Section 2 Temperature and Scheduling

Improving Greenhouse Temperature Optimization and Integration Production Efficiency he rate of plant development (time to flower or the production of roots) is Erik Runkle and Matthew Blanchard TT primarily influenced by the average daily A240C Plant and Soil Sciences temperature. The average daily temperature is Department of Horticulture the mathematical average temperature over a Michigan State University series of 24-hour periods and can be calculated East Lansing, MI 48824 as:

wo primary environmental factors that Average daily temperature = [(day control plant growth and development temperature × hours) + (night temperature × are temperatureerature and light. Although T hours)] ÷ 24 these two factors have distinct effects on plants, they interact in many ways. In order for growers The average daily temperature is important to be able to optimize crop production, to calculate because it determines the rate of knowledge of how these factors influence plant plant development. Generally, the warmer the growth and development is very important. This average daily temperature, the faster a plant section discusses the fundamentals of grows. It’s analogous to how fast you drive your temperature and how this information can be automobile to get to work. The faster you drive, used to improve production efficiency and the earlier you arrive at work. Similarly, the reduce production time. In addition, the effects warmer your crops are grown, the quicker they of plug and liner size on finishing time is also will grow and become ready for market. discussed. Therefore, if you lower the average daily

temperature in the greenhouse, plants will take longer to become marketable. This applies to plugs, flats, potted crops, hanging baskets, and any other size of plant or container. There are also other factors that influence crop timing, including photoperiod and the average daily light integral, both of which are discussed later. How can we use average daily temperature to schedule a crop? Many greenhouse crops Figure 11. The effects of average daily temperatures produce a set number of leaves before flower from 59 to 95 °F (15 to 35 °C) on the development of ‘Grape Cooler’ vinca (Catharanthus roseus). Photo initiation and we are able to track the rate of courtesy of Royal Heins, Michigan State University. progress towards flowering by counting the number of leaves that unfold each day. Easter lily growers are familiar with this leaf counting technique to track plant development and

Temperature and Scheduling 1 ensure that their crop is on schedule. We can when perennials or bulbs are provided with cool control the rate of leaf unfolding and flowering temperature treatments to satisfy a vernalization time by raising or lowering the average daily response. temperature. Figure 11 shows an example of Growers should also know what the vinca (Catharanthus roseus) grown at an optimum temperature is for a crop. The average daily temperature of 59 to 95 °F (15 to optimum temperature is the temperature at 35 °C). At a cool temperature (59 °F or 15 °C) which plant development is most rapid (Figure the rate of leaf unfolding is very slow and time to 12). As temperature increases beyond the flower is >100 days, whereas at a warm optimum value, growth slows as plants show temperature (86 °F or 30 °C), leaf unfolding is symptoms of heat stress. Therefore, in most faster and time to flower is ≈30 days. instances, crops are grown above the base temperature but not above the optimum Base and Optimum Temperature temperature of the crop. The optimum The relationship between average daily temperature can be around 70 °F (21 °C) for temperature and growth and development is cool-season crops such as pansy and alyssum, linear between the base and optimum or as high as 90 °F (32 °C) for warm-season temperature (Figure 12). The base temperature crops such as vinca and hibiscus. Note that the is a cool temperature at which a plant stops optimum temperature for plants is not based on growing. The base temperature can vary plant quality attributes, and thus the optimum considerably from crop to crop. For example, temperature is not necessarily the most the base temperature for seed petunia is about desirable growing temperature. 39 °F (4 °C), which means that at or below this During production, it is important to consider temperature, petunias essentially stop growing. actual plant temperature and not just the For a warm-growing crop such as vinca, the surrounding air temperature. Actual plant base temperature is much higher, around 50 °F temperature is influenced by many factors (10 °C). Experienced growers can often predict including conduction, convection, transpiration, which crops have a low base temperature and radiation and thus plant temperature can be because they are usually grown cooler than several degrees warmer or cooler than air plants that have a high base temperature. temperature. Later in this section, we discuss During the winter and spring, floriculture crops how adding supplemental lighting in the are often grown about 20 to 30 °F (11 to 17 °C) higher than their base temperatures. We rarely want to grow plants at or near the base temperature because plant development is too slow. One of the few times when a growing temperature near the base temperature is desirable is when plants need to be held because the markets are not available to receive plants, which can occur when sales are slow following Figure 12. The rate of plant development (such as leaf unfolding) is linear an extended period of rainy between the base temperature and the optimum temperature. weather. Another example is

