The Greenhouse Structure

The greenhouse structure represents both the barrier to direct contact with the external environment and the containment of the internal environment to be controlled. By design, the covering material allows for maximum light penetration for growing crops. A number of commercial greenhouse manufacturers and greenhouse designs are suitable for greenhouse vegetable crop production. The standard greenhouse design used for vegetable production, is a gutter connect greenhouse.

Rough gutter conned, double poly, vegetable production greenhouse

By design, a gutter connect greenhouse allows for relatively easy expansion of the greenhouse when additions are planned. Gutter connect greenhouses are composed of a number of "bays" or compartments running side by side along the length of the greenhouse.

Typically, these compartments are approximately 120 feet long by 21 to 25 feet wide. The production area is completely open between the bays inside the greenhouse. The roof of the entire structure consists of a number of arches, with each arch covering one bay, and the arches are connected at the gutters where one bay meets the next. The design of a gutter connect greenhouse allows for a single bay greenhouse of 2,500 feet to easily expand by the addition of more bays to cover an area of 1 hectare (2.5 acres) or more.

With a gutter connect greenhouse, the lowest parts of the roof are the gutters, the points where the adjacent arches begin and end. The trend for gutter heights in modern greenhouses is to increase, with greenhouses getting taller.

The reasons for this change are two-fold: firstly, newer vegetable crops like peppers require a higher growing environment. Peppers will often reach 3.5 meters  in height during the course of the production cycle, so taller greenhouses allow for more options in crop handling and training.

Secondly, taller greenhouses allow for a larger air mass to be contained within the structure. The advantage is that a larger air mass is easier to control, with respect to maintaining an optimum environment, than a smaller air mass. Once a grower has established an environment in the larger air mass, it is easier to maintain the environment.

Rough Brothers greenhouse under construction

Typical gutter heights for modern greenhouse structures are 13 to 14 feet and are quite suitable for greenhouse pepper production. The trend for future gutter height is to increase further, with new construction designs moving to 16 to 18 feet.

There arc a number of options for greenhouse covering materials: glass panels, polycarbonate panels and polyethylene skins. Each of the coverings has advantages and disadvantages, the main determining factors usually being the trade-off between cost and length of service. Glass is more expensive, but will generally have a longer service life than either polycarbonate or polyethylene.

The United States vegetable production greenhouses are constructed with double polyethylene sheets. Two layers of polyethylene are used, with pressurized air filling the space between the two layers to provide rigidity to the covering. The life expectancy of a polyethylene greenhouse covering is about four years.

Energy conservation is also an important factor. The covering must allow light into the greenhouse and yet reduce the heat loss from the greenhouse to the environment during the winter.
New coverings are being developed that selectively exclude certain wavelengths of light and, as a result, can help in reducing insect and disease problems.

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