Growing Under Open Roofs By Sven E. Svenson

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 theoutdoor environment. Heating systems are needed when the outdoor environmentmakes the greenhouse too cold. Fans and cooling systems are needed when theoutdoor environment makes the greenhouse too hot (or perhaps the greenhousedesign provides insufficient ventilation). Shading systems are needed when theoutdoor environment provides too much radiation (too much light or heat).Irrigation systems are needed since the greenhouse roof prevents naturalrainfall from watering the crop. Horizontal airflow fans are needed tocirculate the air because the greenhouse prevents exposure to natural breezes.Supplemental lighting systems are needed when the outdoor environment providesinsufficient light (or perhaps the greenhouse glazing does not providesufficient light transmission). All of this technology is expensive andrequires energy and maintenance to operate.

Many cultivars that are healthy when grown outdoors maystretch and become infested with pests and diseases when forced in agreenhouse. Growers are forced to use chemical growth regulators and pesticidesto regulate height and control pests. If growers could use the outdoorconditions that support the growth of healthy, high-quality plants when thoseoutdoor conditions are available, then growers might be able to reduce the useof growth regulators and pesticides.

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

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

An often-repeated statement is: “Growers understandthat 80 percent of problems in a greenhouse can usually be traced to poorenvironmental conditions.” By working with the natural environment asmuch as possible, growers using open-roof structures can expose the crop to thebest 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 oroutdoor-only growing are nearly eliminated.

Using the natural breeze

In traditional greenhouses, the solid roof and side wallsprotect the plant from exposure to damaging winds. Stationary coverings alsoprevent exposure to gentle, natural breezes. If the grower can expose the cropto 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 ofopen roofs and roll-up side walls lets growers capture natural breezes as agrowing tool to develop crops with strong stems and a compact appearance.

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

Naturally compact plants with strong stems and improvedresistance to pests let the grower reduce or eliminate the applications ofchemical growth regulators and pesticides. This reduces all costs and risksassociated with chemical applications, including: cost of chemicals,application labor, application equipment maintenance, applicationrecord-keeping, possible phytotoxicity damage to the crop, and possible workerexposure to chemicals (including possible litigation costs). When your recordsprove you have reduced your use of chemicals, you can also request a lowerpremium on your liability insurance.

For crops like bedding plants or perennials that willeventually 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 plantsto a “hardening-off” area before shipping.

Using UV light

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

Exposure to excessive UV radiation can slow growth or directlydamage many crops. However, exposure to sufficient UV light is needed toencourage the plant to protect itself from radiation. The stronger cell walls,changes in physiology and thicker, waxy cuticles that plants develop to protectthemselves from exposure to UV light also help protect them from pests anddiseases. Many plants also grow more compact when exposed to UV light. So, justlike grower-controlled exposure to natural breezes, careful exposure to UVÁ light can improve crop quality, reduce the use of chemicals and reducecosts. Natural UV light is free, but you need a structure that lets you takeadvantage of it.

For crops like bedding plants or perennials that willeventually be used outdoors where they will be exposed to UV light, plants properly grown in open-roof structures are always acclimated to UV light. Thiseliminates 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 lightavailable to the crop. Stationarycoverings reduce the amount of light all the time, even when a reduction(shading) is not needed. Under cloudy conditions or in the early morning andlate afternoon hours, shading is usually not needed. Open-roof structuresprovide growers with the option to expose the crop to all of the availablelight whenever shading is not needed. The extra light can influence cropscheduling.

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

As is the case with UV light, for crops that will eventuallybe used outdoors where they will be exposed to full sunlight, plants properlygrown 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 formany years. Reducing the difference between the day and night temperatureshelps many crops develop a compact growth habit. Open-roof structures can beeasily operated to keep temperatures warm at night and cool during the daywithout the energy and maintenance expense of a cooling system, therebyreducing the difference between the day and night temperature. The natural,outdoor air temperature is free, but you need to have a structure that can takeadvantage 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. Inopen-roof structures, crops receive more light when the roof is kept open torelease heat. When this “photo:thermal ratio” is increased, plantsaccumulate more dry weight (more carbon) on a daily basis, which can lead tofaster growth or development. So, one way to obtain compact plants using DIFand faster crop development at the same time is to operate the open-roofstructure to optimize the photo:thermal ratio.

Adapting to open-roof growing

When using open-roof structures, grower-regulated exposureto natural breezes, natural UV light, natural full sunlight and DIF allcontribute to crops that are naturally compact, naturally more pest- anddisease-resistant and continuously acclimated to outdoor conditions. Thesegrowing tools may be used all at the same time or whenever one or more isavailable.

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

A computer operation system is an essential component forproper operation of an open-roof growing system. Trying to keep the roof andside walls in the correct position by hand is a full-time job, and other choreswill 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 apermanent, full-time worker.

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

Consider the labor savings

An important reason growers are selecting open-roofstructures is to reduce labor costs. There are significant labor savings fromreduced crop handling. When using open-roof systems, plants need not be movedfrom stationary-roof systems into “acclimation areas” forhardening-off before shipping (the roof is moved instead). Useful growing space is not wasted asacclimation area.

Seasonal greenhouse glazings need not be removed — theroof is moved instead. Eliminating each labor-handling step saves a significantamount of money.

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



Sven E. Svenson

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 sven.e.svenson@orst.edu.



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