parsnips, squash, onions, poultry and red meat, all raised in Auburn. She said she does it not only for the cash but also to put her philosophy into practice.

"I want to provide good fresh food to the public," she said.