I've just wrapped a bunch of tomato cages with 3.5mil plastic sheeting to
create green tepees for my tomatoes. The sheeting is opaque not clear
because that's all that Home Depot had (it's sold in the paint department
not the garden department). Is there special greenhouse plastic sheeting
that would work significantly better or is the drop cloth plastic good
enough? I'm only going to be using the green tepees until the end of May.
The tepees are open at the top so it should be ventilated well enough. I'm
just trying to get an early start, the normal time to plant tomatoes
around here is the end of May, doing it at the start of May is a little
risky. I'll remove the tents in a couple of weeks when the weather becomes
Depending on the plastic, the
frost protection factor associated with a row cover
can vary from 2? to 7? F. Generally the heavier materials
(spunbonded polypropylene) give greater
frost protection but tend to exclude more light.
Greenhouse coverings include long-life glass, fiberglass, rigid
double-wall plastics, and film plastics with 1- to 3-year lifespans. The
type of frame and cover must be matched correctly.
Glass. Glass is the traditional covering. It has a pleasing appearance,
is inexpensive to maintain, and has a high degree of permanency. An
aluminum frame with a glass covering provides a maintenance-free,
weather-tight structure that minimizes heat costs and retains humidity.
Glass is available in many forms that would be suitable with almost any
style or architecture. Tempered glass is frequently used because it is
two or three times stronger than regular glass. Small prefabricated
glass greenhouses are available for do-it-yourself installation, but
most should be built by the manufacturer because they can be difficult
The disadvantages of glass are that it is easily broken, is initially
expensive to build, and requires must better frame construction than
fiberglass or plastic. A good foundation is required, and the frames
must be strong and must fit well together to support heavy, rigid glass.
Fiberglass. Fiberglass is lightweight, strong, and practically
hailproof. A good grade of fiberglass should be used because poor grades
discolor and reduce light penetration. Use only clear, transparent, or
translucent grades for greenhouse construction. Tedlar-coated fiberglass
lasts 15 to 20 years. The resin covering the glass fibers will
eventually wear off, allowing dirt to be retained by exposed fibers. A
new coat of resin is needed after 10 to 15 years. Light penetration is
initially as good as glass but can drop off considerably over time with
poor grades of fiberglass.
Double-wall plastic. Rigid double-layer plastic sheets of acrylic or
polycarbonate are available to give long-life, heat-saving covers. These
covers have two layers of rigid plastic separated by webs. The
double-layer material retains more heat, so energy savings of 30 percent
are common. The acrylic is a long-life, nonyellowing material; the
polycarbonate normally yellows faster, but usually is protected by a
UV-inhibitor coating on the exposed surface. Both materials carry
warranties for 10 years on their light transmission qualities. Both can
be used on curved surfaces; the polycarbonate material can be curved the
most. As a general rule, each layer reduces light by about 10 percent.
About 80 percent of the light filters through double-layer plastic,
compared with 90 percent for glass.
Film plastic. Film-plastic coverings are available in several grades of
quality and several different materials. Generally, these are replaced
more frequently than other covers. Structural costs are very low because
the frame can be lighter and plastic film is inexpensive. Light
transmission of these film-plastic coverings is comparable to glass. The
films are made of polyethylene (PE), polyvinyl chloride (PVC),
copolymers, and other materials. A utility grade of PE that will last
about a year is available at local hardware stores. Commercial
greenhouse grade PE has ultraviolet inhibitors in it to protect against
ultraviolet rays; it lasts 12 to 18 months. Copolymers last 2 to 3
years. New additives have allowed the manufacture of film plastics that
block and reflect radiated heat back into the greenhouse, as does glass
which helps reduce heating costs. PVC or vinyl film costs two to five
times as much as PE but lasts as long as five years. However, it is
available only in sheets four to six feet wide. It attracts dust from
the air, so it must be washed occasionally.
Mad dog Republicans to the right. Democratic spider webs to the left. True
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