Season extension

Season extension in agriculture is any method that allows a crop to be grown beyond its normal outdoor growing season and harvesting time frame, or the extra time thus achieved. To extend the growing season into the colder months, one can use unheated techniques such as floating row covers, low tunnels, caterpillar tunnels, or hoophouses. However, even if colder temperatures are mitigated, most crops will stop growing when the days become shorter than 10 hours, and resume after winter as the daylight increases above 10 hours. A hothouse — a greenhouse which is heated and illuminated — creates an environment where plants are fooled into thinking it is their normal growing season. Though this is a form of season extension for the grower, it is not the usual meaning of the term. Season extension can apply to other climates, where conditions other than cold and shortened period of sunlight end the growing year (e.g. a rainy season). Unheated greenhouses (also known as cold houses) offer protection from the weather, such as sub-optimal temperatures, freezing or drying winds, damaging wind gusts, frost, snow and ice. Unheated greenhouses can extend the growing season of cold hardy vegetables well into the fall and sometimes even through winter until spring. Sometimes supplementary heating is appropriate when temperatures inside the greenhouse drop below 32 degrees Fahrenheit. Passive heated or low-energy greenhouses: Using principles of passive solar building design and including thermal mass will help keep an otherwise unheated greenhouse several degrees warmer at night and on overcast days. Other systems such as ground-coupled heat exchangers, thermal chimneys, thermosiphons, or "climate batteries" can also be used to take ground-stored heat and use it to help heat a greenhouse. Polytunnels (hoop houses): Whereas a greenhouse has a frame and is glazed with glass or stiff polycarbonate sheets, polytunnels are built with thin polyethylene plastic sheeting stretched over curved frameworks, often extending as long "tunnels".