Season extension allows farmers a longer growing or harvesting season than they could otherwise expect. In Minnesota and other northern states, season extension can be a crucial part of a marketing plan: you might achieve a 22-week season, for instance, rather than a 15-week season for a CSA (Community Supported Agriculture) farm. Some Minnesota farmers use greenhouses to offer a winter CSA of salad greens!
Season extension methods include black plastic ground cover, high tunnels, row covers, greenhouses, succession planting, and mulching, to name a few. Frequently, several of these methods are used simultaneously.
Regional Extension Educator
Horticulture, University of Minnesota Extension Service