When March rolls in and springtime is in sight, those green thumbs can start getting mighty itchy. But, although the days are leaning towards balmy, the nights can still be downright frigid. With some preparation, a greenhouse can be just the ticket for getting a jumpstart on those summer seedlings, whether using a simple do-it-yourself job or an expensive rig.
The down and dirty method to create a basic greenhouse is called a cold frame. With some 2×6-inch wood beams, make a rectangle and stack one on top of each other so that it’s 12- to 16- inches high. Next, put a frame of some kind around it; it can be as simple as four posts in each corner (though attaching cross beams would offer some stability). Then stretch plastic polyethylene sheets— easily bought at most hardware stores—over the frame and secure. “And when you put [the sheet plastic] on top of this box, that becomes a greenhouse,” said Bill Miller, professor with the Horticulture Department at Cornell’s College of Agriculture and Life Sciences. “Light can come in, plants can grow, but the heat stays in, and that’s basically what a greenhouse is—a structure that allows light through and retains heat.” These simple units can be good to start seedlings or grow young plants until they’re ready to transplant. “This is probably only something you can do in mid-to-late spring, depending on weather and temp,” to get a jumpstart on the summer growing season. The main disadvantage of a cold frame setup is temperature control; on a hot, sunny spring day the only way to adjust the heat is to prop open the plastic sheet corners, even then it’s not a very accurate or efficient means of temperature control.
Greenhouses with a little more capacity come in two popular forms: a lean-to style or a freestanding unit. The former is attached directly to a house and consists of a sloping, south-facing glass roof. It’s great for wintering house plants, but is less common because if one doesn’t already exist, building an actual addition off the house can get costly and involve permits, especially if there isn’t a south-facing wall that can easily be added to.
￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼The latter is more common for residential use. Do-it-yourselfers can create a functional unit much like an upscale cold frame. Sink corner posts in the ground and build a frame using 2×4 or 2x6ers, making sure to also construct a reinforced roof with some pitch. In general, it will cost about $10 per square foot. Again, polyethylene sheets are the most economical, though they should be battened down, not stapled, so they don’t tear. “These things don’t necessarily look good, they’re kind of crude, but they can be perfectly fine,” said Miller. Garden stores also sell greenhouse kits as small as 48 square feet up to 200. Generally they have aluminum or galvanized steel frames and can come with rigid plastic panels, like a polycarbonate. They can come in several styles, from hoop constructions to shed-like structures, even lean-to designs, and cost anywhere from $400-$600 and up to $3000.
To go truly upscale involves masonry and construction to create an aesthetic piece of architecture, something like Hartley Greenhouse, that can cost anywhere from $30,000 to $250,000. “[You’re talking] much bigger structures that’re engineered so that you can have heat all year round. You can literally have tropical trees growing in it; you can have your own botanical garden in your backyard,” said Miller. “Most backyard gardeners, folks that are interested in growing a few tomatoes and vegetables, and even serious gardeners, the vast majority don’t need to have a fancy engineered installed greenhouse.”
Miller’s main advice: “If you think you need a greenhouse that’s 50 square feet, go 50 percent bigger… As soon as you start putting benches in you have to have walk spaces to get around the thing, so right off the bat, half the space is not going to be useable for growing things.”