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.
Before greenhouses became so popular as they are today, hotbeds were universally employed whenever glass was required to start early plants. Although hotbeds, in many cases, have been replaced by greenhouses, they are still used extensively by growers in all parts of the country. Almost every farm home operates a hotbed in the spring. This structure is of great importance to village gardeners who pride themselves upon the earliness of their products. Many renters also depend upon hotbeds, because they may be forced to vacate the property upon short notice and are naturally unwilling to spend much money in constructing permanent greenhouses. Hotbeds are inexpensive, and the amount of space devoted to them may be increased from year to year without much outlay. In addition, they do not require attention at night, as do the furnaces of greenhouses. On the other hand, hotbeds are inferior to greenhouses in every particular, although with skillful management they produce excellent results.
Hotbeds should be located so that a liberal supply of water is accessible. The most convenient plan so far as water is concerned is to install the overhead system of irrigation (122) and also to make provision for hose connections. Spigots, with underground cut-offs, should be placed between the frames, at intervals not exceeding 100 feet.
The hotbeds should also be convenient to the farm buildings, and to a room which can be made warm and comfortable for the work of sowing and transplanting. The frames require frequent attention some days, and a convenient location is important for this reason.
Protection from severe north and west winds is a great advantage. This may be secured by natural windbreaks, as hills and trees. Buildings may also serve the purpose. A common practice is to plant hedges or to construct board walls 5 or 6 feet high for this purpose. The walls may also be used to support the mats while drying.
South or southeastern exposures are preferable to others. The frames should run parallel with each other, with ample space between them for alleys or roadways, for the handling of mats and sash, and for snow shoveled from the glass. To serve these purposes best there should be at least 10 feet between the frames, but when the land is high priced and limited in area it is economy to make the alleys about 2 feet wide. These alleys are often filled with manure to help retain the heat of the hotbeds.
Most hotbeds are heated by the fermentation of manures in pits excavated for this purpose. The first essential of the pit is good drainage, natural or artificial. Artificial drainage may be provided by running tile from the bottom of the pit. In most soils, however, this precaution is unnecessary. The pit should be dug in the fall before the ground is frozen, and a few inches of leaves or coarse manure placed in the bottom during the winter. It should be of the same width as the frame (156) and of any desired length.
The proper depth of the pit depends upon several factors. In the North it is customary to use 15 to 30 inches of manure. The pit should, therefore, be a few inches deeper than the depth of the manure. For starting early vegetable plants in the North, 18 inches of good manure is ample, while 24 to 30 would not be too much in forwarding the eggplant, which requires a high temperature for seven or eight weeks. Tender plants, like tomato and pepper, also require more manure. Southward, the depths of manure vary from 6 to 12 inches. The kind of manure used and the length of time the hotbed will be needed also determine the proper depth of the pit.
The frame may be made of wood, concrete, brick or stone. The most common material is wood, although concrete is more durable. Of the woods, locust, cedar and chestnut make the most durable frames. A common plan is to use either locust or cedar for the posts, and chestnut or other less durable woods for the sides and crossbars. The frame may be of any desired length, and wide enough to accommodate the sash. That is, the width of the frame at the top should be about « inch less than the length of the sash.