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. The energy budget for heating and cooling greenhouses can be greatly reduced through use of Ground Air Heat Transfer (GAHT) or climate battery technology. Using GAHT, excess heat accumulated during the summer months is stored via underground pipes in the ground underneath the greenhouse and retrieved later for use as needed during the winter months. The ground underneath the greenhouse is surrounded by insulation to ensure the heat stays underneath the greenhouse.
There are many other ways to beat the cold, for earlier spring planting and growing into the fall and winter:
- Row covers: light fabric placed over plants retains heat and can offer up to several degrees of frost protection. In smaller gardens almost any type of cover, including newspaper cones, miscellaneous bits of plastic, etc., can serve the same purpose.
- Hoop houses: plastic sheeting is placed over a frame (usually in an arced shape), to create a type of greenhouse. Hoop houses can be large or small, simple or nearly as functional as greenhouses.
- Cold frames: transparent-roofed enclosure, built low to the ground, used to protect plants from cold weather. Cold frames are found in home gardens and in vegetable farming. They are most often used for growing seedlings that are later transplanted into open ground.
- Hotbeds: a mass of hot compost is used for the heat it gives off to warm a nearby plant. Typically a few centimetres of soil are placed on top of the compost mass, and the plant grows there, above the rising heat.
- Mulches: any material placed on the soil around plants will help retain heat. Organic mulches include straw, compost, etc. Synthetic mulches, typically, plastic sheeting with slits through which plants grow, is used extensively in large-scale vegetable growing.
- Raised beds: beds where the soil has been loosened and piled a few inches to more than a foot above the surrounding area heat more quickly in spring, allowing earlier planting.
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, season extension allows 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 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 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 but, 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. The principle of a well-boat for commercial fishing was the same, keeping the fish alive for market. Such boats are sometimes used today.