The purpose of a greenhouse is capturing solar energy and providing an ideal place for the growth of plants when outside temperatures are too cold for their survival. In some locations, the heat energy the greenhouse traps is sufficient for the protection the plants from freezing, but in others, the unit requires a supplemental heat source. When temperatures outside grow warm, the addition of draperies and the opening of windows and vents transforms the structure into a shade house.
Greenhouses help to extend the growing season for plants. Gardeners bring in potted plants before the danger of the first frost and continue harvesting crops beyond the normal limits of nature. Using a portable greenhouse on wheels, the emperor Tiberius consumed cucumbers every day of the year. In the 1600s northern Europeans used orangeries, which were greenhouses, for extending the regions where oranges grew, and George Washington grew pineapples at Mount Vernon.
Greenhouses also give some vegetables with long growing season an early start. Gardeners plant seeds in pots in the greenhouse several weeks earlier than it is safe to plant them outside due to the danger of frost. Once the danger passes, they transplant the seedlings to the outdoor garden. This early start in the spring provides an earlier harvest.