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Many vegetables are started under protection, there being a number of advantages in using this method.
1) It increases the length of the growing season, many crops being started under cover before it is possible to grow them outside. Where the seasons are especially short this is the only means of maturing such tender long-season crops as tomatoes, peppers, eggplants, and melons. Provided the crops bear continuously, it is obvious that the early started plants may produce larger yields due to extending the length of the growing season. 2) By starting the plants under cover earlier than it is possible to start them outside, the vegetables can frequently be matured at an earlier date. 3) By growing the crops under cover for a portion of their growing season they may require less time to mature in the field. This en¬ables the grower to grow more crops on the same land during the same season.
Various equipment is used for protecting early grown plants. Greenhouses provide the best facilities for growing early vegetables, since the temperature may be more easily controlled by better ventilating facilities. The work¬men can do a much better job and they do not have to stoop down and lean over, nor will there be any danger of exposing the plants to cold temperatures, when they are watered. Hotbeds are artificially heated whereas cold frames have no other source of heat but natural or solar heat. Hotbeds are usually covered with a glass sash as glass admits a maximum of light and has some other advantages.
Cloth and some similar materials have been substituted for glass in making the cover but have proved to be less satisfactory and are but seldom used.

Vegetable Growing Structures
There are several kinds of hotbeds and several methods of heating them.
The pit hotbed is one which is placed in an excavation. The frame if made of wood is usually supported on posts so that the north side is from 12 to 18 inches above the ground. This provides a slope to the south so that the water will drain away from the structure. Obviously, most of the hotbed being underneath the sur-face of the soil, less heat is required. Ten or 12 inches of fresh horse manure are found to be adequate for heating pit hotbeds, whereas from 16 to 18 inches are usually required for surface beds.
The frame of hotbeds may be made from lumber. Some¬times a more permanent material such as concrete or brick or a combination of the two is substituted for lumber in making the frame. Whatever material is used it is essential that the corners be squared accurately and the upper surface of the frame be perfectly level and on the same plane. Otherwise the sash will not make a perfect fit, and it will be difficult to prevent the penetrating of cold air during excessively cold weather. There are various types of heating hotbeds: manure heating, steam heating, hot water heating and electrical heating. Which method to follow is determined by a number of factors.
Cold frames are forcing structures that differ from hotbeds in that they are not provided with any form of artificial heat. The heat used comes from the visible rays of the sun, which pass through the covers and which are changed to longer heat rays when striking the soil or other dark objects within the bed.