In nearly all types of vegetable gardening glass is essential to secure the best returns. It is true that many crops, as sweet corn, cabbage, peas, beans and the root crops, are grown at a profit without the use of glass, although most gardeners regard it as indispensable .for certain parts of their operations. The term glass includes the different types of hotbeds, cold frames, forcing hills and greenhouses.
Glass is often used merely for protection, as for wintering plants in cold frames. The more common uses, however, are to hasten or forward the growth of plants, so the crops will mature before their normal time, and to grow crops to full maturity during the winter season, when it is not possible to produce them in the open in the same latitude.
There are many advantages in starting certain vegetables under glass, then transplanting to the open ground as soon as conditions are right. Some of these advantages are: (1) Crops placed on.the market before their normal season usually command the highest prices. (2) In many sections the summers are too short to mature certain crops, such as eggplants, watermelons and late tomatoes, without the use of glass. (3) In starting the crop early there is less danger of loss by destructive frosts in the fall. (4) By forwarding the plants under glass, two or more crops can often be grown on the same land during the season, (5) Weeds are generally less difficult to combat when plants of good size are set in the open. (6) It may also be the means of avoiding troublesome insects and fungous diseases. (7) Some vegetables, notably the tomato, produce larger crops when started under glass. (8) As the vegetables will be harvested considerably earlier, there may be time to start a cover crop. This is of great advantage in soils lacking humus.