Temperature and Scheduling 2 greenhouse can affect plant temperature and crop development. The best tool to determine the actual plant temperature of your crop is to use an infrared thermometer. Infrared thermometers are very accurate and can be a great investment for any greenhouse grower. As discussed earlier, the average daily temperature of the greenhouse can be adjusted to speed up or slow down the development of a crop. However, the effects of changing the average daily temperature Figure 13. The effect of temperature on time to flower of petunia depends on the species, the (Petunia ×hybrida) from seed and vinca (Catharanthus roseus) from a magnitude of the change, and the small plug. When temperature is decreased, there is a larger delay in original temperature setpoint. For flowering for plants with a high base temperature (vinca) compared to plants with a lower base temperature (petunia). example, the effect of changing the average daily temperature on crop timing of differ in how they respond to lowering the petunia and vinca is illustrated in Figure 13. greenhouse temperature; generally cold- Lowering the temperature by 5 °F has a sensitive plants are more responsive to lowering somewhat small effect at warm temperatures, the greenhouse temperature than cold-tolerant and has a larger effect at cooler temperatures. species. So, if you are determined to lower your For example, lowering the average daily greenhouse temperature set point, you’ll likely temperature by 5 °F from 65 to 60 °F delays a delay crop timing more with cold-sensitive crops. petunia crop (from seed) by about 13 days, and See Table 6 for a list of plants categorized by lowering the temperature from 60 to 55 °F their base temperatures. Ideally, crops with delays petunia by 22 days. The effect of different base temperatures should be grown in lowering the temperature can have a more separate greenhouses with different temperature dramatic effect on cold-tolerant crops. For set points to produce crops in an energy-efficient example, lowering the temperature from 65 to 60 manner. °F increases time to flower of vinca (from a plug) by about 30 days – much longer than the delay Temperature Integration in petunia with the same temperature decrease. The concept of “temperature integration” has been used by many Dutch greenhouse growers Cold-Tolerant and Cold-Sensitive Crops in recent years. This term describes how plants Plants respond differently to temperature respond to temperature over a period of time. partly because they have different base Simply put, the rate of plant development is temperatures. Plants with a base temperature dependant upon the average daily temperature of 39 °F (4 °C) or lower can be called “cold- from the time you plant the crop. This is a very tolerant plants” and those with a base simple but powerful concept. Plants respond to temperature of 46 °F (8 °C) or higher can be the temperature constantly, and they grow called “cold-sensitive plants”. We categorize progressively faster as temperature increases, plants by their base temperature because they and grow progressively slower as temperature

Temperature and Scheduling 3 decreases. The exception to this rule is when Does Lowering Temperature Save cool-season crops are grown very warm, and at Fuel? some high temperature (above the optimum) This is a common question many these plants begin to experience stress and the greenhouse growers ask. As discussed rate of crop development begins to decrease. In previously, lowering the average daily addition, once crops are exposed to temperature can increase the production time of temperatures at or below their base a crop. If you lower the temperature set point, temperature, a further temperature decrease but still plan to finish the crop on the same does not influence crop timing. market date as in previous years, then What is the implication of temperature adjustments will need to be made to your integration? If your day and night are each 12 production schedule. One option is to begin hours long, and if you lower your night production with a more mature crop (such as temperature without increasing your day transplanting from a 128-cell plug instead of a temperature the same amount, your average 588-cell seedling), which will reduce production daily temperature will decrease. Thus, cooler time in the finished container (see our nights without warmer days will increase the discussion on this topic later). A second option time it takes for your crop to become shippable to compensate for the lengthened production or transplantable. If your night temperature time at the lower temperature is to transplant the settings are longer than 12 hours, then you need crop earlier in the year. If you transplant earlier to offset the shorter day temperature set point in the year, chances are you’re going to open up even more so that your 24-hour average the greenhouse earlier in the year, when it is temperature stays the same. colder outside and thus energy consumption for New technology in greenhouse climate heating is relatively high. A simple question controls now utilizes the concept of temperature follows: is it economical to increase the integration to reduce energy consumption for production time to compensate for a lower heating. For example, during conditions when average greenhouse temperature? solar radiation is high and greenhouse During the winter and early spring, it can be temperature naturally increases, climate controls more energy-intensive to grow crops at cooler maintain a higher day temperature. To offset temperatures than to open up the greenhouse the warm day temperature and save on energy, later and use a warmer growing temperature. A the climate control system lowers the night lower temperature set point requires less temperature set point. Although the heating and heating, which translates into less fuel ventilation set points change often, a similar consumption per month. However, a average daily temperature is maintained over temperature reduction also increases crop time, and the crop finishes on schedule. These timing, meaning that plants are in the new climate control systems also incorporate greenhouse longer. A longer production time weather forecasting to make adjustments to the has several negative consequences, including: temperature settings. Growers in The • overhead expenses (cost per ft2 per Netherlands are already using this technology, week) are greater for that crop and we expect similar systems will be used by • the crop takes longer to finish, so you large growers in the United States in the near will turn fewer crops per year future. For more information in this topic, see • a longer crop time means that you will article by Rijsdijk and Vogelezang, 2000. have to heat the crop longer and possibly open up a greenhouse earlier, when it is colder outside.

