Growing Under Open Roofs

March 18, 2002 - 12:18

If someone told you that you could take advantage of optimal light, ventilation and natural DIF, saving on chemical and labor costs, what would you say? All it takes to get started is an investment in an open-roof greenhouse system.

What is the function of a growing structure? To control the environment or to assist the grower with manipulating healthy plant growth? Should the grower and the plants be forced to adapt to the environment the greenhouse provides, or should the greenhouse provide adaptable environments the grower needs to force the crop? Open-roof structures provide more environmental flexibility compared to traditional greenhouse designs.

Working With Nature

Greenhouses create a growing environment in response to the
outdoor environment. Heating systems are needed when the outdoor environment
makes the greenhouse too cold. Fans and cooling systems are needed when the
outdoor environment makes the greenhouse too hot (or perhaps the greenhouse
design provides insufficient ventilation). Shading systems are needed when the
outdoor environment provides too much radiation (too much light or heat).
Irrigation systems are needed since the greenhouse roof prevents natural
rainfall from watering the crop. Horizontal airflow fans are needed to
circulate the air because the greenhouse prevents exposure to natural breezes.
Supplemental lighting systems are needed when the outdoor environment provides
insufficient light (or perhaps the greenhouse glazing does not provide
sufficient light transmission). All of this technology is expensive and
requires energy and maintenance to operate.

Many cultivars that are healthy when grown outdoors may
stretch and become infested with pests and diseases when forced in a
greenhouse. Growers are forced to use chemical growth regulators and pesticides
to regulate height and control pests. If growers could use the outdoor
conditions that support the growth of healthy, high-quality plants when those
outdoor conditions are available, then growers might be able to reduce the use
of growth regulators and pesticides.

Just as there are beneficial insects useful for Á
biological manipulation of pests, there are “beneficial”
environmental conditions useful for manipulation of plant growth. Both are
helpful tools when used properly; both are a waste of money when not properly

The list of useful environmental conditions (see sidebar on
page 30) that are not available when growing in traditional greenhouse designs
is also the list of the advantages of growing under “open” roofs.
In open-roof structures, growers have more control over the development of stem
caliper, insect resistance, disease resistance, transplanting-stress resistance
and root growth development because they have more control of exposure to
natural breezes, exposure to natural UV light, exposure to full natural
sunlight and exposure to cooler daylight temperatures.

An often-repeated statement is: “Growers understand
that 80 percent of problems in a greenhouse can usually be traced to poor
environmental conditions.” By working with the natural environment as
much as possible, growers using open-roof structures can expose the crop to the
best available environment for a longer period of time.

During any crop-growing cycle, the best environmental conditions often occur outside a traditional greenhouse. Open-roof structures allow the grower to expose the crop to beneficial outdoor environments while retaining the ability to protect the crop from sub-optimal outdoor environments (without having to move the crop). The grower has greater control of crop development than can be obtained by growing in traditional greenhouses or outdoor compounds alone (or in combination). The risks associated with greenhouse-only or
outdoor-only growing are nearly eliminated.

Using the natural breeze

In traditional greenhouses, the solid roof and side walls
protect the plant from exposure to damaging winds. Stationary coverings also
prevent exposure to gentle, natural breezes. If the grower can expose the crop
to natural breezes, the crop develops a compact appearance with stronger stems.
The response is similar to the use of “brushing” or
“mechanical conditioning” for height control. Proper operation of
open roofs and roll-up side walls lets growers capture natural breezes as a
growing tool to develop crops with strong stems and a compact appearance.

Exposure to gentle breezes has also been shown to help the
plant develop its own natural resistance to insects and diseases. It also
encourages proper development of waxy surfaces on leaves and stems, which help
reduce water loss and provide some protection against insect feeding and
invasion by germinating disease spores. When the plant’s own defenses are
stimulated, the effectiveness of biological predators is improved. So, compared
to structures with stationary coverings, plants grown in open-roof structures
often require fewer applications of pesticides. Natural breezes are free, but
you need a structure that lets you take advantage of them.

Naturally compact plants with strong stems and improved
resistance to pests let the grower reduce or eliminate the applications of
chemical growth regulators and pesticides. This reduces all costs and risks
associated with chemical applications, including: cost of chemicals,
application labor, application equipment maintenance, application
record-keeping, possible phytotoxicity damage to the crop, and possible worker
exposure to chemicals (including possible litigation costs). When your records
prove you have reduced your use of chemicals, you can also request a lower
premium on your liability insurance.

For crops like bedding plants or perennials that will
eventually be used outdoors where they will be exposed to natural breezes,
plants properly grown in open-roof structures are already acclimated to wind.
This eliminates the labor costs and growing space needed to relocate the plants
to a “hardening-off” area before shipping.

Using UV light

Greenhouse glazing greatly reduces or nearly eliminates
exposure of the crop to UV light. The materials that protect the glazing from
damage by UV light also prevent exposure of the crop to UV light.

