Chapter 1

Greenhouse Structures and Design

Greenhouse Site Selection

Careful planning prior to construction is an essential first step in the development of a successful, profitable greenhouse production system. Before starting, it is important to have an idea of the type of plants you want to raise and sell and a decision as to whether you wish to be retail or wholesale. Sometimes one or two factors are so important that the choice of site is obvious, but more commonly each site has good and bad attributes. If desired, make a list of potential greenhouse sites and compare them using a decision matrix. The following are major factors that should be investigated before greenhouse plans go beyond paper.

Microclimate

The primary limiting factor to crop production in greenhouses is low light intensity during the winter. Areas having frequent fog, inclement weather, or shadows cast by trees or tall mountains are poor for greenhouse crops in general.

Water Availability and Quality

One of the most frequently overlooked requirements in the establishment of a greenhouse is the quantity and quality of the water. A sufficient quantity of high-quality water is extremely important to produce greenhouse crops. The need for frequent irrigation requires careful planning and management, to ensure that operations have sufficient water to maintain adequate supplies for crop production.

Topography

The topography of the site affects where a growing structure is built. (Topography refers to the shape of the land, e.g., hilly, steep, rocky, flat.) The surface of the ground of the greenhouse should be level. Placing a growing structure on a flat surface is efficient because it facilitates easy adjustments to various mechanical controls in the greenhouse, which is economical. The site should be well drained.

Windbreaks

Outside the greenhouse, a windbreak can reduce infiltration of cold air and convection of heat away from the greenhouse. Wind speed greatly affects infiltration rates; 15 mile-per-hour winds can double heat loss in a greenhouse. Well-designed windbreaks can reduce wind speeds by 50 percent, reducing heat losses by 5 to 10 percent compared to an open area. They can also reduce snow accumulation on roofs.

Room for Expansion

A parcel of land larger than the grower's immediate needs should be acquired. The ultimate size of the range should be predicted. Area should then be added to this predicted figure to accommodate service buildings, storage, access drives, and a parking lot. Additionally, extra space should be allotted to cover unforeseen needs. To meet the environmental codes of some municipalities, it is necessary to use holding ponds for water effluent from the range in order to reduce nutrient release into streams. Doubling the area covered by greenhouses would constitute a bare minimum land requirement.

Availability of Labor

Present and future labor needs should be assessed and should be in accord with the labor supply in the area. Procurement of a labor supply has been a perennial problem in the horticulture industry. While the solution has appeared to rest on locating close to an urban area, this brings on a problem of higher wages. The greenhouse owner must also determine if labor is available to perform both routine and harvest-time duties.

Infrastructures

Proximity to transport networks (e.g. roads, railway), access to communication systems (e.g. telephone, internet) and availability of energy (e.g. gas, electricity) must all be considered. Greenhouses also need convenient access to materials for growing plants (growing media, fertilizers, pesticides, etc.). Transportation requirements to the greenhouse site relate directly to the intended operation's size and marketing arrangements.

Market Accessibility

Locating near the market is very important for retail growers who rely on customers getting to the greenhouse. The market potential should be explored before building. The competition should be scouted out and surveys conducted (if possible) as to the greenhouse market demands of the local area.

Legal Considerations

Site selection involves various legal considerations. Permits, licenses, and zoning regulations govern where a greenhouse may be built and often even dictate what type of building materials may be used. Selecting an appropriate site also involves how the greenhouse operation affects its neighbors. If the proposed site is near a school, hospital, or residential community, the greenhouse must cooperate with the zoning rules of these entities. If water from the site drains into parks, farms, or ecological areas, the land may be subject to various state and federal regulations. Some states require the owner to obtain certification to purchase restricted-use pesticides. In addition, the greenhouse owner must also be aware of relevant mandates from the Occupational Safety and Health Administration that ensure employee safety.

Greenhouse Orientation

Quite possibly the largest design consideration when planning a winter or year-round greenhouse is determining the orientation and angle of glazing for the structure. In general, growers optimizing for winter growing should orient their free-standing greenhouse in an east-west orientation, meaning that the longer, glazed side of the greenhouse should face south, with the shorter ends facing east and west. This orientation allows low angle light from the winter sun to enter from the side where it will not be blocked by ribs of the frame. In the orientation of multi-bay greenhouses structural components come into play.

Angle of Incidence

The pitch of a roof means the degree of slant or the angle of divergence from the horizontal. The glass of the roof not only allows the light and heat rays to pass through it, but it alos acts to some extent as a mirror, thus reflecting a part of the rays. The amount lost by reflection is proportional to the angle of incidience. Thus, if the sun’s rays fall upon the roof at right angles, little or none is lost by relfection; but when they fall at a less angle, the amount reflected increases as the angle of incidence increases. The amount of sun’s enegy lost by reflection when the rays strike the roof at various angles is shown in Figure 1.1.

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