Temperature and Scheduling 4 There are other consequences to growing “typical” double-poly greenhouse. crops in a cool greenhouse. One concern is that From winter until mid-summer, the model plants take longer to dry out, so they stay wet predicts that the total amount of energy used to longer. Also, because cool air holds less heat a crop (from transplant to flowering) moisture than warmer air, the relative humidity actually increased as the growing temperature can be higher in a cool greenhouse. Pathogens decreased. In other words, it was more can be more problematic when crops are kept expensive to heat a crop planted earlier in the moist and when the humidity is high. year and grown at a cool temperature compared to opening a greenhouse later and using a Energy Consumption Models higher temperature set point. The opposite was Hiroshi Shimizu at the University of Ibaraki true for crops grown in the fall; an earlier in Japan developed a sophisticated model to planting date and a lower greenhouse predict how much energy is consumed to heat a temperature consumed the least amount of greenhouse to produce a crop. The simulations energy. are complex and depend on environmental A more user-friendly software program factors (outdoor temperature, light levels, and to predict greenhouse energy consumption, wind speed), numerous greenhouse factors Virtual Grower, has been developed by (glazing type, use of thermal curtains, sidewall Jonathan Frantz and colleagues at the USDA- and floor insulation, etc.), the crop grown and ARS Greenhouse Production Research Group in the greenhouse temperature set point. Figure Toledo, Ohio. This software provides the ability 14 illustrates the predicted energy consumption for growers to predict heating costs based on to heat a crop in Michigan with different finish user-defined inputs such as growing dates and three temperature set points. This temperature, greenhouse location and structure, simulation was based on Michigan weather data, time of year, fuel type, fuel cost, etc. Virtual a greenhouse crop with a base temperature of Grower is a great tool for greenhouse growers, 41 °F (5 °C), and several assumptions for a but a limitation to this software is that data on

Figure 14. The estimated amount of energy required to produce a crop at different growing temperatures throughout the year in Michigan. This simulation indicates that the total amount of energy consumed to produce a flowering crop increased as growing temperature decreased from winter through mid-summer.

Temperature and Scheduling 5 crop timing are not included. Future versions of weeks. However, plants forced at a warm Virtual Grower will include specific crop data so temperature had a significant reduction in flower growers can predict both crop timing and energy size. There are some floriculture crops, such as consumption at different temperature set points. hibiscus, that do not perform well at cool For more information on Virtual Grower or to temperatures. For such tropical crops, plant download a free copy, visit quality is highest when grown at a moderately www.ars.usda.gov/Research/docs.htm?docid=1 warm temperature [70 °F (21 °C) or higher]. 1449. Therefore, there is often a trade-off between high quality crops and crop timing. Cooler Temperature Effects on Plant Quality temperatures produce higher quality plants but There is one major benefit to growing crops they take longer to reach maturity and energy relatively cool in the winter and spring, when consumption per crop can be greater. Crops light is limiting in northern latitudes. Crops grown at warm temperatures develop faster and grown cool take longer to flower, and thus they thus have shorter crop times and require less have a longer period of time to harvest light. energy for heating, but the quality of plants is Because of this, many plants (especially cold- often not as high. If a grower is unable get a tolerant crops) are of higher quality when grown higher price for a higher quality crop, then there at moderately cool temperatures. When ready is little incentive to grow cool. for transplant, plugs grown at cool temperatures often have thicker stems, better rooting, and Greenhouse Space Efficiency greater branching. Similarly, finish crops grown s energy costs continue to rise, cool can have more branching and produce greenhouse growers are evaluating the more, larger flowers. The effects of forcing A space efficiency of their production area temperature on flower size of ‘Blue Clips’ to determine if there are opportunities for Carpathian harebell (Campanula carpatica) is improvement. One strategy is to purchase illustrated in Figure 15. At a warm forcing larger plugs or liners for transplanting into temperature (70 °F or 21 °C) plants flowered in 7 finished containers. By purchasing larger liners, to 8 weeks, while at a cool forcing temperature the production time in the finish container is (60 °F or 15 °C), plants flowered after 10 to 11 reduced and the crop is in the greenhouse for a shorter period. This strategy can improve space-use efficiency and provides the opportunity for an additional crop turn. An additional benefit is the savings in energy for greenhouse heating; when starting with larger liners, production can begin later in the spring when less greenhouse heating is required. How much production time is saved by transplanting larger liners versus smaller liners? Research by Paul Fisher at the University of Florida has helped to answer this question. Figure 15. The effects of forcing temperatures from Figure 16 provides an example of how liner size 59 to 81 °F (15 to 27 °C) on plant quality of ‘Blue and age influences the production time for Clips’ Carpathian harebell (Campanula carpatica). At a warmer temperature, plants flowered earlier but finishing Calibrachoa ‘Superbells Red’ grown in flower size was reduced compared to plants forced at 4.5-inch (11-cm) pots. Production time from a cooler temperature. Photo courtesy of Cathy transplant to finish of calibrachoa can be Whitman, Michigan State University. reduced by 17 days by starting with a 40-mm

