Paul James, host of HGTV's Gardening by the Yard, explains the basics of planning for a greenhouse. The ideal location for a greenhouse is a southeast exposure so that it can capture the maximum available light during the day. As with any structure, a good foundation is critical. For a greenhouse like ours, pour a reinforced concrete footing that extends roughly 4" below ground and 4" above. Use bolts to secure the base plate of the greenhouse to the foundation (figure A). For the floor, lay down a single sheet of landscape fabric, and cover it with mulch (figure B). One of the most important things to consider when planning a greenhouse is the type of overhead glazing or covering material used. The least expensive is plastic sheeting, which will need to be replaced every two to three years. Rigid polycarbonate is another choice, and although it lasts much longer than plastic sheeting, it tends to yellow over time. The most durable and expensive of all glazing materials is glass. No other material captures more light. If the greenhouse will be used year-round, consider the climate. In the northern border states, you'll need heat in the cooler months but can probably do without a cooling device for the summer months. In the Deep South, a greenhouse must be cooled in summer but probably won't need to be heated in winter. In the nation's midsection, both heating and cooling are a must for gardening throughout the year. To produce strong, healthy plants and reduce their risk of infection from fungal diseases, install a good fan to ensure adequate air circulation (figure C). You'll also need a minimum/maximum thermometer. You'll need benches of various heights to set the plants on. One should be counter height and designated a potting bench. Some growers prefer wooden benches, but because insects can overwinter in wood, many gardeners have switched to plastic.
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