Section 1: Environmental Parameters of Interest
Controlled environments used in horticultural production may be as simple as saran-covered shade houses or as complex as growth chambers. Although greenhouses are probably the most common example of a controlled environment used in horticultural production, the type of controlled environment and systems that are needed depend upon the climate, time of year, crops being produced, the environmental parameters that must be controlled and the degree to which these parameters need to be controlled. The environmental parameters that may need to be controlled include:
Temperature
One of the primary reasons for producing crops under controlled environments is so that temperature can be optimized for the crops being produced. By controlling temperature, production may be extended into seasons where outdoor production would not be possible, or temperature may be manipulated to improve production and quality, reduce production time, break dormancy or control flowering. It is important to remember that lowering temperature may be as important as increasing temperature depending upon the climate (i.e. subtropical and tropical locations vs. temperate locations), time of year (i.e. summer vs. winter) and crops being grown.
Light
Light is a critical component for controlled environment production. In cases such as greenhouses, the only light source may be the sun, or supplemental light (light added from an artificial source such as high-intensity lamps) might be used to increase the light levels or increase daylength. In some cases, such as during summer months, reducing light levels through the use of a sun screening materials (i.e. saran shade cloth) may be required. In other cases, such as growth chambers and seed germination chambers, all of the light must be provided from an artificial source. It is important to remember that there are 3 characteristics of light that are important for the production of crops in controlled environments (including greenhouses).
Quantity (irradiance/intensity)- To maximize production, the quantity of light (number of particles of light called photons) received by the plant needs to be optimized. Too low light or too high light levels can be detrimental and reduce production or crop quality. Different plant species also have different light level requirements. For example, the optimal light level for most ferns is much lower than that for chrysanthemum or tomato.
Duration (photoperiod) - Photoperiod (more correctly the length of the night) affects plant growth and development. For example, some plants (i.e. poinsettia and chrysanthemum) are short-day plants (actually long-night plants) and only flower when the night exceeds a critical length (i.e. 12.5 hours for poinsettia). Photoperiod is often manipulated to either maintain plants in a vegetative state or to induce flowering. To accomplish this, blackout shade cloth might be pulled in the late afternoons to essentially create a longer night (short day) or lights might be turned on at night to essentially create a shorter night (long day).
Quality (wavelength/color) - Visible light quality may be expressed in terms of the light's wavelength (in nanometers) or the color that it appears to the human eye (think of the colors of light in a rainbow). Optimal plant growth requires light of specific wavelengths. For photosynthesis, light in the range of 400 - 700 nanometers (nm) is most effective for driving photosynthesis. This wavelength range is referred to as photosynthetically active radiation (PAR). While red (i.e. 660 nm) and blue (i.e. 460 nm) are the wavelengths of light used most efficiently in photosynthesis, red, far-red (i.e. 730 nm) and blue light have significant effects on plant photomorphogenesis (changes in plant growth due to exposure to different wavelengths of light). For example, a light source used for night interruption to create long days and prevent flowering in short day plants (i.e. poinsettia) must have some red light supplied from the light source to be effective. Plants grown under high levels of blue light tend to be shorter while plants growing under conditions with high far-red light tend to be taller and elongated. Light supplied only in a narrow wavelength band can be detrimental to plant growth. Understanding how light quality affects plant growth is very important when selecting a light source to be used to supplement the light available for photosynthesis or to manipulate photoperiod.
Water
One of the most essential components for any successful horticulture operation is water. This is particularly true for production in controlled environments such as greenhouses where intense cropping requires not only significant volumes of water, but water with suitable chemical characteristics. Characteristics such as the water's alkalinity, electrical conductivity and concentration of potentially phytotoxic ions such as fluoride or sodium are particularly important.
Fertility
Plants require mineral elements such as N, P, K, Ca, Mg and others for proper growth. The goal in controlled environment agriculture(including greenhouse crops production) is to provide the required elements in the appropriate concentrations and ratios to maximize production. Mineral element deficiencies, toxicities or imbalances can be detrimental to plant growth.
Substrate (root medium)
Naturally occurring soils (field soils) are typically not used for the production of crops in controlled environments (including greenhouse). Instead, substrates (also called root media) composed of materials such as Sphagnum peat, perlite, vermiculite, composted barks, coir or rice hulls are formulated to create a substrate with the physical and chemical characteristics desired for the specific crop and environment. The substrate must not only provide physical support for the plant, but it must also serve as a reservoir for water and nutrients. Additionally, the substrate must allow for gas exchange so that oxygen is available to the roots.
Humidity
Depending upon the objective, increasing or decreasing the relative humidity experienced by the crop may be desired. During propagation, mist or fog systems might be used to increase the relative humidity and reduce water loss through transpiration. During crop production, venting of water-saturated air may be required to lower the relative humidity and reduce the possibility of foliar disease development (i.e. Botrytis).
Atmosphere
Although the atmosphere may include several environmental parameters, of particular interest are such factors as carbon dioxide concentration, ethylene and substances such as phytotoxic volatiles given off by certain wood preservatives and herbicides. Carbon dioxide is an important component in photosynthesis. Often times in controlled environments (including greenhouses) the carbon dioxide level is below optimal and is limiting to plant growth. In such a case, the carbon dioxide level can be increased by injecting carbon dioxide into the atmosphere. Ethylene gas and other volatile compounds can cause undesirable effects such as malformed growth and flower abortion.
Airflow
Airflow can be manipulated in order to provide a more uniform temperature through the environment or to remove hot air from the environment. Airflow may also be manipulated to control the relative humidity experienced by the crop.
Insects/Diseases
Although insects and diseases are not an environmental parameter in the true sense of the word, they do affect the quality of the crops being produced, and they need to be maintained at or below economic threshold levels.
