It's about this time of year, when container plants are still lush but ever-shorter days signal that killing frosts can't be too far off, that gardeners are apt to sigh and mutter, "I wish I had a greenhouse."

If only we had a greenhouse, we figure, we could keep tender plants alive until next year, extend the fall season, get a jump on spring, and even -- maybe best of all -- create a warm, green oasis to sustain us through the winter.

Now, apparently, more gardeners are making the wish a reality.

"Greenhouse gardening is growing at an exponential rate," said Mike Helle of Sunshine GardenHouse, a company in Longview, Wash., that ships greenhouse kits nationwide. Although the number of gardeners who own greenhouses is still quite small, "the growth in the last few years has been in the range of 15 to 20 percent a year," he said.

Considering the boom in all aspects of gardening over the past couple of decades, it's not surprising that everyday gardeners might see greenhouse growing as the next frontier. Often, though, the concept has seemed too complicated and too expensive.

It doesn't have to be either, Helle said.

"I have been trying to demystify greenhouse activity," he said. After all, he said, "if you want to be complicated, even lawn care can be technical."

And although greenhouse gardening used to be thought of as something reserved for the wealthy, good-looking kit greenhouses now "are affordable on a level that is still an investment, but not outside the realm of what many gardeners would be willing to spend."

Helle's company sells greenhouse kits starting at $1,299 for a 6-by-4-foot model.

To make greenhouses more user-friendly, Helle said, some of the major sellers of kits are getting together to increase awareness and discuss ways of providing a kind of "tech support" system for potential greenhouse owners.

In England, he said, "one of every five homes has some kind of greenhouse. . . . They've grown up with them, they know how to do it. That's not true here. Greenhouses are not part of our gardening history. But we're getting started on a new adventure here."

Some people have been growing plants in greenhouses for decades. Stuart Hughes of Maple Glen, Pa., for example, built his first greenhouse in 1972 -- a Lord & Burnham lean-to model with single-pane glass -- and used it for 20 years before he took it down to make way for a big Patio Enclosures solarium, in which he grows about 200 orchids.

Over the years, Hughes, a retired biology teacher who produces a newsletter for the Delaware Valley Chapter of the Hobby Greenhouse Association, also built a conservatory that serves as a cool greenhouse, and a sunroom with two window walls that he calls "the cold room," where cold-tolerant plants spend the winter. He also added a large Pella window greenhouse to his kitchen.

Outside is his only stand-alone greenhouse, a removable TwinWall polycarbonate model that in winter protects just one plant, a Washington navel orange tree that has grown in the ground for eight seasons.

"In spring, the orange blossom smell permeates the whole garden," Hughes said.

About 800,000 people nationwide purchased sunrooms or greenhouses last year, up about a third over five years ago, said Bruce Butterfield, research director for the nonprofit National Gardening Association.

"Many people dream of a greenhouse, but often it's hard to swallow what they cost," Butterfield said.

What they cost, though, depends on what you want.

"We can do anything that anyone wants us to do," said Bob LaRouche, owner of Glass Enclosures Unlimited in Wayne, Pa., who deals mostly with custom designs. He has built Victorian conservatories, greenhouses with wine cellars below, and one with a virtual-reality golf range and a movie theater underneath.

Michigan-based FlowerHouse offers tent-like pop-up units that company founder Scott Wehner calls "self-erecting portable greenhouses." The 8-by-8-foot DreamHouse model in a plastic Gro-Tec material costs $300 delivered, and "you could put it up now and take it down in May and hang it from a hook in the garage until next fall," Wehner said.

Because it's made of plastic, you'd have to pay careful attention to your choice of supplemental heat. Wehner suggested dark flooring and dark-colored containers filled with water that absorb the sun's heat by day and release it by night, or a small ceramic heater with its own thermostat.

Between the extremes of greenhouses are numerous choices of varying complexity.

"A free-standing kit greenhouse is a project that could be undertaken by the average, rather handy person, but a greenhouse that is attached to your home should be undertaken very carefully and should probably be built by a professional," LaRouche said.

Therese Lundvall is a greenhouse consultant with Charley's Greenhouse & Garden in Mount Vernon, Wash., which has specialized in greenhouse kits and accessories for nearly 30 years. She thinks there are several reasons for the increased interest in greenhouse gardening.

"We noticed after 9/11 that a lot of people felt very unsettled, and wanted to be able to control their food more," she said. "They were worried about different kinds of attacks, so we had a lot of people say, 'We want to grow our own stuff.' And it was also a comfort thing; instead of going on vacations, they wanted to stay home and nest a bit. Greenhouses really jumped after that date.

"And one of the major things people say when we are recommending greenhouses is that they want to raise their own bedding plants, either by seed -- people are lured by that 99-cent packet of seeds -- or by cuttings, because they don't want to lose their favorite plants."

Then there's the orchid factor.

"More people are growing orchids. It's America's most popular houseplant now, and people realize they have cultural needs that require more than some houses can give them," Lundvall said.

Many orchid enthusiasts, she said, "seem to have a bit of an addiction. . . . They need a greenhouse."

Stuart Hughes's observations seem to support that.

"In my poverty-stricken days, I used to have the concept that everyone on the Main Line," a string of affluent towns in the Schuylkill River valley west of Philadelphia, had a greenhouse, Hughes said. "But only people who get the bug get them . . . and off the top of my head, I think more than half are orchid growers."

Hughes has orchids growing in all his attached greenhouses. He likes the idea of attached structures because they conserve energy and they're easily accessible in bad weather -- he sits among his plants in the solarium to read the paper every morning.

"A lot of people are making their greenhouses big enough to do more than just grow plants," Lundvall said. "They have little water features, a bistro set, they pipe in music, some people have a coffeemaker out there. They are using this as an environment to relax in."

Helle agreed.

"A greenhouse is a wonderful place to decorate," he said. "The handles on my greenhouse door are old trowels, I have twinkle lights around for the holidays, and I have a wicker chair and a CD player. . . . It's a wonderful outdoor room."

Retired teacher Stuart Hughes tends to plants in his conservatory, one of several greenhouses he has at home in Maple Glen, Pa.