Bigger, sturdier, ever more luxurious versions - and custom jobs - can cost thousands, tens of thousands, and more.
"As with anything else, they can be as expensive as you want them to be," says Egolf, who built a 14-by-20-foot custom greenhouse adjoining his master bedroom 18 years ago. The materials cost $8,000.
In 2012, an estimated 1.7 million households purchased greenhouses, compared with 800,000 in 2000, according to the National Gardening Association.
Sounds like a big jump. But Bruce Butterfield, the association's market research director, offers perspective: That still represents only about 1 to 2 percent of gardening households.
"Of course, there are greenhouses, and there are greenhouses," he adds, warning consumers away from "rickety" models that have flooded the market in the last few years. They're made of cheap materials and often have no foundation.
"It's like heating the outdoors," says Butterfield, who recommends kits that offer "some fairly rugged barrier that separates frozen ground from nonfrozen ground."
Egolf's greenhouse has a full insulated foundation below the frost line and an insulated shingle roof. He heats the inside with propane, which costs about $180 per winter.
Egolf, who advises newbies to start with a kit and to seek advice from the Hobby Greenhouse Association (hobbygreenhouse.org), has been known to start 400 tomato plants a year from seed in his greenhouse. ("I went overboard," he acknowledges, so last year he cut back to 120.) His wife, MaryAnn, starts about 300 violas, pansies and daylilies.
Together, they landscape their 31/3 acres with flowers and fill 20 raised beds with vegetables every spring, which still sounds a bit "overboard."
"The greenhouse is about three-quarters full right now, and I haven't even started my seeds yet," says Egolf, whose tomatoes make a sea's worth of soup, paste, salsa, ketchup, and sauce.
Next month, Sam Moll, seasonal greenhouse manager and plant production specialist at the Rodale Institute's Experimental Farm in Kutztown, will begin planting the first wave of thousands of seeds in a 30-by-100-
foot, propane-heated greenhouse and a new, unheated, high tunnel, also known as a hoop greenhouse, which measures 30 feet by 75 feet.
Moll will start "cold crops" - greens (lettuce, kale, arugula, spinach), broccoli, cabbage, herbs, beets, carrots - for a plant sale in April; the "hot crops" - tomatoes, peppers, squash, eggplant - will be sold in May.
It all sounds yummy, which is why Moll cautions anyone thinking of starting vegetable seeds in winter: "Do a personal reflection on your vegetable appetite."
In other words, plant what you like to eat. Don't go crazy, as Moll's father did this year.
"My dad just started gardening, and he's planting eggplant and all this strange stuff that he doesn't like," Moll says. "I think he's romanticizing about growing his own food."
Not a bad thing, but Moll is a practical fellow.
Before you buy a greenhouse, think about how much time you want to put into it, he says, "and make it easy to get to, 'cause it's cold outside."
Lynn Cook and Troy Ray are both romantic and practical.
Their custom-built greenhouse is large - 28 feet by 50 feet - and situated just across the driveway from their fully restored, 18th-century home in southern Chester County.
Their first greenhouse, a $1,700 kit, was 8 feet by 6 feet and a great learning experience. Their current greenhouse, built in 2007, has a concrete floor with center drain, four circulation fans, and a control system that manages air vents and a shade curtain. It's heated with natural gas and uses well water.
Cook and Ray declined to share the cost of their grand greenhouse. Cook says only that it is a priority for them: "We are totally obsessed."
Cook, a retired nurse-educator, and Roy, a telecommunications consultant, spend more than 1,000 hours a year working in their greenhouse just on plants they'll enter in Philadelphia Flower Show competitions. (Last year, that was 309.) Basic maintenance takes a few hours a day.
"By the time these babies get into the flower show, they've had maybe 4,000 hours of work," Ray says, as he shows off those "babies" - rare orchids, ferns, dwarf conifers, succulents, and cacti.
The hours of work have paid off with flower show and plant society awards. Walls in the house are covered with citations, and loose-leaf binders are jammed with ribbons and rosettes.
"You have to love it," Ray says.
Butterfield, of the National Gardening Association, agrees - with a caveat.
"While you may be a fairly skilled gardener out of doors, you've got to become an elite gardener to grow in a greenhouse," he says, citing "Mother Nature's unwillingness to cut you a break."
"You've got to control the temperature, the moisture, soil, ventilation, hours of light and sometimes, pollination," he says. "I think it's tricky."