Temperature and Scheduling 6 purchasing larger liners outweigh the savings from reduced production time in the finished container? Paul Fisher has performed a financial analysis to answer this question. The simple answer is that if the savings in cost per square foot week from starting production later are greater than the cost of purchasing a larger liner, then it makes economic sense. However, the amount of savings will be dependent on the greenhouse location, time of year, and labor, overhead, and heating fuel costs. For an example of how to calculate the potential savings from starting with a larger liner, see article by Fisher, 2006.

Figure 16. The effect of liner size on time to produce a finished rooted liner of Calibrachoa ‘Superbells Red’ from a direct-stuck cutting and time from transplanting a rooted liner to a finished 4.5-inch (11-cm) pot. Plants were grown at 70 °F (21 °C) under a 16-hour photoperiod and an average daily light integral of 9.3 mol·m−2·d−1. Photo courtesy of Paul Fisher, University of Florida. liner (50-count tray) versus a 20-mm liner (144- count tray). For a complete list of finishing times for various bedding plants, see chapter 16 in Styer and Koranski, 1997. Paul Fisher has also shown that a similar production time can be achieved by substituting time in the liner stage for time in the finished container. For example, when starting with small liners (105-count tray) that are 4 weeks old, plants require 8 weeks to finish in a 12-inch hanging basket, whereas only 4 weeks are needed to finish the hanging basket when Figure 17. The effects of liner size on finishing time in 12-inch (31-cm) hanging baskets with five liners starting with large liners (18-count tray) that are per basket. Cuttings were stuck into 25-mm (105- 8 weeks old (Figure 17). In both scenarios, the count), 40-mm (50-count), or 70-mm (18-count) liner total production time is similar, 12 weeks. For a trays and transplanted into hanging baskets after 4, 6, or 8 weeks, respectively. Photographs of liners complete summary of this research project, see were taken at the time of transplant into hanging article by Fisher and colleagues, 2006). baskets. Photo courtesy of Paul Fisher, University of Although starting with larger liners can Florida. reduce production time in the finished pot, large liners can be costly to purchase and ship. The most important question is: Does the cost of

Temperature and Scheduling 7

Sources for More Information

Fisher, P. 2006. The most profitable liner size? Greenhouse Grower 24(12):36−40.

Fisher, P. and E. Runkle. 2004. Lighting Up Profits: Understanding Greenhouse Lighting. Meister Media Worldwide, Willoughby, Ohio. Available at www.meistermedia.com.

Fisher, P., H. Warren, and L. Hydock. 2006. Larger liners, shorter crop time. Greenhouse Grower 24(11):8−12.

Rijsdijk, A.A. and J.V.M. Vogelezang. 2000. Temperature integration on a 24-hour base: A more efficient climate control strategy. Acta Hort. 519:163−170. Available at www.actahort.org/books/519/519_16.htm.

Runkle, E.S. 2005a. 10 ways to lower your spring heating bill and save money. Greenhouse Management and Production 25(12):59−60.

Runkle, E.S. 2005b. Optimize your temperatures. Greenhouse Management and Production 24(12):65−67.

Runkle, E. 2006. Temperature effects on floriculture crops and energy consumption. Ohio Florists’ Association Bulletin 894:1−8.

Runkle, E. 2007. Manage temperatures for the best spring crops. Greenhouse Management and Production 27(1):68−72. Available at www.GreenBeam.com.

Runkle, E. and P. Fisher. 2006. Growing crops cooler. Greenhouse Grower 24(3):84−85. Available at www.meistermedia.com.

Runkle, E.S. and R. Heins. 2001. Timing spring crops. Greenhouse Grower 19(4):64−66.

Runkle, E., H. Shimizu, and R. Heins. 2002. How low can you go? GrowerTalks 65(10):63−68.

Styer, R.C. and D.S. Koranski. 1997. Plug and Transplant Production: A Grower’s Guide. Ball Publ., Batavia, Illinois. Available at www.ballpublishing.com.

