Plastic Greenhouses – advantages
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. 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 costs little, 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 little money 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.
