J. Raymond Kessler, Jr.
Greenhouses are not free of influences from the outside environment: 1) Solar radiation warms inside surfaces and the air, 2) air moves in and out through ventilation and leakage, 3) wind robs a greenhouse of heat, 4) precipitation reduces inside light intensity. An important part of being a greenhouse grower is to watch the weather and learn to respond to it not only on a day-by-day basis but also season-by-season.
By its very nature, the structure of a greenhouse influences its internal environment. The structural materials have an effect on air temperature, solar radiation, relative humidity, and atmospheric gasses. Indirectly, the structure influences soil temperature and moisture. A grower should be aware of how structural materials influence the environment and now to use the greenhouse environmental control equipment to regulate the environment.
One of the major reasons for a greenhouse is to control temperature. Most of the crops grown in a greenhouse are of tropical or subtropical origin or are grown at inclement times of the year. The greenhouse is used to maintain an optimum night temperature (above freezing). This is usually accomplished with some kind of heating system. During the day, however, the greenhouse structure can actually interfere with temperature control by trapping heat. Therefore, various cooling methods must then be used.
Temperature is a measure of heat energy. The energy balance or movement of energy into, within, and out off a greenhouse occurs by three different methods:
Conduction: diffusion of thermal energy through a continuous medium, the rate of which depends on the properties of the medium. Heat energy movement is always from a region of high temperature to lower temperature.
Convection: diffusion of thermal energy between two dissimilar materials, usually between a gas and liquid, gas and solid, or liquid and solid. Heat energy movement is always from a region of high temperature to lower temperature.
Radiation: radiation heat transfer occurs when electromagnetic energy leaves one object and is intercepted and absorbed by another. This differs markedly from conduction and convection. All objects emit radiation, warmer objects more so than cooler objects. The wavelengths usually considered to be involved in radiation heat transfer are in the infrared band.
Radiation Cooling: On clear, cold nights, plants and other objects within a greenhouse will lose heat to the outside by radiation cooling. Because objects within the greenhouse are much warmer than those outside, they lose heat by emitting infrared radiation through the glazing and into the clear sky. Under such conditions, the foliage temperature may be 5°F cooler than the surrounding air. Radiation cooling occurs very little when skies are cloudy.
Condensation: When the foliage of plants in the greenhouse are cooler than the surrounding air, moisture in the air may condense on the leaf surfaces. This is most common in the spring and fall when days are bright and warm and nights are cool and clear. The moisture that collects on the leaves serves as an ideal medium for the germination of several disease-causing spores, mainly powdery mildew. Condensation also occurs on the inside pf poly-covered greenhouse when the inside air is warm and humid and the outside air is cold. Under these conditions, condensation can rain-down on the foliage creating an environment for diseases.
Greenhouses heat up during the day for two reasons: 1) the greenhouse effect, 2) because the greenhouse is an enclosed space. How much a greenhouse heat up during the day depends on how much solar radiant energy is coming in through the glazing, what happens to that energy, and how much is retained.
The Greenhouse Effect
When radiant energy strikes an object, one of three things can happen: 1) the light is reflected, 2) the light is absorbed, or 3) the light is transmitted. When solar radiant energy enters the greenhouse, some is reflected from various surfaces and passes out of the greenhouse, some is absorber by plants, soil, benches, walks, etc. and converted to latent heat, and the rest is absorbed and re-emitted as long wave radiation. Much of the sunlight entering a greenhouse is shortwave radiation. When matter absorbs radiation, some of the energy is lost as heat. The rest is re-emitted as longer wave radiation, mostly infrared. Water in the greenhouse atmosphere absorbs infrared and converts most of it to latent heat. In addition, glass is transparent to shortwave radiation but opaque to long wave radiation. So a lot of the energy of sunlight is trapped in a greenhouse as heat.
Plant Physiological Processes
Some physiological effects of temperature relating to greenhouse production:
– Influences the rate of photosynthesis (dark reactions).
– Influences respiration rate.