Temperature and Scheduling 8 Tables

Table 6. Plants can be categorized by their base temperature, which is the temperature at or below which plant development ceases. “Cold-tolerant crops” are those with a base temperature of 39 °F (4 °C) or lower, “intermediate crops” are those with a base temperature of 40 to 45 °F (4 to 7 °C) and “cold-sensitive crops” are those with a base temperature of 46 °F (8 °C) or higher. Information based on research at Michigan State University and published research-based articles. Cold-sensitive crops [base temperature of 46 °F (8 °C) or higher] Angelonia gardnerii (Angelonia) Begonia ×semperflorens-cultorum (Fibrous begonia) Caladium bicolor (Caladium) Capsicum annuum (Pepper) Catharanthus roseus (Vinca) Celosia argentea (Celosia) Colocasia antiquorum (Elephant ears) Euphorbia pulcherrima (Poinsettia) Gazania rigens (Gazania) Hibiscus spp. (Hibiscus) Impatiens hawkeri (New Guinea impatiens) Musa ornata (Banana) Pennisetum setaceum ‘Rubrum’ (Purple fountain grass) Phalaenopsis spp. (Phalaenopsis orchid) Rosa ×hybrida (Rose) Saintpaulia ionantha (African violet) Salvia farinacea (Blue salvia) Intermediate crops [base temperature of 40 to 45 °F (4 to 7 °C)] Calibrachoa ×hybrida (Calibachoa) Coreopsis grandiflora (Coreopsis) Dahlia pinnata (Dahlia) Oenothera fruticosa (Sundrops) Impatiens wallerana (Seed impatiens) Salvia splendens (Red salvia) Cold-tolerant crops [base temperature of 39 °F (4 °C) or lower] Ageratum houstonianum (Ageratum) Antirrhinum majus (Snapdragon) Campanula carpatica (Campanula) Diascia spp. (Twinspur) Gaillardia ×grandiflora (Blanket flower) Leucanthemum ×superbum (Shasta daisy) Lilium longiflorum (Easter lily) Lilium spp. (Asiatic and Oriental lily) Lobularia maritima (Alyssum) Nemesia strumosa (Nemesia) Pericallis ×hybrida (Cineraria)

Temperature and Scheduling 9 Petunia ×hybrida (Petunia) Rudbeckia fulgida (Black-eyed Susan) Scabiosa caucasia (Pincushion flower) Schlumbergera truncata (Thanksgiving cactus) Tagetes patula (French marigold) Viola ×wittrockiana (Pansy) Zygopetalum spp. (Zygopetalum orchid)

Temperature and Scheduling 10 Greenhouse Temperature greenhouse. These techniques are further discussed in this article. Management Ventilation A.J. Both reenhouses can be mechanically or Assistant Extension Specialist naturally ventilated. Mechanical Rutgers University G ventilation requires (louvered) inlet Bioresource Engineering openings, exhaust fans, and electricity to Dept. of Plant Biology and Pathology operate the fans. When designed properly, 20 Ag Extension Way mechanical ventilation is able to provide New Brunswick, NJ 08901 adequate cooling under a wide variety of [email protected] weather conditions throughout many locations in http://aesop.rutgers.edu/~horteng the United States.

Natural ventilation (Figure 18) works based Introduction on two physical phenomena: thermal buoyancy ne of the benefits of growing crops in a (warm air is less dense and rises) and the so- greenhouse is the ability to control all called “wind effect” (wind blowing outside the aspects of the production environment. O greenhouse creates small pressure differences One of the major factors influencing crop growth between the windward and leeward side of the is temperature. Different crop species have greenhouse causing air to move towards the different optimum growing temperatures and leeward side). All that is needed are these optimum temperatures can be different for (strategically located) inlet and outlet openings, the root and the shoot environment, and for the vent window motors, and electricity to operate different growth stages during the life of the the motors. In some cases, the vent window crop. Since we are usually interested in rapid positions are changed manually, eliminating the crop growth and development, we need to need for motors and electricity, but increasing provide these optimum temperatures throughout the amount of labor since frequent adjustments the entire cropping cycle. If a greenhouse were are necessary. Compared to mechanical like a residential or commercial building, ventilation systems, electrically operated natural controlling the temperature would be much easier since these buildings are insulated so that the impact of outside conditions is significantly reduced. However, greenhouses are designed to allow as much light as possible to enter the growing area. As a result, the insulating properties of the structure are significantly diminished and the growing environment experiences a significant influence from the constantly fluctuating weather conditions. Solar radiation (light and heat) exerts by far the largest impact on the growing environment, resulting in the challenge maintaining the optimum growing temperatures. Fortunately, several techniques Figure 18. Natural ventilation in a glass-glazed can be used to reduce the impact of solar greenhouse. Photo courtesy of A.J. Both, Rutgers radiation on the temperature inside a University.

