One typical, small, prefabricated, plastic greenhouse comes packed in a do-it-yourself kit. The 6- by 8- by 7-foot model sells for under $80.00. Extra 4-foot sections are available so you can extend the house as much as you wish. The manufacturer claims it will stand up under 50-degree-below-zero weather. He recommends for it a $15.00 electric heater.
A manufacturer of prefab greenhouses puts out a delightful 10- by 12-foot Fiberglas house with an aluminum frame. This house eliminates hail damage, glass replacement, and periodic painting.Still another manufacturer of prefab greenhouses includes with his greenhouses a Fiberglas roof.
Crops for the Transparent Plastic House:
Many growers use the plastic greenhouse just as they would the cold greenhouse. They grow spring bedding plants and vegetables in it and, during summer, use it to store large specimen plants. Other growers use the plastic greenhouses the year round for growing potted African violets, wax begonias, geraniums, and so forth.
The care of plants in a plastic greenhouse, if it is well ventilated, approximates that of under-glass plants. If there is only side-ventilation, you should install a fan to circulate fresh warm air in winter and to bring in cooler summer air.
If your crops need shading, do not use a material with a linseed oil base. This soaks into the plastic and can never be removed.
Other Uses for Plastic:
Both transparent and corrugated plastic can also be used for glazing a cold frame or lath house. In these structures you can harden off annual and perennial seedlings, getting them ready for spring sales. Or you can start any of the plants recommended for growing in a glass-glazed cold frame.
And there are many uses for plastic inside the glass house. I use thin sheets of transparent plastic to cover flats of seedlings or cuttings. It helps to maintain constant moisture and cuts down the time spent on watering the plantings. I like plastic bags for storing tubers and bulbs. Also, I fasten sheets of transparent plastic inside the glass house to provide insulation against cold and so cut down on heating costs.
Fiberglas House in Illinois:
Mr. and Mrs. F. O. Reh of Belleville, Illinois, have pioneered in Fiberglas greenhouses, and their experience is valuable for every grower. In their first plastic greenhouse, the Rehs used 40- by 140-inch sheets of corrugated plastic for the roof.
A clear mastic was used to seal the plastic sheets together. (The sheets can also be "cemented" together with a weatherproof adhesive tape.) An overlap of one corrugation was used. The sheets were fastened to the various supports with galvanized screws, set with lead washers. In this greenhouse, there are no provisions for vents in the roof, but four screened ventilators at ground level allow air intake. Top ventilation is provided by a thermostatically controlled low-speed fan. This fan makes an air change in the house every 7 minutes without creating any drafts.
The fibers of glass in the plastic and its crinkled surface diffuse the light without casting shadows. In this greenhouse every inch of space is available for growing plants. Eighty-five per cent of the visible light is admitted but a large percentage of the infrared (burning rays) are screened out. The sunlight in this plastic greenhouse is like the light in a glasshouse on a slightly cloudy day.
Flat plastic sheets make up the side walls of the Reh greenhouse. These sheets, measuring 36 by 72 inches, are fitted with edges butted; the seams are sealed with the sticky mastic and covered with cypress molding. The greenhouse does not have a solid foundation wall; the plastic material extends to ground level giving full-length light for growing plants.
Belleville is located in the south western part of Illinois. The seasons are variable. Although the temperatures never dip as low as in Minnesota, it is necessary to make some provision there for heating a greenhouse. The Reh heater is a propeller fan type with steam entering a finned coil through which the fan draws air. The adjustable heater blade "pushes" the warm air in any desired direction. This circulating heater, controlled by a thermostat and an aqua stat, is suspended from the roof. It collects and circulates the cooling humid air from under the roof before moisture condenses, and fans the air back to the plants in the form of a warm, humid breeze. Since the heating unit is up near the roof, it also eliminates drip, which in conventionally heated glasshouses is caused by the condensation of moist air when it strikes and collects on the chilled glass roof. More than annoying, a regular drip of water falling on plants is a disease hazard.
Mechanical controls of temperature and humidity in this plastic setup are largely automatic, and the effects are indeed different. On the warm summer day when I visited the Rehs, the outside temperature was in the high nineties. Inside the plastic house the temperature was 85 degrees, and the air as balmy as on a spring day.
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