Exposure to excessive UV radiation can slow growth or directly
damage many crops. However, exposure to sufficient UV light is needed to
encourage the plant to protect itself from radiation. The stronger cell walls,
changes in physiology and thicker, waxy cuticles that plants develop to protect
themselves from exposure to UV light also help protect them from pests and
diseases. Many plants also grow more compact when exposed to UV light. So, just
like grower-controlled exposure to natural breezes, careful exposure to UV
Á light can improve crop quality, reduce the use of chemicals and reduce
costs. Natural UV light is free, but you need a structure that lets you take
advantage of it.

For crops like bedding plants or perennials that will
eventually be used outdoors where they will be exposed to UV light, plants properly grown in open-roof structures are always acclimated to UV light. This
eliminates labor costs and growing space needed to relocate crops to a
“hardening-off” area before shipping.

Using full sunlight

All greenhouse glazings reduce the total amount of light
available to the crop.  Stationary
coverings reduce the amount of light all the time, even when a reduction
(shading) is not needed. Under cloudy conditions or in the early morning and
late afternoon hours, shading is usually not needed. Open-roof structures
provide growers with the option to expose the crop to all of the available
light whenever shading is not needed. The extra light can influence crop

Compared to the taller, softer growth with weak stems that
some plants develop when grown in traditional greenhouses, careful exposure to
full-sun conditions when grown in open-roof structures encourages plants to
develop stronger stems, waxy surfaces and a compact growth habit. So, just like
grower-controlled exposure to natural breezes and UV-light, careful exposure to
full sunlight can improve crop quality, reduce the use of chemicals and reduce
costs. Natural, full sunlight is also free, but you need a structure that
allows you take advantage of it.

As is the case with UV light, for crops that will eventually
be used outdoors where they will be exposed to full sunlight, plants properly
grown in open-roof structures are already acclimated to full-sun conditions.
This eliminates labor costs and growing space needed to relocate crops to a
“hardening-off” area before shipping.

Optimizing natural DIF

Growers have been using DIF to manipulate plant growth for
many years. Reducing the difference between the day and night temperatures
helps many crops develop a compact growth habit. Open-roof structures can be
easily operated to keep temperatures warm at night and cool during the day
without the energy and maintenance expense of a cooling system, thereby
reducing the difference between the day and night temperature. The natural,
outdoor air temperature is free, but you need to have a structure that can take
advantage of it.

When open-roof structures are operated to reduce DIF
(maintaining cool daylight temperatures), the relationship between the amount of light the plant is exposed to each day (daily light integral) and the average daily temperature is altered. In traditional greenhouse systems, the temperature is controlled by the cooling system, but light levels remain unchanged. In
open-roof structures, crops receive more light when the roof is kept open to
release heat. When this “photo:thermal ratio” is increased, plants
accumulate more dry weight (more carbon) on a daily basis, which can lead to
faster growth or development. So, one way to obtain compact plants using DIF
and faster crop development at the same time is to operate the open-roof
structure to optimize the photo:thermal ratio.

Adapting to open-roof growing

When using open-roof structures, grower-regulated exposure
to natural breezes, natural UV light, natural full sunlight and DIF all
contribute to crops that are naturally compact, naturally more pest- and
disease-resistant and continuously acclimated to outdoor conditions. These
growing tools may be used all at the same time or whenever one or more is

Compared to traditional growing structures, the timing and
concentration of growth regulators needed when growing in open-roof designs are
different. In some situations, they may no longer be needed. Similarly, the
timing and amounts of pesticides, fertilizers and irrigation water also differ.
Temperature recipes used to schedule crops differ. When growers switch to
open-roof structures, they will have to carefully reapply their skills to
manipulate crops as necessary. However, they will have many more tools and
alternatives available to them to help them be successful and profitable.

A computer operation system is an essential component for
proper operation of an open-roof growing system. Trying to keep the roof and
side walls in the correct position by hand is a full-time job, and other chores
will usually cause the person in charge of the structure to “fail”
on a regular basis. A one-time purchase of a good computer control system
(typically about $20,000 purchased and installed) is much less expensive than a
permanent, full-time worker.

No matter how sophisticated, growing structures do not
operate by themselves. Remember that the computer controller does not operate
the system or grow the crop for you. This is computer-assisted growing, not
computer-controlled growing. You must still know your crop and its needs, and
make daily, routine or seasonal adjustments to the computer settings as needed.

Consider the labor savings

An important reason growers are selecting open-roof
structures is to reduce labor costs. There are significant labor savings from
reduced crop handling. When using open-roof systems, plants need not be moved
from stationary-roof systems into “acclimation areas” for
hardening-off before shipping (the roof is moved instead). style="mso-spacerun: yes">  Useful growing space is not wasted as
acclimation area.

Seasonal greenhouse glazings need not be removed — the
roof is moved instead. Eliminating each labor-handling step saves a significant
amount of money.

For some crops, such as florist azaleas, open-roof
structures can eliminate from 2-5 handling steps. Depending upon your crop and
your current handling system, you can eliminate as much as 50 percent of your
handling labor costs by  growing
under an open-roof system.

About The Author

Sven E. Svenson is an assistant professor at the North Willamette Research and Extension Center at Oregon State University, Aurora, Ore. He may be reached by phone at (503) 678-1264 ext. 14 or via E-mail at

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