Snow surrounds one of Bob Jones' high tunnels on his farm in Huron, Ohio. High tunnels are popping up in Ohio fields and around the country as farmers try their hand at tunnel farming. The idea is to shield the crops from the elements and trap the heat of the sun, extending the growing season and increasing production.

DAYTON, Ohio — They look like greenhouses — hutlike structures swathed in plastic that serve as cocoons for growing tomatoes, lettuce, peppers, carrots, strawberries and bushels of other fruits and vegetables.

Called high tunnels, the buildings are popping up in Ohio fields and around the country as farmers try their hand at tunnel farming.

The idea is to shield the crops from the elements and trap the heat of the sun, extending the growing season and increasing production.

"The tunnels provide a microclimate around the crops that essentially fools them into thinking it is a different season than it really is," said Matt Kleinhenz, extension vegetable specialist for Ohio State University's Ohio Agricultural Research and Development Center.

A tunnel-farming workshop sponsored by Ohio State in November drew farmers from five states and 40 ZIP codes in Ohio. Hundreds of Ohio farmers are now using the tunnels, Kleinhenz said.

David and Lisa Schacht have covered more than half an acre with three connected tunnels shielding strawberries at their farm near Canal Winchester in central Ohio.

The couple, who also grow vegetables, hope the tunnels will enable them to begin selling strawberries the first of May — four weeks before their competitors do. They expect to command a higher price and create a following of loyal customers who will return.

"Our school of thought is to moderate the environment just enough to improve our yields and our quality and our season," Lisa Schacht said.

High tunnels can be large or small. Many are about 90 feet long and 20 feet wide, with wooden and aluminum frames. Unlike greenhouses, tunnels are covered with plastic, lighted only by the sun, and lack automated heating and cooling systems.

They protect plants from extreme cold, wind, and rain — which can keep soil wet for days and keep farmers from cultivating it — and they exclude or reduce pests and diseases.

The tunnels can extend the growing season by up to four months, with two months at each end, Kleinhenz said. And they make it easier to grow certain crops better adapted to zones farther south.

"The biggest challenge is the tremendous amount of management it takes," said Paul Wiediger, who farms near Bowling Green, Ky. "It becomes a balancing act."

He uses his five tunnels, covering 10,000 square feet, year-round to grow tomatoes, lettuce, spinach, carrots, beets, turnips, potatoes, eggplants and melons.

Wiediger has to juggle the timing of planting, growing and harvesting the various crops to make sure they're ready when he needs to take them to market. He is at the mercy of the sun; overcast days slow growth.

But he said the quality, quantity and size of his tunnel crops is far superior to what he's grown outside. And the extended season brings cash when he normally wouldn't have it.

"They help take some of the risk out of a tremendously risky business," he said.