If you're a farmer or an adventurous gardener, growing in high tunnels - those metal-framed structures covered in polyethylene plastic - just got a little less stressful.

Gov. Corbett recently signed into law a bill ensuring that owners of high tunnels, which provide a warm, dry environment for growing vegetables, fruits, herbs, and flowers in the offseason, will no longer have to worry about complying with the state building code.

No one was systematically cracking down on high tunnels. But a few municipalities in rural Pennsylvania were requiring tunnel owners to get a building permit, which meant hiring architects and engineers and installing fire-suppression sprinklers, foundations, and handicapped access.

"The common concern we have, and we've seen it in the past, is that when one municipality comes in with an extra regulation, another municipality sees that, and soon it becomes the standard," said the Pennsylvania Farm Bureau's Joel Rotz, who was among supporters who lobbied for the law.

The new law, which had no legislative opposition, is an attempt to head off such overtures. It also adds the increasingly popular high tunnel to the list of traditional greenhouses and other agricultural and horticultural structures that are exempt from the building code. (They are also exempt from property tax assessments, but subject to local zoning regulations.)

Over the last decade, high tunnels - also known as hoop houses or "poly houses" - have allowed the growing season to be extended at both ends, from February to December. They've been a boon to growers, who can charge higher prices in the offseason, and consumers, who now enjoy a wide variety of produce at farmers' markets practically year-round.

Early-spring markets bring greens and root vegetables. Tunnel-grown tomatoes and basil get to summer markets three weeks earlier than those grown outdoors. And at this time of year, market tables overflow with local potatoes, onions, and more root crops and greens.

"The growth of high tunnels has been exponential, not only in Pennsylvania, but around the country," says Pennsylvania State University professor of vegetable crops William J. Lamont Jr., who helped popularize the technology in the late 1990s.

About three years ago, Jerome Shabazz, executive director of the Overbrook Environmental Education Center in West Philly, proactively contacted the city Department of Licenses and Inspections about getting a building permit for a high tunnel. Shabazz said he did so because he wanted to make sure the structure - unlike other tunnels, elevated from the ground - was safe and because the center strives to be a model for other urban farmers.

L&I then asked for design plans and other paperwork, which had to be drawn up by architects, and a building permit was issued for the 17-foot-by-37-foot tunnel. The process cost the nonprofit about $2,500.

That, actually, was a modest fee charged by a "very socially conscious architectural firm," said Shabazz, who nonetheless favors the new law because "anything that takes away restrictions from nonprofits and the small-scale farmer is a good thing."

High tunnels are much cheaper than traditional greenhouses, about $2.50 to $3 per square foot, excluding labor, according to Penn State. Unlike greenhouses, where plants grow in pots or on benches, tunnel crops are in the ground. And tunnels are usually unheated and lack permanent ventilation systems; to circulate air, growers manually or mechanically roll up the sides.

Although the metal frames can last about 25 years, the plastic typically lasts four or five. Greenhouses, depending on their quality, are much longer-lived.

For centuries, greenhouses, glass jars or cloches, and cold frames have helped growers game the season. These days, even repurposed windows are enlisted.

But high tunnels, long popular in Europe, Asia, and the Middle East, are relatively new here. They were introduced by researchers at the University of New Hampshire in the late 1980s and a decade later by Lamont and others at Penn State.

Lamont attributes this to "the fact that we had so much expansive land that we could use for growing. People didn't really think about being much more intense in agricultural production."

But they're on board now. And Lamont and others worried that if municipalities across the state started requiring building permits for high tunnels, "they'd put the kibosh on that initiative. They'd make it so cumbersome, people would get frustrated and throw their hands up."

Over the last three years, salesman Bryan Saylor of Haygrove Tunnels in Mount Joy, Pa., has seen interest in high tunnels skyrocket. Tunnel-grown tomatoes don't crack like their outdoor cousins, he says, and fruit grown this way not only tastes great but also has a longer shelf life.

Then there's the peace of mind.

Saylor knows a farmer who lost 70 acres of outdoor tomatoes to rain. An additional 20 acres in high tunnels were fine. "With high tunnels," he says, "you're taking most of the weather out of the equation."

That's true over at the Overbrook Environmental Education Center, too. December arrives momentarily, but the tunneled tomato and pepper plants are still producing. Collards and kale are just coming in.