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Erik Runkle
Horticulture
Rising energy prices are prompting many greenhouse growers to consider lowering their greenhouse temperature setpoints to reduce their monthly heating costs. Temperature influences crop timing, plant quality and energy consumption, as well as other issues discussed in this special MSU Greenhouse Alert e-newsletter. When determining a growing temperature, it is important to understand how temperature influences plant growth and development so that growers can optimize their production schedules and still produce high quality plants on time.
Within the temperature range of most greenhouses during the winter, plants develop leaves and flowers progressively faster and faster as temperature increases. Thus, turning down the thermostat during the day or night will delay crop timing. In other words, if you grow plants at cooler-than-normal temperatures, production time will increase. This means that, to finish a crop on the same time as last year, you must begin growing the crop earlier in the year at the cooler temperature compared to the date you would need to start your crop at a warmer temperature.
As temperature decreases, there is some temperature at which a plant ceases to develop. This temperature is called the base temperature, and varies from crop to crop. For example, the base temperature for seed petunia is about 39°F (4°C), which means that at or below this temperature, petunias stop growing. For seed vinca (Catharanthus), the base temperature is much higher, around 50°F (10°C). Vinca placed in a 45°F (7.5°C) greenhouse will not develop leaves or flowers, but a petunia will continue to grow (just slowly).
As temperature increases above the base temperature, plants grow faster and faster. Figure 1 illustrates the time it takes for petunia and vinca plants to flower at various temperatures. As temperature increases above the base temperature, a small increase in temperature can make a big difference in the time to flower. As we get to warmer temperatures, the same increase in temperature has a smaller effect on accelerating flowering.
As we see in the example with petunia and vinca (Figure 1), lowering the temperature by 5 degrees has a somewhat small effect at warm temperatures, and has a larger effect at cooler temperatures. The effect of lowering the temperature also depends on the crop. For example, lowering the temperature from 65 to 60°F would take petunia about 13 days longer to flower, and would take vinca about 30 days longer to flower.
The reason for the difference in how temperature influences crop timing is related to the base temperature of a plant. Plants with a low base temperature can be considered “cold-tolerant plants.” Those with a high base temperature can be called “cold-sensitive plants.” See Table 1 for a list of some examples of these two plant classifications based on their base temperature. Cold-sensitive plants are more sensitive to a lower greenhouse temperature than cold-tolerant species. There are also plants that fall between these categories (base temperature between 39°F and 46°F), such as red salvia. For more information on this topic, see the article entitled Optimize your temperatures that appeared in the December 2004 issue of GMPro magazine.
All plants respond to temperature during all stages of development. For example, seedlings of Salvia ‘Vista Red’ grew faster as temperature increased from 57 to 79°F (14 to 26°C) (Figure 2). At these temperatures, plugs grown in 288-cell trays took approximately 6.5 weeks to finish at 57°F and about four weeks at 79°F. Note the differences in both roots and shoots. Plants also respond to temperature during the finish stage. For example, under the low light conditions of winter, time from transplant to first flowering of Salvia ‘Vista Red’ took 12 days longer at 63°F than at 73°F (Figure 3).
For many crops, plant quality at the same stage of development increases as growing temperature decreases. If plants are grown at similar light intensities but at different temperatures, marketable plants grown at cooler temperatures often have thicker stems, greater branching, more roots, and more, larger flowers. Exceptions to this are plants that grow best in warm conditions, such as hibiscus. Therefore, one of the benefits of growing at cool temperatures is that overall plant quality could be improved even though crop timing is delayed. However, remember that some plants, especially warm-season plants, can experience chilling injury at cool temperatures. |
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Erik Runkle
Horticulture
Fuel costs for many greenhouses will be substantially higher than they’ve been in recent history, and perhaps ever. I have compiled a list of suggestions for how greenhouse operations can reduce energy consumption, accelerate crop development or improve space efficiency. Some of these suggestions require capital investment, but I believe in most situations, the return on investment can be significant, especially in cold climates.
As discussed in Temperature effects on crop timing and plant quality in this Greenhouse Alert issue, crop timing increases as temperature decreases. If you plan to market the plants on the same date as in the past, then you need to begin growing the crop earlier, meaning you will have to start heating your greenhouse earlier. Energy consumption per crop grown in the spring can be higher when crops are grown cool because you have to heat the greenhouse for a longer period of time.
