In agriculture, season extension refers to anything that allows a crop to be grown and harvested beyond its normal outdoor growing season and harvesting window.
For colder climates, the fully heated and artificially lit greenhouse is the ultimate season extension device, allowing some crops to be grown year-round, through sub-zero winters. An energy-expensive approach. There are many other ways to beat the cold, for earlier spring planting and growing into the fall and winter:
Season extension techniques are most effective when combined with crop varieties selected for the extended growing conditions. Many approaches are used in large-scale agriculture, as well as in small-scale organic farming, and home gardening.
Using unheated, unlit methods, depending on the crop, up to several weeks of productivity can be added, where shortened period of sunlight and cold weather end the growing season.
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).
Growth, harvest, or both
In its more passive forms with minimal warming, what season extension accomplishes is to allow plants to remain metabolically idle (with very slow growth) but avoid dying. This allows growers to make a conceptual distinction between extending the growing season and extending the harvest season. The reality is a spectrum, and the same farm can do both (on a crop-by-crop or field-by-field basis), but nonetheless these two conceptual poles can be distinguished. The former type of extension is what a hothouse does, such as producing tropical fruit in winter by totally conquering the outdoor climate at high expense. In contrast, the latter type of extension can be viewed not as a way to escape the context of seasonality entirely but rather, working within the context of seasonality, to achieve food preservation in a way that uses life itself for preservation instead of using technology applied to dead plant or animal tissue (as mechanical refrigeration, freezing, or canning would). From this viewpoint it can even be compared to ancient norms of meat preservation, which relied on the principle that a good way to avoid meat spoilage is to keep the animal alive until just before consumption. In other words, the best postharvest strategy sometimes can be to confine the postharvest period to hours or days instead of weeks or months (by keeping the plant or animal alive as long as possible—the ultimate form of preservation). In the meat example, in preindustrial contexts livestock were brought to market on the hoof, and sailing ships on months-long voyages would keep turtles in forced semihibernation until each one was slaughtered to make a meal.