As the description of the different types of growing structures indicates, the materials used to cover (glaze) the structures are glass or some type of plastic. While glass has the longest history as a glazing material, plastics now dominate in the commercial industry. They are available in both soft and rigid forms. The choice of glazing material will directly affect plant growth because materials differ in the amount of light they let into the house and the amount of heat retained or lost. Also affected by the choice of glazing material is the cost of constructing the house initially, the amount of maintenance required to keep the glazing in good condition, and the frequency of replacement required.
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pt’ ^ TABLE 19-1. |
A Comparison of Materials Used to Cover Growing Structures |
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|
Material |
Length of Service |
Amount of Sunlight Passing Through |
Maintenance Required |
|
Glass |
Ten to twenty years depending on material used as glazing bars |
Full sunlight less that blocked by settled dust and shading of the superstructure; glass gives greatest amount of light |
Wood glazing bars require glass to be removed, new caulking applied, and the bars repainted every ten years. Aluminum glazing bars need little maintenance except periodic resetting of glass to correct slippage |
|
Fiberglass |
Five to ten years depending on quality of fiberglass, maintenance, and intensity of sunlight causing clouding |
75 to 85 percent of that transmitted by glass |
The fiberglass must be scrubbed down every other year to remove dirt that collects on exposed glass fibers |
|
Soft plastic (polyethylene) |
Six months to two years depending on quality of plastic and intensity of sunlight causing clouding |
50 to 80 percent of that transmitted by glass depending on type of plastic, number of layers, age, and degree of clouding caused by ultraviolet rays |
More frequent replacement needed than with other coverings; plastic clouds and is broken down by ultraviolet rays in sunlight |
|
Shade fabric |
Normally used for part of a year, then stored until next season |
30 to 50 percent of that transmitted by glass; multiple layers of fabric used for heavier shading |
None of importance |
|
Soft plastic (vinyl) |
Four to five years depending on thickness |
85 to 90 percent of that transmitted by glass |
The material attracts dust, which must be washed off |
|
Soft plastic (polyvinyl fluoride) |
Ten years or more |
92 percent of that transmitted by glass |
Less than the other soft plastics |
|
Acrylic rigid panels |
Ten years or more |
83 percent of that transmitted by glass |
Periodic washing to remove dust and dirt |
|
Polycarbonate rigid panels |
Ten years or more |
75 to 80 percent of that transmitted by glass (less as it ages) |
Periodic washing to remove dust and dirt |
Glass, the classic and traditional covering, has one great advantage over the plastic alternatives: the high level of light transmission that it permits. It also has a longer life than the plastic coverings. In regions of the country where heavy snow loads may collapse a plastic covered house, the framework of a glass greenhouse allows it to stand strong against the weight and the wind. While a few panes of glass may be lost,
the greenhouse will survive. However, it is not without its disadvantages. Vandals and hail storms can damage glass greenhouses more easily and at greater cost than plastic houses.
Plastic is available in rolls or as rigid panels. It may be applied to the framework of the greenhouse as a single layer or as double sheets separated by a layer of air to improve heat retention. The initial reason for the popularity of plastic over glass was its lower cost. That advantage has lessened with the unexpected sharp rise in the cost of petroleum – based products and narrowed the cost differential between glass and the plastics. Plastic glazings have the disadvantages of reduced light transmission, especially during the gray days of winter, and a tendency to sustain a higher humidity within the greenhouse. That can lead to added disease problems. Table 19-1 compares the glazing materials used to cover greenhouses and other growing structures. The most recent statistics from the Agriculture Statistics Board indicate that glass is the glazing of choice for about 13% of the nation’s greenhouses, rigid plastic panels cover about 19%, and soft plastics are used for over 68% of the production structures.