Most of the energy consumed by a greenhouse is used for heating, and most (perhaps 80%) heating occurs at night. Deploying a retractable shade/energy curtain at night can significantly reduce heat loss by providing another insulating layer to the greenhouse.
A majority of seedlings and cuttings are produced in late winter and early spring, when natural light levels in the northern half of the
United States and Canada are low. Increasing the light level can accelerate crop development by increasing plant temperature and by reducing the number of leaves formed before the first flower develops.
Many bedding plants and perennials are long-day plants, meaning that they flower earlier when grown under a long photoperiod. Common examples of long-day plants include ageratum, blue salvia, dianthus, pansy, petunia, rudbeckia, snapdragon, and tuberous begonia. During the spring, the photoperiod is naturally short until April, so flowering of early long-day crops is delayed unless artificial long days are provided.
Look for gaps near fans, pads and doors; make sure there are no holes or gaps in your roof; and consider adding an extra layer of insulation to your north wall. Just be mindful not to reduce incoming light too much – or the quality of your crops might be compromised.
Not all plants respond to temperature the same way. Plants like vinca and celosia grow very slowly at 60°F (16°C) while other plants such as ageratum, pansy and ivy geranium continue to grow moderately well at this low temperature. Whenever possible, grow cold-tolerant crops in one greenhouse and cold-sensitive crops in a separate greenhouse.
Once a greenhouse is opened for use in the early spring, heat needs to be used whether it is full of crops or not. If you have multiple ranges, try to schedule your spring crops so that you fill each greenhouse when first opened.
During the finish stage, there are fewer plants per square foot of greenhouse space compared to during the plug stage. Thus, heat and lighting costs per plant are lower when plants are grown at the higher plant density during the plug stage. By using a larger plug size, you could increase your plug stage duration and reduce the time of your finish stage.
Horizontal air flow fans not only mix warm air with cool air, but also improve the uniformity of temperature within the greenhouse. If you already have horizontal air flow fans installed, make sure they are all operating and are well positioned (i.e., not angled down towards crops or upwards towards the roof).
You want to avoid frequent cooling/heating cycles, and the best way to do this is to increase the temperature deadband during the winter and spring. A slightly larger increase in the venting setpoint will allow you to take advantage of heating from the sun. Remember to monitor your average daily temperature and make adjustments as necessary to stay on track with your production schedule.
A more complete version of this article will appear in my Growing Trends column in the December 2005 issue (Page 59) of GMPro magazine. For more information on shade curtains, supplemental lighting guidelines, and photoperiod and irradiance responses, consider purchasing the “Lighting Up Profits” book, available at Ball Publishing. |
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Ryan Warner
Horticulture
High fuel costs make it imperative to produce greenhouse crops as efficiently as possible. One tool that far too few growers take advantage of is manipulating the greenhouse light environment to accelerate flowering. Light impacts time to flower of greenhouse crops in several ways. First, many plants are photoperiodic. That is, they flower in response to day length. Although many growers are familiar with closing blackout cloths to provide the short days necessary for flowering of poinsettias or mums, only a small percentage of growers are controlling day length to accelerate flowering of spring bedding plant crops. However, many bedding plant crops are long-day plants (see the Internet link at the end of this article). If you are not providing any supplemental lighting as a night-interruption or a day extension, the natural photoperiod is not long enough to induce flowering in these long-day plants until late March or later. Taking advantage of a crops’ response to photoperiod can reduce production time by several weeks or more, and therefore reduce production costs.
An example of this is ‘Wave’ petunias. Many growers have noticed that ‘Wave’ petunias will put on excessive vegetative growth without flowering, outgrowing their container or taking over a mixed container, before finally flowering in late spring. ‘Wave’ petunias have a very strong requirement for long days and will flower much earlier if provided with long-day lighting.
Conversely, short-day plants such as Cosmos and Zinnia will flower earlier if grown under short day lengths (less than 12 hours). In fact, if short-day crops are started early in the season, they may flower prematurely and fail to fill out the container. For these crops, it may actually be desirable to prevent premature flowering by providing night-interruption lighting for the first few weeks of growth.
In addition to photoperiod, some bedding plant crops flower earlier when grown under high total light quantity (daily light integral, or DLI). High DLIs impact crop timing in two ways. First, flowering occurs earlier in development (i.e. the plant forms fewer leaves below the first flower) for some crops. Second, high DLI will increase plant temperature, which increases the plants’ development rate.
