BACK TO THE LAND
GREENHOUSES
The back to lander/homesteader can buy a prefabricated greenhouse(as shown in photos below) or choose to build one to meet specific needs and budgets. This article provides information about the more simple, more affordable home constructed versions.
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1#
A Small Backyard Greenhouse For the Home
Gardener
Home gardening and landscaping are enjoyable hobbies for many people
in Virginia. If you are
among them, you might benefit from a small backyard greenhouse. A greenhouse
can be built easily
and inexpensively in several hours using simple hand tools and materials
available at most building
supply stores. This publication presents plans and instructions for
an easily constructed greenhouse that costs about $100 and may be used
for many purposes. Measuring 12 feet by 14 feet (168 square feet), it is
small enough to be unobtrusive but large enough to meets the needs of the
serious grower.
Selecting a Site
Choose a level, well-drained plot for the greenhouse. If it will be
used primarily for plant propagation in the summer, place it in partial
shade to minimize heat buildup. A good location is the north side of a
large deciduous tree. If a partially shaded site is not available, you
can use a shade cloth or a white
plastic cover to control the amount of sunlight reaching the interior.
If the greenhouse will be used for starting transplants or growing plants
to maturity, it will need
maximum exposure to the sun. It should also be located where air drainage
is good; avoid low areas
surrounded by woods or buildings. Easy access to running water and
electricity is also important.
A backyard greenhouse constructed from inexpensive materials.
Constructing the Greenhouse
The greenhouse consists of a wooden frame to which are attached bows
of polyvinyl chloride (PVC)
pipe. The bows hold up the clear plastic film covering.
All lumber for construction should be treated for ground contact with
an environmentally safe
preservative such as CCA. Avoid wood preserved with chemicals such
as pentachlorophenol or
creosote. Lumber treated for ground contact has a higher concentration
of preservative and therefore will last much longer than untreated or lightly
treated wood. Except for the covering, the structure should last 10 to
12 years.
Unless the site is very flat, you will need to level the foundation
boards. This is best done by digging a trench on the high side instead
of raising the low side, as the foundation boards should contact the
ground at all points to ensure a good seal. To keep rainwater out,
however, the top of the foundation
board should not be below ground level, particularly around the door.
the purpose of the
4-inch-by-4-inch post at each corner is to anchor the greenhouse
and prevent it from moving in high winds.
The PVC pipe bows are attached to the side boards of the foundation
with electric metallic tubing
(EMT) clamps It is best to attach the clamps loosely to the side
boards with wood screws
before you secure the side boards to the end boards and corner blocks.
They may be tightened after
the PVC pipe has been slipped into place.
PVC pipe may be purchased in 12- or 20-foot lengths in either schedule
40 or schedule 80 weights.
Schedule 80 PVC pipe has a thicker wall and is stronger than schedule
40 pipe. It is therefore
recommended for greenhouse construction. The 12-foot width of the greenhouse
was chosen so that
each half of the rib section of pipe is 10 feet long. PVC pipe may
be easily cut with a hand say,
although a neater cut can be made with a pipe-cutting tool specially
designed for that purpose. Care
must be taken during assembly because the cement used to fasten PVC
is fast-acting and permanent.
It should not be used when the temperature is below 50 degree Farenheight.
Check the directions on
the can.
It is best to lay all parts out on a flat surface for assembly. For
the ribs to be 2 feet apart, the mid-rib
pipe must be cut into pieces about 22 1/2 inches long. After
all joints have been cemented and allowed to harden for a few minutes,
position the entire assembly over the foundation boards, bend the ribs,
and secure them in place with the EMT clamps. Assembly is best done by
two or more people to prevent over stressing the cemented joints. Ten feet
of galvanized EMT positioned inside the mid-rib will give added strength
to the structure.
Construction Details
After all the PVC ribs have been secured, the end frames and door
may be constructed. Although
there is a door on only one end, both frames are essentially the same.
The two diagonals on each end give strength to the structure and make it
easy to secure the cover. The top of the end frame may befastened to the
end ribs with either a wood screw of a small metal strip bent into a U
shape. If wood screw are used, the PVC pipe must be predrilled to prevent
splitting.
Select the type of cover material according the intended use of the
greenhouse. Clear 4- or 6-mil
plastic greenhouse film is generally best if the house will be used
for growing plants. If it will be used
exclusively for propagating or overwintering plants, consider using
4- or 6-mil milky or white copolymer film. White copolymer film reduces
the amount of heat and light within the house and therefore limits the
fluctuations in these factors, keeping conditions more nearly constant
during propagation or over the winter. However, shading clear plastic with
a 30 to 50 percent polypropylene shade cloth or greenhouse whitewash available
from horticultural supply companies will produce nearly the same effect.
Foundation side and end boards are attached to 4-by-4 posts to anchor
the
structure. EMT clamps on the side boards hold the PVC pipe ribs.
The cover may be attached with small tacks, but staples are much faster
and are more secure. Allow
the cover to extend several inches past the bottom of the foundation
boards and cover the extra
material with packed soil to prevent the entrance of moles and rainwater.
An inch of coarse sand or
fine gravel on the floor will reduce problems with mud and weeds.
