Greenhouse structures
Open roof greenhouses provide a natural environment for plant
growth when the outdoor weather is suitable and an artificial
environment when it is too hot or cold. Opening the roof over the
plants increases light intensity which can help to control the growth
habit, flowering and crop timing. It also reduces electricity costs as
expensive fan cooling is not needed.
Roll-up roof
Several methods are used to open the roof. Some
manufacturers make a roof that opens by rolling up the single or double
layer of flexible plastic glazing that runs the length of the greenhouse
bay. A small gear motor rotates a shaft that winds the plastic on to it
like a window shade. A light, second framework over the structure
secures the plastic from bellowing out during windy weather. Opening
and closing the roof can be either manual or automatic. Each side of
the roof can be controlled independently for flexibility in cooling.
Folding roof
Folding roof greenhouses work well in snowy climates as they
can be tightly closed during cold weather. Most designs use standard
vent hardware. Some have panels that hinge at the gutter and open
upward. Opening is almost 100%. Others have panels that are hinged at
the ridge and one gutter and slide sideways on teflon bearings. Opening
is about 85%. Most designs use rubber gasketing to seal the joints.
Glazing can be glass, polycarbonate or film plastic. Some manufacturers
provide a movable gutter to collect rainwater when the roof is partially
open. Wind sensors should be installed to close the roof in stormy
weather. Movable shade is frequently installed with the open roof
design. It reduces the heat load by reflecting the suns rays back out.
The shade curtain should be of a porous design to allow heat to escape.
In northern climates an energy blanket may also be installed to reduce
heat loss during the winter.
Retractable roof
These structures consist of a steel frame, flexible glazing
and cable support. Woven UV stabilized polyethylene film creates a
watertight glazing. Depending on the cropping system, bracing of the
structure can be external cables attached to deadmen, internal
compression braces or trusses with cable X bracing. Flat roof designs
are used where there is little rain or snow. A-roof designs shed the
rain and snow to an internal gutter system. Designs that will carry up
to 35 lb/sq ft snow load and 100 mph wind loads are available. The roof
opens in sections by moving the leading edge of the curtain. One gear
motor will handle up to 50,000 sq ft of roof Heating is more difficult
than in a conventional greenhouse due to the single layer plastic and
greater infiltration through gaps and cracks in the seals.
High tunnel
These low-cost, unheated poly covered hoophouses can extend
the growing season or provide overwinter protection to plants. A couple
of manufacturers make a gutter-connected model. Ventilation is manual,
by rolling up the sides, opening the doors or in the case of the
gutter-connected design, pushing up the roof plastic. Cost is usually
less than $1/sq ft.
Basic principles of natural ventilation
Retractable roof designs can provide better dormancy maintenance,
plant hardening and insect screening through ventilation control.
Natural ventilation systems operate on the principle that heat is
removed by a pressure difference created by wind and temperature
gradients. Wind plays the major role. For a well designed greenhouse,
wind speeds of 1 mile/hour are adequate to keep the inside temperature
within two degrees of outdoor ambient. Weather records show that there
are very few days that the wind is less than 1 mph, especially if the
outdoor temperature is above 80°F.
Buoyancy, the effect from heated air getting lighter and rising also aids ventilation. The trend toward taller greenhouses has helped this in that it gets the hot air higher above the plants. The standard gutter height is now 12 and taller greenhouses are used for some crops.
Natural cooled greenhouses provide more uniform temperature throughout the greenhouse as compared to fan cooling where the temperature between the intake louvers and fans may be as much as 15 deg F. Natural ventilation systems also reduce energy costs by eliminating the 0.5 to 1 kilowatt hour/sq ft/year of electricity needed to operate a fan system. In snow country, installing small fans with a capacity of 1 2 cfm/sq ft of floor area will allow ventilation when there is snow in the gutters and the roof can not be opened.
Shadehouses
Shade structures are used to provide protection against wind and
solar radiation. They are a useful tool for modifying the environment
and extending the growing season, both in cold and warm weather.
In nursery operations, a shadehouse can provide temperature and weather protection year-round. It can also reduce irrigation needs during the summer. In some areas, the reduction in animal damage will help to pay for the structure.
Why shading?
Incoming solar energy is converted to heat energy when it
strikes plant leaves. This can result in excessive air, leaf and soil
temperatures. Placing plants under a 30 to 50% shade in the middle of
the summer can lower leaf temperature by 10°F or more. This along with
reduced wind speeds can significantly reduce transpirational water
losses during the growing season.