While installing high-pressure sodium lamps to increase DLI is a major investment, lighting to provide a long-day photoperiod is not. Only low light levels (about 10 footcandles) are required for photoperiodic lighting. Long-days can be provided either as a night-interruption (also known as “mum lighting”), with lights on from 10:00 PM to 2:00 AM or as a day-extension. If lighting as a day-extension, lights should be on long enough so that the total day length (natural plus extension) is 14 hours.
Long-day lighting can be provided with either incandescent bulbs or high-pressure sodium lamps. Because only low light levels are needed for photoperiodic lighting, if high-pressure sodium lamps are used, they can either be hung higher in the greenhouse so that a greater area is lighted, or used in a Beamflicker-type fixture. Some growers have had success by mounting halogen lamps to an irrigation boom and running the boom back and forth over a crop to provide night-interruption lighting.
For crop-specific information on which crops are photoperiodic or flower earlier under high DLI, read the “Fundamentals of Flowering” articles listed at: http://www.greenhousegrower.com/grower_tools/index.html |
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Dean M. Krauskopf
Southeast Michigan Greenhouse Educator
Don’t forget that cooler air temperatures can mean cool media temperatures. Media temperatures control nutrient and water uptake and can have a powerful effect on plant growth. This effect is most noticeable with plugs and small plants
Optimum media temperature varies by type of plant and especially by plant age. Young plants require higher media temperatures than older, almost fully developed plants. Optimum temperatures are usually in the 60° to 65° range for most crops. Focus your efforts at providing optimum temperatures in the propagation/germination areas and early in the crop. As the crop matures, cool media temperatures will have less of an effect on plant growth.
Media temperature may be up to 10°F lower than air temperature with overhead heating systems when the plants are on benches. The difference between air and media temperature can be even greater when the plants are on the ground or have just been watered with cold water. The best way to determine media temperature is by using a media/soil thermometer. These thermometers are rugged and inexpensive but require calibration just as with thermostats and other temperature sensing equipment. Another way to check media temperature is by using an infrared thermometer. These units are more expensive but can measure temperatures from a distance, a useful feature when checking the middle of prop beds or hanging baskets. You should check them against a calibrated media/soil thermometer because the color of the media and/or pot can influence their readings.
Temperature gradients between air and media are caused by a number of reasons. Water evaporating from the media surface or the surface of the pot or plug tray cools the media especially in propagation or germination beds. With overhead heating systems warm air may be blocked before it reaches the media by the top of the plants or because the containers are placed on the ground or a solid bench surface. The ground is a huge heat sink and is very slow to warm up, especially in cloudy weather. Raising pots or flats just an inch or two off the ground or solid bench increases temperature significantly by preventing movement of heat to the cold ground/bench and increasing air movement around the roots.
Probably the greatest reason for cool media temperatures is watering with cold water. When the plant is young, the greatest component (by weight) of the media is water. Water absorbs a great deal of heat without changing temperature and the movement of energy (heat) from air to water isn’t very efficient. Since air doesn’t hold a great deal of energy (heat) per cubic foot you must circulate a lot of air around the pot/plug to increase media temperature. The most efficient way to increase media temperature is to water with tempered water. Seventy degree F water coming out of the end of the hose usually means the hot water heater is set at 100°F or slightly higher. Also consider reducing evaporation cooling by using bed covers or tenting instead of overhead misting in propagation beds.
Most of us are familiar with Phosphorus (P) deficiency symptoms (stunting, purpling of stems, leaf petioles and undersides of leaves) caused by cold media but low media temperatures also influence the uptake of water and all other nutrients. When the roots are not active because of low temperatures, water isn’t taken up and the plants can wilt even when the media has adequate water available. This is common when the sun suddenly comes out after a period of low light. Make sure your employees are trained to check media moisture levels rather than just react to the sight of wilted plants by watering. Calcium (Ca) moves with the water being transpired. If the root isn’t active, water isn’t moving into the plant and neither is Ca. Low temperatures also cause higher relative humidity around the plant which slows the transpiration of water, and therefore, the movement of Ca into growing points. Below 60°F media temperature Ammonium Nitrogen (NH4) is not converted into Nitrate Nitrogen (NO3-) by bacteria in the media. NH4 can build up to toxic levels if you don’t increase the percentage of NO3- in your fertilizer.