Temperature and Scheduling 11 ventilation systems use a lot less electricity and horizontal airflow fans are frequently installed to produce (some) noise only when the vent ensure proper air mixing. The recommended fan window position is changed. When using a capacity is approximately 3 cfm per ft2 of natural ventilation system, additional cooling can growing area. be provided by a fog system. Unfortunately, natural ventilation does not work very well on Humidity Control warm days when the outside wind velocity is low ealthy plants can transpire a lot of (less than 200 feet per minute). Keep in mind water, resulting in an increase in the that whether using either system with no other H humidity of the greenhouse air. A high cooling capabilities, the indoor temperature relative humidity (above 80-85%) should be cannot be lowered below the outdoor avoided because it can increase the incidence of temperature. disease and reduce plant transpiration. Due to the long and narrow design of most Sufficient venting, or successively heating and freestanding greenhouses, mechanical venting can prevent condensation on crop ventilation systems usually move the air along surfaces and the greenhouse structure. The use the length of the greenhouse (the exhaust fans of cooling systems (e.g., pad-and-fan or fog) and inlet openings are installed in opposite end during the warmer summer months increases walls), while natural ventilation systems provide the greenhouse air humidity. During periods with crosswise ventilation (using side wall and roof warm and humid outdoor conditions, humidity vents). control inside the greenhouse can be a In gutter-connected greenhouses, challenge. Greenhouses located in dry, dessert mechanical ventilation systems inlets and outlets environments benefit greatly from evaporative can be installed in the side- or end walls, while cooling systems because large amounts of natural ventilation systems usually consist of water can be evaporated into the incoming air, only roof vents. Extreme natural ventilation resulting in significant temperature drops. systems include the open-roof greenhouse Since the relative humidity alone does not design, where the very large maximum tell us anything about the absolute water holding ventilation opening allows for the indoor capacity of air (we also need to know the temperature to almost never exceed the outdoor temperature to determine the amount of water temperature. This is often not attainable with the air can hold), a different measurement is mechanically ventilated greenhouses due to the sometime used to describe the absolute very large amounts of air that such systems moisture status of the air: the vapor pressure would have to move through the greenhouse to deficit (VPD). The VPD is a measure of the accomplish the same results. difference between the amount of moisture the When insect screens are installed in air contains at a given moment and the amount ventilation openings, the additional resistance to of moisture it can hold at that temperature when airflow created by the screen material has to be the air would be saturated (i.e., when taken into account to ensure proper ventilation condensation would start; also known as the rates. Often, the screen area is larger compared dew point temperature). A VPD measurement to the inlet area to allow sufficient amounts of air can tell us how easy it is for plants to transpire: to enter the greenhouse. higher values stimulate transpiration (but too Whichever ventilation system is used, high can cause wilting), and lower values reduce uniform air distribution inside the greenhouse is transpiration and can lead to condensation on important because uniform crop production is leaf and greenhouse surfaces. Typical VPD only possible when every plant experiences the measurements in greenhouses range between 0 same environmental conditions. Therefore, and 1 psi (0 to 7 kPa).

Temperature and Scheduling 12 Shading nvesting in movable shade curtains is a very smart idea, particularly with the high energy I prices we are experiencing today (Figure 19). Shade curtains help reduce the energy load on your greenhouse crop during warm and sunny conditions and they help reduce heat radiation losses at night. Energy savings of up to 30% have been reported, ensuring a quick payback period based on today’s fuel prices. Movable curtains can be operated automatically with a motorized roll-up system that is controlled by a light sensor. Even low-cost greenhouses Figure 19. Example of an internal shade system in a can benefit from the installation of a shade greenhouse. Photo courtesy of A.J. Both, Rutgers system. The curtain materials are available in University. many different configurations from low to high shower or the swimming pool but before you shading percentages depending on the crop have a change to dry yourself off). This heat requirements and the local solar radiation (energy) is provided by the surrounding air, conditions. Movable shade curtains can be causing the air temperature to drop. At the same installed inside or outside (on top or above the time, the humidity of the air increases as the glazing) the greenhouse. Make sure that you evaporated water transitions into water vapor specify the use when you order a curtain and becomes part of the surrounding air mass. material from a manufacturer. When shade The maximum amount of cooling possible with systems are located in close proximity to heat evaporative cooling systems depends on the sources (e.g., unit heaters or CO2 burners), it is humidity of the air you started with (the drier the a good idea to install a curtain material with a initial air, the more water can be evaporated into low flammability. These low flammable curtain it, the more the final air temperature will drop), materials can stop fires from rapidly spreading as well as the initial temperature of the air throughout an entire greenhouse when all the (warmer air is able to contain more water vapor curtains are closed. compared to colder air). This section will

investigate in more detail how evaporative Evaporative Cooling cooling can be used to help maintain target set hen the regular ventilation system point temperatures during warm outside and shading (e.g., exterior white conditions when the ventilation system alone is W wash or movable curtains) are not not sufficient to maintain the set point. able to keep the greenhouse temperature at the desired set point, additional cooling is needed. In homes and office buildings, mechanical Pad-and-Fan System Two evaporative cooling systems are refrigeration (air conditioning) is often used, but commonly used in greenhouses: the pad-and- in greenhouses where the quantity of heat to be fan and the fog system. Pad-and-fan systems removed can be very large, air conditioning is are part of a greenhouse’s mechanical often not economical. Fortunately, we can use ventilation system (Figure 20). Note that swamp evaporative cooling as a simple and relatively coolers can be considered stand-alone inexpensive alternative. The process of evaporative cooling systems, but otherwise evaporation requires heat (recall how cold your operate similarly as pad-and-fan systems. For skin can feel shortly after you get out of the