– Influences rate of metabolic synthesis and degradation.
– Influences rate of transpiration.
– Influences timing of crop maturity.
– Influences flower initiation and/or development in some species.
– Influences post harvest of flowers.
– Soil temperature influences water and nutrient absorption.
– Influences seed germination.
– Influences rooting of cuttings.
The day/night temperature at which crops are grown in a greenhouse are those that have been found to maximize growth, yield, and quality. Temperature influences the rate of photosynthesis or production of high energy compounds. Part of this food material is used by respiration to provide energy for synthesis processes. The rest is used to produce cellular components. Plants grow only when the supply of food material exceeds the requirements for respiration.
The rates of photosynthesis and respiration are influenced by many factors, e.g. light, temperature, carbon dioxide, relative humidity, etc. Therefore, temperature cannot be considered in isolation and any statement about an optimum temperature for growth of a given crop cannot be made.
The interaction of factors affecting plant growth is explained based on Blackman’s Principle of Limiting Factors: the rate of a process influenced by many separate factors is limited by the pace of the slowest factor.
In the greenhouse, specific day/ night temperatures are maintained for each flower crop to obtain profitable growth and market quality. Night temperatures have traditionally been stressed in recommendations because plants grow more at night than during the day.
The growth of many plants can occur over a wide range of temperatures. This range may be defined at three basic levels: 1) a minimum temperature below which no growth occurs, 2) an optimum temperature at which the greatest growth occurs, and 3) a maximum temperature above which no growth occurs. Growth rate increases above the minimum temperature until an optimum is reached, then declines until the maximum temperature is reached. The minimum, optimum, and maximum temperatures varies widely among plant species.
Most plants do not respond in the same way to temperature at all stages of growth. Generally, seed germination and early seedling growth occurs most rapidly at warmer temperatures. These same warm temperatures may be detrimental to growth as the plant matures. Young plants have a large leaf area (photosynthetic tissues) compared to stem and root area (respiration tissues). High relative photosynthetic area and warm temperatures favors carbohydrate production and utilization for growth. However, when plants get older, there is more stem and root area (respiration tissues) to leaf area so cooler temperature favor growth by reducing respiration. Plants in a vegetative stage of growth generally have a warmer temperature optimum than those in a reproductive stage. Different parts of the same plant may also have different optimum temperatures for growth. Thus, root growth may show a different response from shoot growth.
As a general rule, greenhouse crops are grown at day temperature 5-10° F higher than night temperatures on cloudy days and 15° F higher on clear days. Therefore, optimum day temperature for growth generally decrease as solar radiant energy decreases. This should be kept in mind as the seasons change. Night temperatures may be 50-70° F, 60-65° F is a good starting point for warm season crops and 50-55° F for cool season crops. Plants generally grow better if the day temperature is warmer than the night (except African Violet). Diurnal variation in temperature is termed Thermoperiodicity. The exact temperature levels change with light intensity and plant age.
Temperature is a valuable tool to modify and regulate crop timing. Many Floriculture crop are marketed for specific Holidays such as Thanksgiving, Christmas, Valentine’s Day, Easter, and Mother’s Day. The market value of crop targeted for these dates drops if the crop is not ready in the week or two before a Holiday. Crops may be speeded-up or slowed-down by increasing or decreasing the temperature (especially night). Keep in mind that extreme temperature changes can have an adverse affect on crop quality.
Over the last 40-50 years, plant height has been controlled using chemical growth retardants. Concerns about the environment and human health has lead to efforts to control plant height using other means. Recently researchers have uncovered a practical relationship between plant height and day/night temperature. This relationship can be expressed as the difference in the day and night temperature, abbreviated DIF:
DIF = day temperature (DT) - night temperature (NT)
For example, DIF values of +10°, 0°, and -10°F are derived from 70°F DT - 60°F NT, 65°F DT- 65°F NT, and 60°F DT - 70°F NT, respectively.
The principle of DIF can be applied in the greenhouse to control plant height and reduce the need for chemical growth retardants.