Lastly, the fastest way I know to kill roots is by drowning them. Cool media doesn’t dry out quickly and roots can be starved for oxygen for long periods of time. Make sure your media is highly aerated, with lots of large pores to allow water to drain and oxygen to penetrate. However, highly porous media doesn’t hold a lot of water so adjust your watering practices during warmer weather. |
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Jeanne Himmelein
Southwest
Michigan Extension Educator
Scientists study plant development rates based on environmental factors with a key factor being temperature. Entomologists study insect lifecycle changes based also on temperature (rate of metamorphosis). In most disciplines, the rate of plant and insect development is measured and recorded utilizing growing degree days. Growing degree days are the accumulation of heat over time and is a tool used to predict plant growth, as well as insect development to improve IPM strategies.
Scientists have developed models representing insect development utilizing growing degree days. The models have been primarily designed for outdoor production of fruits, vegetables and field crops. Utilizing weather station data and models in the production area, farmers can schedule IPM activities based on what stage primary pests are present. For some examples of models and the impact of temperature on insect populations outdoors, visit: www.ipm.ucdavis.edu/WEATHER/ddretrieve.html
With greenhouse production, we are able to manipulate temperature, and thus it is not appropriate to utilize growing degree days based on outdoor weather. I put together some information on insect development at specific temperatures to help you understand what insects you should be looking for depending on your growing temperature.
Insect |
Development begins above this temperature |
Western flower thrips |
50°F |
Green peach Aphid |
39°F |
Twospotted spider mite |
50°F |
Whitefly |
47°F |
Develop at temperatures above 50°F. At a temperature of 78°F to 82°F, the completed life-cycle from egg to adult is 12 to 14 days. As temperature decreases, time from egg to adult increases.
A model has been designed for the development of green peach aphid, and we know that their development occurs when temperature is above 39°F and the rate of development increases until 89°F.
Development begins above 50°F. The optimum development temperature is between 85°F and 95°F, which explains why this pest is particularly a problem during warm weather and in hot greenhouses.
A model has been developed for the greenhouse whitefly, which states that development begins at a temperature above 47°F. Although the greenhouse whitefly and the silver leaf whitefly are different species, I believe the development temperatures are fairly close. There is a seven-day difference in time to development from egg to adult at 70°F in these two species.
The differences in the base temperature of each insect (the temperature at which development begins) enables us to determine the impact of lowering greenhouse temperature on insect populations. With a lower greenhouse temperature, insects with a lower base temperature would be impacted relatively less than insects with a higher base temperature. Therefore, populations of Western flower thrips and twospotted spider mites would increase at a slower pace at a lower greenhouse temperature compared to populations of green peach aphids. So, if you’re tempted to lower your greenhouse temperature, keep your eye out for aphids, as they will continue to develop relatively quickly compared to the other three insect pests. |
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Tom Dudek
District Extension Horticulture and Marketing Agent
The following publications contain information that will be useful to greenhouse growers in Michigan faced with the high cost of energy. While there may be some duplication of information, the concepts are the same as growers from across the county are facing the same issue. If you would like further assistance in this matter, please feel free to contact your area Greenhouse MSU Extension Educator.
Energy Conservation for Greenhouse Growers by Dr. David Tatum, Extension Horticulture Specialist, and Dr. Jimmy Bonner, Extension Assistant Specialist. Information Sheet 1617; Mississippi State University Extension. A two page pdf downloadable document focusing on items that growers can implement to conserve energy.
Go to: http://msucares.com/pubs/infosheets/is1618.html for further information.
Greenhouse energy Conservation Checklist by John W. Bartok, Jr.; Agricultural Engineer
Natural Resources Management & Engineering Department; University of Connecticut, Storrs CT. A two and half page checklist that growers can use to evaluate where they are in determining their energy conservation efforts. Go to: http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_
management/jb_energy_cklst.htm for a down loadable copy.