Temperature and Scheduling 13 (reducing pad efficiency), it is a common practice to bleed approximately 10% of the returning water to a designated drain. In addition, during summer operation, it is common to ‘run the pads dry’ during the nighttime hours to prevent algae build-up that can also reduce pad efficiency. As the cooled (and humidified) air exits the pad and moves through the greenhouse towards the exhaust fans, it picks up heat from the greenhouse environment. Therefore, pad-an-fan systems experience a temperature gradient between the inlet (pad)

Figure 20. Evaporative cooling pad installed along and the outlet (fan) side of the greenhouse. In the inside of the ventilation inlet opening. Photo properly designed systems, this temperature courtesy of A.J. Both, Rutgers University. gradient is minimal, providing all plants with similar conditions. However, temperature pad-and-fan systems, an evaporative cooling gradients of 7-10 °F are not uncommon. pad is installed in the ventilation opening, The required evaporative pad area depends ensuring that all incoming ventilation air travels on the pad thickness. For the typical, vertically trough the pad before it can enter the mounted four-inch thick pads, the required area greenhouse environment. The pads are typically (in ft2) can be calculated by dividing the total made of a corrugated material (impregnated greenhouse ventilation fan capacity (in cfm) by paper or plastic) that is glued together in such a the number 250 (the recommended air velocity way as to allow air to pass through it while through the pad). For six-inch thick pads, the fan ensuring a maximum contact surface between capacity should be divided by the number 350. the air and the wet pad material. Water is The recommended minimum pump capacity is pumped to the top of the pad and released 0.5 and 0.8 gpm per linear foot of pad for the through small openings along the entire length four and six-inch thick pads, respectively. The of the supply pipe. These openings are typically recommended minimum sump tank capacity is pointed upward to prevent clogging by any 0.8 and 1 gallon per ft2 of pad area for the four debris that might be pumped through the system and six-inch pads, respectively. For evaporative (installing a filter system is recommended). A cooling pads, the estimated maximum water cover is used to channel the water downwards usage can be as high as 10-12 gpd per ft2 of pad onto the top of the pads after it is released from area. the openings. The opening spacing is designed so that the entire pad area wets evenly without Fog System allowing patches to remain dry. At the bottom of The other evaporative cooling system used the pad, excess water is collected and returned in greenhouses is the fog system (Figure 21). to a sump tank so it can be reused. The sump This system is often used in greenhouses with tank is outfitted with a float valve allowing for natural ventilation systems (i.e., ventilations make-up water to be added. Since a portion of systems that rely only on opening and closing the recirculating water is lost through strategically placed windows and do not use evaporation, the salt concentration in the mechanical fans to move air through the remaining water increases over time. To prevent greenhouse structure). Natural ventilation an excessive salt concentration from creating systems generally are not able to overcome the salt build-up (crystals) on the pad material additional airflow resistance created by placing

