irrigation needs; and the use of irrigation rather than reliance on rains may reduce insect and disease incidence.

In other words, a greenhouse with appropriate environmental equipment can provide nearly optimum growing conditions for plants on nearly a year- round basis. It generally does this, however, at a substantial financial cost- especially if artificial lighting must be provided.

BASIC ECONOMIC RELATIONSHIPS

Greenhouse production is nearly always more expensive per unit of product than field cultivation in the same area during the same time period- provided efficient field culture is possible. This is a big if, and is at the heart of the economic rationale for climate control in greenhouses. The purpose of greenhouse production is to provide a product either (1) where local field production is of lower quality or even higher cost, or (2) when local produc tion is not possible. An example of the former is summer tomato production in northern Europe; an example of the latter is winter production of vegetables in the northern United States or in northern Europe. The situation is of course more complicated than this because of the presence of imports from other producing areas.

While nearly any crop, including tree fruits, can be and probably has been grown in a greenhouse, the process has proved commercially economic for only a few. The bulk of current food production from greenhouses is accounted for by only three vegetable crops: tomatoes, cucumbers, and lettuce. Why has production been so confined? It is a combination of the production and market characteristics of these three vegetables, such as their ability to come into bearing soon, their ability to respond with heavy yields to intensive green house culture systems, the perishable nature of the product, the high value per acre, and other factors.

While greenhouse production lessens the climatic risks involved in produc tion, it does not reduce the quality of management needed, and has probably increased economic risks. The economic problems are largely due to the heavy fixed investment which is necessary, the relatively limited range of alternative food crops available, and the constant competition from imported field products. Challenges facing the greenhouse grower today are not much dif ferent from those noted by Liberty Hyde Bailey in 1897:

The growing of vegetables under glass for the winter market is one of the most special and difficult of all horticultural operations, . . .there are comparatively few varieties of vegetables particularly adapted to winter forcing, and the markets are less extensive and more unstable.

To succeed with forced vegetables requires great skill in the management of glass houses, close attention to every detail, and the complete control of all the conditions of plant growth. To these requirements must be added a thorough knowledge of the markets, and the ability to have the crop ready at any given time.

To reduce their economic vulnerability, some vegetable growers in the more settled greenhouse areas are diversifying into cut flowers and bedding plants, sometimes as part of a vegetable rotation.

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