Greenhouses are in far more general use among market gardeners than they were 10 or 15 years ago. They have become especially numerous near railroad lines affording satisfactory shipping facilities and large cities providing good markets for forced vegetables. When a grower learns that a greenhouse is a profitable investment the usual tendency is gradually to increase the area of glass, as many examples of such expansions will prove. The first house is perhaps very small and built for the purpose of starting early vegetable plants, for which it is found convenient and satisfactory ; but the owner is often unwilling to have it idle more than half the year, and, therefore, he tries a forcing crop. If his efforts in the production of crops under glass prove successful, the greenhouse area is increased and new houses are built from year to year, until the grower is known as a vegetable forcer rather than a market gardener. The greenhouses furnish better conditions for starting early plants and they may be used 10 or 11 months in the year if the establishment is properly handled. It is not uncommon for market gardeners to operate an acre or two of greenhouse space, while a much larger proportion of growers have from 1,000 to 10,000 square feet of glass.
If forced vegetables can be made profitable in connection with market gardening, there should be no hesitation in constructing greenhouses. They enable the grower to keep in touch with the market the year round, and they provide employment in the winter when it is often difficult to find sufficient work to keep men busy. Then, again, a greenhouse adds mate-rially to the pleasures of rural or urban life, as it insures summer conditions the year round on some part of the place.
For starting early vegetable plants the greenhouse possesses decided advantages over the hotbed. These may be enumerated as follows: (1) It is cheaper to heat glass structures by means of coal than by manure. (2) Proper soil and atmospheric conditions are better controlled in greenhouses. The gardener spends much of his time inside of the house, and he has abundant opportunity to note every change. If the soil is too dry, it is detected before any injury has been sustained by the plants. If the air is too close, the grower soon discovers it, and the ventilators are opened. (3) Fresh air may be admitted in the severest weather without cold drafts striking the plants. This is impossible in hotbeds. (4) The daily care of greenhouses is less laborious and, therefore, less expensive than in the management of hotbeds. There are no sash to be handled separately several times a day, no mats to move twice a day and no sash to raise when watering. When the Skinner system of irrigation is used in the greenhouse, the labor of watering is so small as to be scarcely worth considering.
Greenhouses are employed for a variety of purposes by market gardeners besides that of forcing crops to maturity. The growing of seedlings for transplanting in cold frames is one of the largest uses. The more tender plants, as tomato, pepper and eggplant, are often transplanted 1« or 2 inches apart after the seedlings are three to five weeks old. If space permits, any of the vegetable plants may be kept in the greenhouse until time for planting in the open ground. If desirable, they may then be shifted to the cold frame for a short time to harden.
The proper size of a greenhouse must be determined by a number of factors. It is never a safe policy to build a large house or an extensive range of houses without thorough experience in greenhouse work and full knowledge of local conditions and market facilities. A house 30 × 100 feet is probably as large as any market gardener should start with, and a smaller structure would be desirable where both capital and experience are limited. A width of 30 feet has been given, because this is the minimum width for economical construction, heating and operation; narrower houses do not provide as uniform atmospheric conditions, and the plants are more likely to be injured by direct cold drafts.
While the natural protection of woods or hills on the north and the west sides is highly desirable, greenhouses should not be constructed where they will be shaded by other buildings.
The position of the house with reference to the points of the compass is apparently of little importance. A three-quarter span house should run east and west to get the full benefit of the sun, while the even span house should probably run north and south in order to secure a uniform distribution of light. Some growers compromise and build their houses northeast and southwest. With the modern house, about 95 per cent of which is glass, ample light will enter the house whatever its position may be.
Serviceability, durability, and economical construction and operation are the main points to keep in mind when building greenhouses. No one denies that the full iron form of construction is the most durable and that it is also highly satisfactory when in operation, but the cost of construction is beyond the means of most vegetable growers. Again, it is doubtful whether full iron construction is the most economical in the end; the first cost is from one-third to one-half greater than for semi-iron construction, and this additional expense may exceed the cost of repairs in other types of construction. With proper care and painting the wood parts in a well-built house will last 25 years, and they could then be renewed at a cost which would not be burdensome to a grower who had harvested profitable crops for a quarter of a century. Iron pipe, concrete and thoroughly dried cypress are the most important materials needed in the construction of a modern commercial greenhouse.