Not all plants require full sunlight to grow. Most plants can only utilize a limited amount of light called the light saturation level. African violets loose chlorophyll at an intensity of 1500 foot-candles (ft-c). Foliage plants may be burned at a level over 2000 ft-c. Red oak and Douglas fir have a saturation level of about 3000 ft-c. Chrysanthemum and geranium plants will take around 4000 ft-c. Rose and carnation plants will take full summer light intensity of up to 10,000 ft-c. The science of shading is really an art as the level of light that you allow to reach the top of the plant is reduce significantly by the time it reaches the bottom leaves.
Lath houses
The original shadehouses were called lath houses as they were
frame structures covered with wood lath. Most were made with poles set
into the ground with 2 framing lumber nailed to the poles to support
the lath. A 50% shade was created by leaving a space equal to the width
of a lath between adjacent laths.
When woven polypropylene was first introduced to the greenhouse industry, shadehouses took on a new appearance. Wire or cable was adequate to support the lightweight material. Today most shade structures are covered with either polypropylene, polyethylene, polyester or a composite fabric which usually contains aluminized polyester strips.
Most greenhouse manufacturers can supply a shadehouse. They can be either fixed roof or retractable roof design. Fixed roof designs are either rigid frame or cable frame.
Cable frame shadehouses
The cable frame shadehouse probably evolved from the shade
tobacco industry where several thousand acres are covered annually in
Connecticut and other states to modify the environment to produce tender
tobacco leaves for the wrapper of the best quality cigars. Posts
surrounded by concrete are set into the ground on an approximate 20 x
20 spacing. Height can be 8 to 16. Deadmen located around the
perimeter provide the bracing for the tension in the wires. Stainless
steel cable with adjustable turnbuckles are strung between the posts to
support the cloth shade material. In the tobacco shadehouses the edges
of the material are sewn around the wires with a strong thread. In the
nursery shadehouses, clips or hooks are used. Shade material hung on
the sidewall around the perimeter is attached to the upper wire and
usually buried in the soil. It provides wind protection to the plants.
Due to the variables in construction, cable shadehouses usually do not
carry a design wind or snow load.
Rigid frame shadehouses
In rigid frame shadehouses the cable is replaced by pipe or
rollformed truss members. This supports the shade cloth. Instead of
deadmen, diagonal knee braces both horizontal and vertical create the
rigid frame. Post spacing is less than the with the cable system,
usually 10 to 18. Shade material can be attached with tek screws or
clips.
Retractable roof
shadehouses
Retractable roof shadehouses use the same technology as is
used in greenhouses. They are available from several manufacturers in
several widths. They can have either cable or truss supports and
usually carry a design wind load.
As solar radiation varies considerably over the day and from season to season the main advantage of the retractable design is the ability to regulate the amount of sunlight that reaches the plants. Increased growth of the plants results as ventilation can be controlled to reduce temperature. Ventilation can also reduce the incidence of disease. Reducing the intensity of sunlight can lower irrigation needs as both the plants and the soil are kept cooler.
Both cable and truss style retractable roof designs utilize standard energy blanket technology for opening and closing the shade. One gear motor can handle up to 50,000 sq ft of growing area. The shade material is usually stored at the post line. For areas that receive considerable snowfall, the roof is retracted and snow allowed to cover the plants providing insulation. The shade material is stored under a protective hood so that it doesnt get covered with snow.
For a grower that is now utilizing the conventional 14 wide overwintering hoophouse covered with white poly for protection of perennials, herbs and nursery stock, a retractable roof structure can give better temperature control. It can also reduce plant handling cost as the larger area under one roof and the vertical sides allow the use of mechanized handling equipment.
Sidewall and endwall ventilation
In most greenhouses and shadehouses, it is advantageous to
have sidewall ventilation. First, it can be used as a first stage of
cooling. Second, in larger structures, it can supply most of the intake
air and the roof vents act as the outlet.
Sidewall and endwall covering can be fixed poly, roll-up curtains or rigid polycarbonate.
Manual and motorized rollup systems are available. These use a conventional roll-up mechanism and small gearmotor. Ventilation rate is controlled by the size of the opening. The drop-down system works better in cooler weather as the air is introduced above the plants. Restraining cables or guides are installed to keep the detached sidewall curtain from blowing on windy days.
Publications
These publications provide additional information and resources. They are available from the Department of Natural Resources Mgt. & Engr., 1376 Storrs Road UConn, Storrs CT 06269-4087. Make check payable to UConn. Prices include postage and handling.
Greenhouse Engineering NRAES-133, 212 pages, $30.00.
Energy Conservation for Commercial Greenhouses NRAES-3, 100 pages, $20.00.
Greenhouses for Homeowners and Gardeners NRAES-137, 200 pages, $30.00.
Herbaceous Perennial Production: A Guide from Propagation to Marketing NRAES-93, 200 pages, $30.00
John W. Bartok, Jr., Agricultural Engineer,
Natural Resources Mgt. & Engr. Dept.
University of Connecticut, Storrs CT
5/05
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