Temperature and Scheduling 14 determine the maximum temperature drop resulting from the operation of an evaporative cooling system, it is important to review a few key physical properties of air: • Dry bulb temperature (Tdb, °F): Air temperature measured with a regular (mercury) thermometer • Wet bulb temperature (Twb, °F): Air temperature measured when the sensing tip is kept moist (e.g., with a wick connected to a water reservoir) while the (mercury) thermometer is Figure 21. Top-down view of a fog nozzle delivering a moved through the air rapidly small-droplet mist for evaporative cooling. Photo courtesy of A.J. Both, Rutgers University. • Dew point temperature (Td, °F): Air temperature at which condensation an evaporative cooling pad directly in the occurs when moist air is cooled ventilation inlets. The nozzles of a fog system • Relative humidity (RH, %): Indicates the can be installed throughout the greenhouse, degree of saturation (with water vapor) resulting in a more uniform cooling pattern • Humidity ratio (lb/lb): Represents the compared to the pad-and-fan system. The mass of water vapor evaporated into a recommended spacing is approximately one unit mass of dry air 2 nozzle for every 50-100 ft of growing area. The • Enthalpy (Btu/lb): Indicates the energy water pressure used in greenhouse fog systems content of a unit mass of air. is very high (500 psi and higher) in order to • Specific volume (ft3/lb): Indicates the produce very fine droplets that evaporate before volume of a unit mass of dry air the droplets can reach plant surfaces. The water (equivalent to the inverse of the air usage per nozzle is small: approximately 1-1.2 density). gph. In addition, the water needs to be free of any impurities to prevent clogging of the small As mentioned before, the maximum amount nozzle openings. As a result, water treatment of cooling provided by evaporative cooling (filtration and purification) and a high-pressure systems depends on the initial temperature and pump are needed to operate a fog system. The humidity (moisture content) of the air. We can usually small diameter supply lines should be measure these parameters relatively easily with able to withstand the high water pressure. a standard thermometer (measuring the dry-bulb Therefore, fog systems can be more expensive temperature) and a relative humidity sensor. to install compared to pad-and-fan systems. Fog With these measurements, we can use the systems, in combination with natural ventilation, psychrometric chart (simplified for following produce little noise compared to mechanical example and shown in Figure 23) to determine ventilations systems outfitted with evaporative the corresponding wet bulb temperature at the cooling pads. This can be an important benefit maximum possible relative humidity (100%). for workers and visitors staying inside these Once we know the corresponding wet bulb greenhouses for extended periods of time. temperature, we can calculate the difference (also called the wet bulb depression) that Psychrometric Chart indicates the theoretical temperature drop In order to use a handy tool (the provided by the evaporative cooling system. psychrometric chart, Figure 22) to help

Temperature and Scheduling 15 0.040 Relative Humidity, % 100% 80% 60% 60 0.036 50% 14.5 0.032 50 Specific Volume, cu ft/lb 0.028 40% Enthalpy, Btu/lb 0.024 40 14 0.020 30 0.016 13.5 20% Humidity Ratio (lb/lb) 0.012 20 13 0.008

0.004

0.000 30 40 50 60 70 80 90 100 110 120 Temperature (°F) Figure 22. Psychrometric chart used to determine the physical properties air. Note that with values for only two parameters (e.g., dry bulb temperature and relative humidity, or dry and wet bulb temperatures), all others can be found in the chart (some interpolation may be necessary).

0.040 100%

0.036 50% Relative Humidity, % 0.032

0.028

0.024

0.020 Specific Volume, cu ft/lb

0.016 Enthalpy, Btu/lb 13.5 25 Humidity Ratio (lb/lb) Ratio Humidity 0.012

0.008

0.004

0.000 Td Twb Tdb 30 40 50 60 70 80 90 100 110 120 Temperature (°F) Figure 23. A simplified psychrometric chart used to visualize the evaporative cooling example described in the text.

Temperature and Scheduling 16 Since few engineered systems are 100% situation can occur with fog systems: installing efficient, the actual temperature drop realized by more fog nozzles may not necessarily result in the evaporative cooling system is more likely in additional cooling capacity, while system inputs the order of 80% of the theoretical wet bulb (installation cost and water usage) increase. In depression. general however, fog systems are able to In understanding Figure 23, it was assumed provide more uniform cooling throughout the that the initial conditions of the outside air were: growing area and this may be an important a dry bulb temperature of 69 °F and a relative consideration for some greenhouse designs and humidity of 50% (look for the intersection of the crops. It should be clear that, like many other curved 50% RH line with the vertical line for a greenhouse systems, the design and control temperature of 69 °F). From this starting point, strategy for evaporative cooling systems we can determine all other environmental requires some thought and attention. It is parameters from the list shown above: the wet recommended to consult with professionals who bulb temperature equals 58 °F (from the starting have experience with greenhouse cooling in point, follow the constant enthalpy line [25 Btu/lb your neighborhood. in this case] until it intersects with the 100% relative humidity curve), the dew point temperature is just shy of 50 °F, the humidity ratio equals 0.0075 lb/lb, the enthalpy equals 25 Btu/lb, and the specific volume equals 13.5 ft3/lb. Hence, the wet bulb depression for this example equals 69 – 58 = 11 °F. Using an overall evaporative cooling system efficiency of 80% results in a practical temperature drop of almost 9 °F. Of course, this temperature drop occurs as the air passes through the evaporative cooling pad. As the air continues to travel through the greenhouse on its way to the exhaust fans, the exiting air may well be warmed to its original temperature (but is no longer saturated).

In Conclusion When evaporative cooling pad systems appear to perform below expectation, it is tempting to assume that an increase in the ventilation rate would improve performance. However, increased ventilation rates result in increased air speeds through the cooling pads, reducing the time allowed for evaporation of water. As a result, the overall system efficiency can be reduced while water usage increases. Particularly in areas with water shortages, this can become a concern. In addition, increased ventilation rates may result in a decrease in temperature and humidity uniformity throughout the growing area. A similar

Temperature and Scheduling 17