Initially, the quest to produce warm-loving crops in temperate regions (and to extend the growing season of local crops) didn't involve any glass at all. The design of the modern greenhouse is strikingly different from its origins in the middle ages.
Each metre square of glass, even if it's triple glazed, loses ten times as much heat as a wall. Heating a building that's entirely made of glass is very energy-intensive, because glass has a very limited insulation value. The high energy use is hardly surprising. In the Netherlands, which is the world's largest producer of glasshouse grown crops, some 10,500 hectares of greenhouses used 120 petajoules (PJ) of natural gas in 2013 - that's about half the amount of fossil fuels used by all Dutch passenger cars. This makes greenhouse-grown crops as energy-intensive as pork meat (40-45 MJ/kg in the USA). A heated greenhouse requires around 40 megajoule of energy to grow one kilogram of fresh produce, such as tomatoes and peppers. The modern glass greenhouse, often located in temperate climates where winters can be cold, requires massive inputs of energy, mainly for heating but also for artificial lighting and humidity control.Īccording to the FAO, crops grown in heated greenhouses have energy intensity demands around 10 to 20 times those of the same crops grown in open fields. (This article has been translated into French, Spanish, Polish and Dutch). Picture: fruit walls in Montreuil, a suburb of Paris.