In the Garden:
Mid-Atlantic
January, 2007
Regional Report

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Aaron Steil discusses pros and cons of Longwood Garden's 1954-built Idea Garden shed and greenhouse.

Considering a Home Greenhouse

Less than enthusiastic about a bleak, nongardening winter, in September I started toying with the idea of buying a greenhouse. Nothing fancy, something small -- just somewhere to overwinter my fancy geraniums, New Zealand flax (Phormium), elephant ears, pitcher plants, ginger, bougainvillea, basil, etc. A place to start my favorite tomatoes and eggplants. Even better, an opportunity to grow fresh lettuce, spinach, Swiss chard, and beets in February!

End-of-season sales sweetened the pot; discount deals seemed too good to pass up. At first glance, the idea looked simple. The more I learned, though, the more complicated the decision. Fortunately Longwood Gardens held a Home Greenhouse Design and Construction course that answered several immediate questions and raised many more. I'm still considering the idea -- more realistically and long-term. Here is some basic information.

Not All Greenhouses Are Alike
Seeing a beautiful redwood frame Sunshine Greenhouse at this September's Garden Writers Association Symposium started me on this hunt. I wanted that 6' X 4' model then and there. It was a stand-alone, gable-roof design with Dutch doors, temperature-controlled vents, and twin-wall, polycarbonate construction. This comes as a kit. For the Longwood class, it took two people four hours to construct without the foundation.

There are other design styles, as well as framing and glazing materials. Frames come in wood, aluminum, galvanized steel, or plastic. Aluminum and wood are the most popular and common. Aluminum is the most expensive. "For good reason," says Longwood instructor Aaron Steil. "It's the best material to use -- attractive, durable, long-lasting, lightweight, doesn't rust."

Redwood and cedar framing have strong aesthetic appeal and low heat loss compared to metal. The downsides are limited lifetime in a greenhouse's damp atmosphere, and heavy shadows mean less light. Greenhouse covering (glazing) comes in polyethylene (best insulating), polycarbonate (best utilitarian use), glass, plastic, and fiberglass.

Besides the gable-roof style, there's the lean-to design that's best attached to the south side of a building for maximum sunlight. It's easy to build and heat.

The hoop house or quonset hut design is the most economical. Designed for form and function, the hoop house is usually made with bent water pipe and covered with polyethylene film. This design is very energy efficient and inexpensive but not so attractive.

The gothic arch design is similar to the gable roof but doesn't have walls. It has more headroom than a hoop house, which makes for a more comfortable gardening work environment.

Foundation
The foundation, the most important aspect of greenhouse construction, is complicated! Unless you're very handy with leveling and squaring, I suggest getting professional help for this crucial stage.

Bigger IS Better
"I have never met a greenhouse gardener who wishes their greenhouse was smaller," Steil said. If someone enjoys being in the greenhouse, they ALWAYS want more room! Better to buy the largest greenhouse you have space for and can afford.

The larger the greenhouse, the less cost per square foot, though overall cost will be more, of course. Larger also means higher heating and future maintenance costs.


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