Energy Conservation for Commercial Greenhouse. NRAES-3 2001 revision. Revised by John W. Bartok, Jr.; Agricultural Engineer; Natural Resources Management. & Engineering Department; University of Connecticut, Storrs CT. An 84-page booklet written by agricultural engineers that understand the greenhouse environment. Originally written in 1978 when we had the first energy crisis, this booklet has been revised four times since then to reflect changes in the industry. I would recommend that every grower obtain a copy and review it with their greenhouse in mind. It contains 10 chapters that focus on every aspect of the greenhouse structure and environment as it relates to energy. To obtain the current price and an order form go to: www.nraes.org or contact the Natural Resource, Agriculture, and Engineering Service (NRAES); Cooperative Extension; 152 Riley Robb Hall; Ithaca NY 14853-5701; Telephone: 607-255-7654. |
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Mary Hausbeck
Plant Pathology
Stem, leaf and flower blights caused by the fungus, Botrytis cinerea, can limit all phases of ornamental production. Botrytis is well known for its ability to produce large masses of gray conidia (also called spores, see below) that may be picked up and carried on air currents and transported to healthy plants where blight can become established. Monitoring the occurrence and build-up of this inoculum in the greenhouse can signal the need for implementing control measures. On bedding and stock plants, Botrytis typically becomes established and produces spores on aging lower leaves that are near the moist soil surface and under the plant canopy. In addition, Botrytis readily infects the broken or cut stem surface of stock plants and progresses downward, causing a dieback of the entire stem. This diseased tissue offers another source of nutrients necessary to produce spores.
Fungicides are often important in managing Botrytis and should be chosen carefully. Each year,
Michigan State University tests products for control of Botrytis blight. Geraniums are good test plants because they seem to be a “magnet” for this disease. All fungicides are applied and allowed to dry prior to introducing Botrytis spores. Each time a test is conducted, fungicides that are considered especially effective are included for a comparison. Over the years of testing fungicides for Botrytis control, I consider Decree, Chipco 26 GT, and fungicides containing chlorothalonil (Daconil and Echo) standards because they consistently provide effective control. We were interested in comparing the fungicides Compass, Terraguard 50W, and Fungo 50WSB against these standards. Compass is a fairly new product and is often recommended for control of powdery mildew. Terraguard is also a strong powdery mildew fungicide when applied as a spray. It can also be used as a drench for some root rots. Fungo 50WSB has thiophanate-methyl as its active ingredient and is effective against a number of foliar blights and root rots.
In our trial, Botrytis progressed and produced spores on nearly 40 percent of the foliage that was left untreated (see accompanying graph). Overall, the standards, including Echo 720, 26 GT, Daconil Weatherstik, and Decree 50WDG, provided good protection and kept disease to less than 10 percent. Compass + Latron B-1956 offered Botrytis control but was not always as effective as the standard fungicides. It is helpful to know that Compass is not only a strong powdery mildew fungicide but can also suppress Botrytis. Terraguard 50W, even at a high rate, was not especially helpful in this trial in managing Botrytis blight. The inability of Fungo 50WSB to limit Botrytis in our trial may be due to resistance of our particular Botrytis strain to this fungicide. Due to the frequency of Botrytis isolates resistant to benzimidazoles (examples are Cleary’s 3336, Fungo), these fungicides are no longer recommended as the primary tool for controlling Botrytis. Although resistance to the systemic dicarboximide fungicides (Chipco 26 GT) has also been documented, resistance does not appear to be widespread, and if used wisely, they should continue to be effective. Rotating dicarboximide fungicides with protectant fungicides (Daconil, Decree, Phyton-27, Exotherm Termil, Echo, Dithane are examples) is a traditional way of delaying the buildup of resistant isolates.
Protectant fungicides do not appear to be at great risk of developing resistance. However, these protectant fungicides may not be as effective as the dicarboximide fungicides. Moorman and Lease (1992, Plant Disease 76:374-376) at The Pennsylvania State University determined that tank mixtures of systemic and protectant fungicides provided good control of Botrytis and lasted longer than applications of single fungicides.
Fungicides cannot do the job by themselves; environmental manipulation and sanitation are necessary partners in a successful program. Since Botrytis requires specific environmental conditions in order to cause disease and produce conidia, control can be achieved by manipulating the environment, making it unfavorable for Botrytis establishment.
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Grow plants in a well-ventilated, low relative humidity (less than 85%) environment.
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Increase plant spacing. A less dense plant canopy will allow air circulation and thorough fungicide coverage.
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Minimize the hours that the foliage is wet by watering in the morning so the foliage dries by the evening.
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Reduce the relative humidity for a minimum of 24 hours immediately following the harvesting of cuttings to help “dry” the wounded stems and thereby limit stem blight.
Sanitation also plays a role in disease management. Botrytis readily colonizes and produces conidia in senescent or dead plant debris. Therefore, remove plant debris on and underneath plant benches that could serve as a reservoir for sporulating Botrytis.
This research was funded by the USDA/ARS through a Specific Cooperative Agreement as part of the Floriculture and Nursery Research Initiative. |
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Christmas trees
Special edition of the Greenhouse Alert
