MATERIALS USED TO COVER THE STRUCTURES

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.

pt’ ^ TABLE 19-1.

A Comparison of Materials Used to Cover Growing Structures

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.