A greenhouse is a structure with transparent walls and roof that creates a controlled environment for plant growth. Greenhouses trap solar radiation and retain heat, allowing year-round cultivation in climates with cold winters or short growing seasons. They also protect plants from wind, rain, and pests while allowing precise control of temperature, humidity, and light. Greenhouse structures range from simple cold frames to fully automated commercial buildings.
Hoop houses or high tunnels are the most common type on homesteads. They consist of a series of arched metal hoops covered with polyethylene film. Hoop houses are relatively inexpensive: a 9 by 15 meter (30 by 50 foot) structure costs approximately $1,500 to $4,000 for materials. They provide season extension of 4 to 8 weeks on each end of the growing season.
Cold frames are small, unheated structures with a transparent lid, typically built from wood and glass or polycarbonate. They are used for hardening off seedlings, overwintering tender plants, and early spring starting. The standard cold frame is 1.2 by 1.8 meters (4 by 6 feet) with a sloping lid that faces south.
Permanent greenhouses are constructed from wood, aluminum, or galvanized steel with glass or polycarbonate glazing. They provide year-round growing capability when equipped with supplemental heating and ventilation. These structures require Concrete Foundations and building permits in most jurisdictions.
Ventilation is provided by roll-up sidewalls on hoop houses, ridge vents, or automatic vent openers. A temperature difference of 3 to 6 degrees Celsius (5 to 10 degrees Fahrenheit) between inside and outside is typical. Shade cloth reduces solar gain during summer months. Supplemental heating, using propane, natural gas, or wood, is required for winter production in USDA zones 6 and colder. Thermal mass, such as water barrels or concrete flooring, absorbs heat during the day and releases it at night.
Greenhouses are used for seedling production, year-round vegetable growing, and overwintering of tender perennials. Winter greens production in an unheated hoop house is possible in USDA zones 5 through 8 using cold-tolerant crops such as spinach, kale, lettuce, and Asian greens. Raised Beds inside greenhouses improve drainage and soil temperature.