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4. Plastic Greenhouses

Plastic, as a substitute for glass in greenhouse glazing, is here to stay, but it is still in the experimental stage. It probably always will be, as long as new materials and different uses con­tinue to appear, so you will have to keep yourself abreast of new developments. Two types of plastic generally are used: smooth transparent sheets of polyethylene and vinyl film, and corrugated and special types of plastic such as Fiberglas, Al-synite, Mylar, Filon, and Corrolux. Commercial builders are finding the smooth film plastic increasingly useful, especially when they want economical, rapid construction for additional plantings or a quick seasonal crop and aren't too concerned with the lasting qualities of the glazing. (See Photo 12.) The more expensive corrugated types have great endurance. Tests indicate they may last 25 years without deteriorating.

If at first you can't afford to build a regulation glass green­house, try one of the transparent plastic types. The investment is low, and growing plants in any kind of a greenhouse is a good way to gain experience.

Those who have had experience with plastic glazing are gen­erally agreed on the following resume: "Plastic glass has excep­tionally high strength, with low weight, is shatterproof, trans­lucent to light, resists weathering, is not attacked by rodents or termites, will not rot, and transmits approximately only 40 per cent as much heat as glass." The last point means that plastic greenhouses are usually cooler in summer and warmer in winter.

Advantages and Disadvantages

Polyethylene, Vinyl, Renolon, or other types of transparent plastic cost about 4 to 16 cents per square foot, depending on thickness. Such plastic is easily attached to the greenhouse framework and all puttying is avoided. It's possible to con­struct a small 6- by 8-foot free-standing greenhouse for $60.00 or less—providing, of course, that you do all the work.

The modern, improved plastics successfully resist strong winds and heavy snow, and transmit up to 98 per cent of the sun's visible light energy—as much as passes through good glass. Some plastics also transmit up to 80 per cent of the ultra­violet rays, which is greater than through glass.

Two layers of plastic—one inside, one outside of the glazing frame—with an inch of space between them, provide excellent insulation, thus decreasing the cost of heating.

Transparent plastic usually deteriorates rapidly and must be replaced every year or two. Certain new plastics being mar­keted are said to last up to 5 years.

The corrugated types are expensive. However, these have a long life, never need shading, and are not damaged by hail, snow, or such pests as termites. They admit up to 85 per cent of the sun's visible rays. They are not ideal for the real sun-loving crops—annuals, geraniums, cacti, etc.—but fine for or­chids, African violets, anthuriums, rex begonias, ferns, and sim­ilar plants requiring medium- to low-light intensities.

Ventilation for plastic greenhouses is somewhat of a problem. Most of them are ventilated through hinged sash on the walls rather than the roof, but a better system is to have sliding panels in the gable ends to allow more thorough ventilation.

Corrugated plastic, slightly rust-tinted in appearance when new, darkens as it ages, gradually shutting out some of the light.

Building the Transparent Plastic Greenhouse

You can use plastic on any type of greenhouse structure— lean-to, span, partial span, or free-standing.

If you are using plastic as temporary glazing (until you can afford glass), you will want to construct the greenhouse frame so it will eventually hold glass. However, if the whole thing is to be but a temporary arrangement for a season or two, you can cut construction costs by building with low-priced, smaller-dimension lumber.

The sheet plastic comes in 100-foot rolls in widths of 10½ to 20 feet. It takes two people to put it on. One tacks the plastic at one gable next to the top of the greenhouse. The other holds the roll of plastic, releasing enough to cover three or four sash bars while pulling tightly on the roll. The first person then tacks on lath down to within 8 inches of the bottom of the strip of plastic. The second length is placed under this 8-inch lap, so there is a lap of about 6 inches, and the second length is then tacked in the same way. If the lap is about 6 inches and the plastic is pulled tight, there is no need to seal the laps since they will stay together even in strong winds. If you want a double-layer covering—and it's advisable—the inside layer can be attached with paper disks and tacks. Be sure to allow an inch or so between the layers for "dead air" insulation.

In my greenhouse, ventilation is obtained through doors and louvers in each end. Other greenhouses are ventilated by doors and side drop vents.

12. Lightly-framed plastic greenhouses  (made of one of the inexpensive films)   offer a good yet low-cost solution for the grower who needs a temporary extra growing area or an easy-to-erect plant shelter in an emergency.  (Photograph by Gottscho-Schleisner)

An experimental house 18 by 24 feet, built by the University of Kentucky, cost $24.00 for the outside layer of .0015-inch plastic. It was estimated that the framework would cost from $150.00 to $250.00. A blueprint of this plastic greenhouse is available for 10 cents through the Agricultural Engineering Department at the University of Kentucky in Lexington.

Prefab Plastic Houses

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. (See Chap­ters 5 and 13.)

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 7½ 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 southwestern 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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