– The primary effect of DIF is to influence internode elongation. A negative DIF may influence the biosynthesis of GA3 since spray applications of GA3 can cause normal internode elongation under negative DIF.
– Plant height can be decreased by decreasing the day temperature or increasing the night temperature or both. Achieve a close to zero or negative DIF. Conversely, to increase plant height, increase the day temperature or decrease the night temperature.
– The magnitude of the response to DIF is not the same across all DIF values. The increase in internode length as DIF increases (more positive) is greater than the decrease in internode length as DIF decrease (more negative).
– The difference in the day and night temperature determines internode length regardless of the absolute day or night temperature.
– DIF works best when plants are in a rapid stage of growth. Response to DIF is rapid, often as soon as 24 to 48 hours.
– Extremely negative DIF can have adverse affects on plants resulting in yellow foliage. If a negative DIF is applied for a short period and plants are returned to a positive DIF, green color usually returns to the leaves. However, young seedling treated for an extended period may remain yellow and stunted.
– DIF affects internode elongation, plant height, leaf orientation, shoot orientation, chlorophyll content, lateral branching, and petiole and flower stalk elongation.
During warm times of the year, dropping the day temperature close to the night temperature may not be possible throughout the day. Recent work has shown that a temperature drop or rise for 2-3 hours at the beginning or end of the light period has a strong affect on internode elongation. In the greenhouse a temperature drop is usually applied by turning on the fans and/or opening vents 20-30 min. before dawn then returning to a normal venting pattern 2-3 hours later. This sensitivity during specific points in the photoperiod may be related to endogenous rhythms.
Flower Initiation and Development
Many Floriculture crops require a specific temperature regime for floral initiation and/or flower development. Temperature requirements for floral initiation:
Qualitative Temperature Requirement - If the temperature requirement is absolute, plants will not flower unless exposed to a specific temperature range for a minimum length of time.
Quantitative Temperature Requirement - A quantitative requirement exists when flowering is modified by exposure to a specific temperature regime, e.g. time to flower or number of nodes on which the first flower is initiated. In this case plants may eventually flower regardless of temperature.
The effect of temperature may be direct or inductive, meaning the floral induction occurs during or after the temperature treatment. The inductive effect of low temperature on floral initiation is known as vernalization.
In other plants, floral initiation occurs late in the growing season when temperature are still warm. The role of vernalization is to satisfy a requirement to allow elongation and development of the flower and flower stalk.
The apical meristem is the site of temperature perception for floral initiation. To perceive the cold stimulus, the cells of the meristem must be metabolically active. In addition, the plant must reach a certain stage of growth before the apical meristem is sensitive to temperature. Some plants can be de-vernalized by high temperatures immediately after vernalization. However, after a specific time interval elapses, high temperatures are no longer effective and the plant remains vernalized.
Classification of Floriculture crops based on response to temperature:
1. Biennials - plants that grow vegetatively the first growing season, initiate flowers during the winter, and flower the second season (complete the life cycle in two growing seasons), e.g. Foxglove. The cool temperature period is necessary for floral initiation and to prepare the flower stalk for elongation when temperature become favorable for growth in the spring. Flowering in biennials is often called “bolting”.
2. Low-Temperature Plants - Plants in this group initiate or develop flowers only when the temperature drops below a critical level for a minimum length of time. The exact level and length of time is species and cultivar dependent. Examples; Cineraria, Calceolaria, Hydrangea, and many Cymbidium orchids.
3. High-Temperature Plants - Plants in this group initiate flowers only when the temperature is increased above a critical level for a minimum length of time. The exact level and length of time is species and cultivar dependent. Examples; Azalea, Clarkia, and annual Larkspur.
4. Bulbs - Many bulbs require a series of temperature treatments for forcing in the greenhouse. Temperature regulates floral initiation, stem elongation, and bulb dormancy. Bulbs must attain a minimum size or weight before they are sensitive to inductive temperature treatment.