Maintaining environmental control of the atmosphere within a greenhouse might be as simple as the opening of a ridge vent (as shown in figure one) or as comprehensive as regulating the operation, number and operating speed of exhaust fans (figure two) required to keep the greenhouse interior within predetermined conditions.
The atmospheric factors that must be controlled within a greenhouse are air temperature, relative humidity and carbon dioxide (CO2) content. Latitude and the months the greenhouse will be in operation will be the factors that determine whether heating or cooling will be the primary consideration or whether a combination of both will be required.
Remember that a greenhouse is an efficient solar collector, so cooling—even when the outside air temperature is near or even below freezing—will always be required. The challenge is to determine how the internal environment is to be either heated or cooled, the relative humidity controlled and the CO2 content maintained or increased, regardless of prevailing climatic conditions outside the greenhouse. In some instances supplemental lighting might also be required to sustain plant growth under low light conditions or during short days.
A greenhouse (also called a glasshouse) is a building in which plants are grown—which is a pretty simple definition that covers a wide range of building types. Greenhouses can be small or large and can vary wildly in shape and structure—ranging from very simple to very complex in design and operation—and can be constructed from either wood, plastic, aluminum or steel.
Greenhouses can be covered with plastic sheets, film, glass or a combination of different transparent materials, coverings often referred to as cladding or glazing. The relative ‘tightness’ of a greenhouse structure presents both advantages and disadvantages—being tightly built keeps insects, disease organisms and unconditioned air out, but it can also cause cracks and breakage during sudden temperature changes if the structure is too rigid.
Screening of all air paths into the structure is an essential element of greenhouse design in order to ensure efficient pest control. For the hobby or non-commercial grower, design selection factors will be based on intended use and budget considerations, while for the commercial grower the design elements required to efficiently produce a desired commodity will determine the structure and operating equipment of the greenhouse.
For cooling the air that enters the greenhouse, outside air can be pulled through a moistened pad (figure three), taking advantage of the principle that air will be cooled by water absorption. If the air to be cooled already has a high relative humidity level, however, this cooling effect will be minimal.
Keeping air moving over plant leaves also has a cooling effect—regardless of its temperature—although its relative humidity will still determine the extent of the cooling effect. High relative humidity within the plant canopy increases the potential for disease infestation and reduces the rate of leaf transpiration, which can slow growth and affect the elemental nutritional status of the plant.
The relative humidity of the air within the greenhouse is not easily or economically reduced, but its effect on plants can be moderated by keeping the air moving within the plant canopy, or even within the entire greenhouse structure. Since plants do not grow well in stagnant air, you’ll need to keep the air within the greenhouse always moving during daylight hours anyway.
Exhausting air from the greenhouse structure is more efficiently accomplished if the air is drawn the shortest distance possible—such as by having exhaust fans placed on the sides rather than at one end, having to pull air all the way across the greenhouse (as shown in figure four).
Within a plant canopy or in an un-ventilated greenhouse the concentration of CO2 can become depleted to such an extent that it reduces the rate of plant photosynthesis.
Carbon dioxide supplementation can enhance plant growth—and there are certain plant species that benefit from constantly maintained elevated CO2 concentrations—but there is evidence that suggests that it is the maintenance of CO2 at ambient levels (300 to 320 ppm) that provides the maximum benefit in most situations.
Carbon dioxide supplementation can be difficult to sustain, however, if the greenhouse air is being periodically exhausted as a way of maintaining temperature control.
Light passing through any glazing material will be reduced in intensity as well as changed in wavelength composition. Light of longer wavelengths will heat the atmosphere inside the greenhouse as well as heating up the greenhouse itself.
A portion of the light entering the greenhouse will reflect from the interior surfaces, with a portion trapped inside as heat and another portion radiated back through the glazing material and out into the surrounding atmosphere.
With this altered distribution of wavelengths due to the characteristics of the glazing material, plant growth characteristics—such as internode length, leaf shape and foliage color intensity—will be affected to an extent determined by wavelength distribution percentage and plant species.
Supplemental lighting might be required for optimal plant growth in periods of low light intensity or during seasons with short days. In most situations, the maximum benefit from supplemental illumination is normally obtained by extending the hours of light, rather than by adding to daylight intensity. Light fixtures (round lamps) can be spaced in the gable area of your greenhouse (as shown in figure five).
Depending on the frequency of periods of intense solar radiation, optimal plant production might require the use of moveable shades placed inside the greenhouse—covers that can be easily pulled over the plant canopy and then as easily retracted (figure five).
Pulling shade cloth over the top of a greenhouse (as shown in figure six) can be beneficial during months of high solar radiation, but even in summer there are days with low light conditions and shading your greenhouse during these periods can result in reduced plant growth.
Warming the air within your greenhouse structure can be accomplished with either convection heating or—more commonly—hot air distribution systems. Using fuels such as natural gas or liquid petroleum, air is passed through a heat exchanger for distribution through a holed plastic pipe placed in the gable of the greenhouse (as shown in figure seven).
The operation of the heat exchanger requires monitoring to prevent the emission of incomplete combustion products like ethylene (C4H4) and carbon monoxide (CO), which adversely affect both plant and human life.
For optimal control of the atmosphere, air conditioning should be accomplished in an air-handling building adjacent to the greenhouse. Conditioned air should be introduced into the greenhouse from openings in the greenhouse floor or from floor ducts so that the conditioned air moves upward through the plant canopy to be collected in the greenhouse gable for return to the air-handling building.
The greenhouse should have only one opening to the outside—a double-door attachment for operator access. With such a design, maintaining constant optimal atmospheric conditions in your greenhouse is possible, leading to maximum plant performance.