Plastic Greenhouses
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.
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
Types of transparent plastic cost little, depending on thickness.
Such plastic is easily attached to the greenhouse framework. 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.
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 percent of the sun's visible rays.
They're 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 can be 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
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.
The sheet plastic usually comes in 100-foot rolls in widths of 10½
to 20 feet. It takes two people to put it on. One person 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
