The vegetable garden, once only in rural areas, has become more common in urban areas with the interest in fresh food, desire to limit the carbon footprint of the food on the family table and rise in food prices. In addition to establishing community gardens, urban gardeners are building their own greenhouses to extend the growing season.
A greenhouse is a building whose walls and roof are built of transparent or translucent materials. Greenhouses provide the controlled atmosphere necessary for the successful growth and propagation of plants. The concept of a sunny space with a controlled climate extends growing seasons and makes agricultural and botanical research practical.
Greenhouses may be window-sized boxes, simple hoop-frame huts or room-sized pipe and poly with a peaked roof. Larger commercial or research greenhouses, built upon brick half-walls for strength and insulation, are glass and steel with arching tops located on an east-west axis to maximize sunshine. England's Royal Kew Conservatory, typical of many of the 19th century, is a veritable glass palace. Its palm house has hundreds of small windows connected by mechanisms that open and close whole walls at a time. Milwaukee's Mitchell Park Conservatory's domes resemble elongated geodesic domes of the last part of the 20th century.
Greenhouses date back to Roman times but the intersection of the English Industrial Revolution, economic growth, the flowering of the natural sciences and the development of modern systems of scientific classification established them firmly in the university, scientific exploration and the English countryside. Glass boxes brought plants back from the voyages of the H.M.S. Beagle and rose in remarkable majesty at Kew Gardens. Today, any suburban gardener can use a cold frame or specially designed windows as an individual greenhouse.
The challenge of the greenhouse is to control the environment under the glass or plastic. Space heaters or free-standing heating plants support temperatures in temperate areas and fans and roof panels tilt open to allow built-up solar heat to escape. Shades shelter delicate plants from direct summer sun and blankets insulate roofs and keep heat in on cold nights.
Every greenhouse, whether a humble nursery shed or expansive conservatory, depends on several manmade environmental systems to maintain the perfect environment for its inhabitants. The passive solar system provides light for growth, warmth for health and, occasionally, electricity for power. Fans, vents and heaters supplement and control the heat and light and a water system provides irrigation and supplements humidity levels. Computers control schedules for complex mechanical systems in conservatories but the backyard greenhouse is most often managed with plastic sheeting and watering cans by a gardener armed only with a thermometer and hygrometer to check humidity.