The mid-rib pipe is cut into sections about 22 1/2 inches long and rejoined with PVC crosses to which the ribs are attached. Ten feet of EMT is placed inside the mid-rib for strength.
(example of inside a type of bowed-PVC greenhouse; this one is
also an attached greenhouse- see following article)
Operating the green house
It is essential that the house be as tight as possible so that it retain
both heat and humidity. Problems
with overheating can usually be solved by simply opening the door.
Supplemental heat can be provided if needed during very cold weather or
at night by one or more heat lamps or a small electric heater. Any electrical
devices used in the damp environment of a greenhouse should be connected
only to a circuit equipped with an approved ground fault current interrupter
(GFCI) outlet or circuit breaker.
Other source of heat can be used, but even very small gas or oil heaters
will usually overheat a
greenhouse this small and may not burn well if not properly ventilated.
The greenhouse will satisfactorily support up to 4 inches of dry snow.
If the snow is deeper or
unusually wet, one or more temporary supports should be provided along
the mid-rib to prevent
collapse. If a larger greenhouse is required, the length may be increased.
Do not increase the width of the house without increasing the size of the
ribs. Doing so will seriously reduce the ability of the house to withstand
snow and wind loads.
Uses
A small greenhouse can be quite useful throughout the year for many
different purposes. In the early
spring, it can be used to germinate and grow bedding and vegetable
plants earlier than would otherwise be possible. In the summer it can be
used either with or with out a mist system or humidifier as a propagation
house for root cutting. It can be used in the fall to start winter vegetable
plants. During the winter, it can protect patio plants and other hardy
but containerized woody plants.
#2
Attached Solar Greenhouses
(an example of an attached greenhouse)
In most climates, a well-constructed solar greenhouse collects more energy on a clear winter day than it needs for greenhouse heating and has the potential to supply a substantial amount of heat to the space adjoining it. A south wall that receives unobstructed sunlight from approximately 9 am until 3 pm is well suited for the addition of a solar greenhouse or sunspace.
In colder areas of Virginia, the greenhouse should have between 0.65
and 1.5 square feet of
south-facing double glass for each square foot of adjacent building
floor area to be warmed. In warmer areas of Virginia, use 0.33 to 0.9 square
feet of glass for each square foot of adjacent floor area. This area of
glazing will collect enough heat during a clear winter day to keep both
the greenhouse and adjoining space at an average temperature of 60 to 70F
during the day.
However, it is also important to utilize enough thermal mass to absorb
direct sunlight and dampen
interior temperature fluctuations. A thermal-mass wall for storing
collected solar energy should be
located between the greenhouse and the adjacent space with a large
surface area of thermal wall
exposed to direct sunlight. The wall can be constructed with several
different materials at
recommended thicknesses.
Material Recommended Thicknesses
Solid Masonry 8 - 12 inches
Concrete 12 - 18 inches
Water in Containers 1 cu. ft. (7.5
gal.) for each sq.ft. of south facing glass
For maximum heat retention, the surface of the wall should be a medium or dark color and care should be taken to not block direct sunlight from reaching it. Small vents or operable windows should be located in the wall to allow heat from the greenhouse directly into the building during the daytime. Operable exterior vents and shading devices to prevent a heat buildup in the greenhouse in the summer are required.
An alternative to thermal-mass storage of heat is to actively take heat
from the greenhouse during the
day and store it in the building for use at night. Heat can be taken
from the greenhouse by a fan and
stored in a rock bed located in the crawl space under the floor of
the building. The advantage of this
system is that space is not lost in the greenhouse to a thermal-mass
wall. The greenhouse should
receive enough heat back from the building at night through the common
wall and glass to keep it at a temperature average between the indoor heated
space and outdoor temperature. However, it is
important to use operable windows or a door between the greenhouse
and building to ensure that
during periods of extremely cold weather the greenhouse can receive
direct heat from the building to
keep freezing temperatures from affecting the plants.
For adequate passive heat transfer from the rock bed to the adjacent
space to be heated, 50 to 75
percent of the floor must act as a heat transfer area. The system moves
warm air to the rock bed and
returns cool air to the greenhouse from the bottom of the rock bed.
In temperate climates, 3/4 to 1-1/2 cubic feet of fist-sized rock for each
square foot of south-facing greenhouse glass should be used.
Solar greenhouses in Virginia require double glazing to prevent undue heat loss. Insulating glass may be used as well as plastic products such as fiberglass or polycarbonate sheets. Plastic films are best used as the second layer of glazing inside the glass or rigid plastic glazing. The cost of glazing may account for nearly half the total cost of the greenhouse structure. The average cost of a solar greenhouse is, in most cases, one third that of standard construction.
When the primary function of the greenhouse is to heat an adjacent building,
taking heat from the
greenhouse by mechanical means and storing it for use in the building
will increase the efficiency of the system. However, the greenhouse will
drop in temperature to about 40 to 45F at night in temperate and cool climates
and considerably lower in very cold climates. Additional sources of heat
will be necessary if plants are to be healthy and productive. Most tropical
plants will drop their leaves if subjected to long periods below 60F, and
few vegetables will produce a yield at low temperature.
