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Mark E. Uchanski, Kulbhushan Grover, Dawn VanLeeuwen, and Ryan Goss

–plant science majors alike. We add to this educational research by including students’ assessment of group work and their own participation in the activity. An agricultural science research project was designed to assess the impacts of hoop house glazing on

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Jonathan M. Frantz, Bryon Hand, Lee Buckingham, and Somik Ghose

observed greenhouse conditions such as malfunctioning or missing vent covers, gaps, or tears in the glazing, and uninsulated partitions between greenhouse sections. Baseline conditions were simulated for greenhouses containing these conditions, and air

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Kristen Hanson, Tilak Mahato, and Ursula K. Schuch

process ( Mahrer et al., 1987 ), especially in cooler coastal climates ( Larson, 2007 ). Although temperatures in soils covered with PE mulch will be highest in a glass house, a structure with PE glazing will also be effective in raising soil temperatures

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Gene A. Giacomelli and William J. Roberts

The diversity of coverings for the greenhouse and other plant production structures has increased dramatically during the past 4 decades. This has resulted from the availability of new types of covering materials and enhancements of previously existing materials, as well as the demands for technological improvements within the expanding controlled environment agricultural industry. The types of coverings currently available are dominated by plastics. These range from traditional glass to the recent advent of polymer plastics, such as thin films or multilayer rigid thermoset plastic panels. Available enhancements such as ultraviolet radiation (UV) degradation inhibitors, infrared radiation (IR) absorbency, and anti-condensation drip surfaces, as well as their physical and spectral properties are discussed. The selection of specific covering alternatives has implications for the greenhouse superstructure and its enclosed crop production system.

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Matthew G. Blanchard, Erik S. Runkle, Arend-Jan Both, and Hiroshi Shimizu

radiating surfaces. In temperate climates during winter nights, outside temperatures are low and the greenhouse glazing temperature can be considerably lower than the inside air temperature. As glazing temperature decreases, LWR emitted by the glazing

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Robert C. Hochmuth and George J. Hochmuth

The evolution of plastic uses (excluding glazing) in the production of greenhouse vegetables is presented. Plastics are used in almost every aspect of crop production, including providing a barrier to the soil, lining crop production troughs, holding soil and soilless media, and providing a nutrient film channel. Irrigation systems have become very elaborate, with various plastic products used to transport water and nutrients and to provide a means of emitting nutrient solution to the crop. The greenhouse environment is managed from several plastic components, including air distribution tubes, shade materials, and energy curtains. Plastics are now common in greenhouse vegetable crop training, insect monitoring, postharvest handling, storage, and marketing.

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Royal D. Heins and James Faust

Photoperiod studies in a greenhouse usually require that the natural photoperiod be modified to increase or decrease the daylength. Modification involves using lights to extend the daylength or using some opaque material (e.g., black sateen cloth or black plastic) to shorten the photoperiod by excluding light. Air temperatures under the material can deviate from those of the surrounding air. It is common knowledge that when plants are covered by the cloth prior to sunset, solar radiation will increase the temperature under the it. It is not as widely known that temperature under the cloth will be lower than surrounding air temperature during the night. Radiant cooling of the material occurs when the greenhouse glazing material is cooler than the air temperature, resulting in cooling of the air and plants contained under the material. We have observed radiant cooling exceeding 150 W·m-2 when glazing is cold (-7°C), resulting in a temperature reduction under the material of up to 4°C. The difference in temperature between short-day and normal- or long-day treatments can lead to incorrect conclusions about the effect of photoperiod on plant development rate. Data will be presented with a sample control system to correct the problem.

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James E. Faust and Royal D. Heins

An energy-balance model is described that predicts vinca (Catharanthus roseus L.) shoot-tip temperature using four environmental measurements: solar radiation and dry bulb, wet bulb, and glazing material temperature. The time and magnitude of the differences between shoot-tip and air temperature were determined in greenhouses maintained at air temperatures of 15, 20, 25, 30, or 35 °C. At night, shoot-tip temperature was always below air temperature. Shoot-tip temperature decreased from 0.5 to 5 °C below air temperature as greenhouse glass temperature decreased from 2 to 15 °C below air temperature. During the photoperiod under low vapor-pressure deficit (VPD) and low air temperature, shoot-tip temperature increased ≈4 °C as solar radiation increased from 0 to 600 W·m-2. Under high VPD and high air temperature, shoot-tip temperature initially decreased 1 to 2 °C at sunrise, then increased later in the morning as solar radiation increased. The model predicted shoot-tip temperatures within ±1 °C of 81% of the observed 1-hour average shoot-tip temperatures. The model was used to simulate shoot-tip temperatures under different VPD, solar radiation, and air temperatures. Since the rate of leaf and flower development are influenced by the temperature of the meristematic tissues, a model of shoot-tip temperature will be a valuable tool to predict plant development in greenhouses and to control the greenhouse environment based on a plant temperature setpoint.

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Jane M. Petitte and Douglas P. Ormrod

The effects of SO2 and NO2, singly and in combination, on the growth and physiology of nontuberizing Solarium tuberosum L. `Russet Burbank' plants were studied in controlled conditions. Plants were exposed to 0.11 μl SO2 and/or 0.11 μl NO2/liter for 24 hours a day up to 10 days. Statistically significant effects were observed mainly in the SO2+ NO2 treatments compared with the control plants. Leaf area was reduced from day 2 onward, and root fresh and dry weights were reduced from day 4 onward. Significant reductions in leaf and stem dry weights occurred on day 6. Net CO2 exchange rates were reduced for SO2 exposed compared with control plants beginning on day 3, while water loss rates were increased with SO2 + NO2 beginning on day 3. The increases in water loss rate were possibly due to the development of cuticular injury observed as abaxial glazing on the upper and middle canopy leaves. Leaf osmotic potential (π) of plants with SO2 + NO2 became more negative within the first 24 hours of the exposure. This reduction was accompanied by an increase in reducing sugar concentration. Xylem water potential was reduced in the mature and expanding leaflets by day 2 of the SO2 + NO2 exposure. The most sensitive aspect of the action of SO2 + NO2 appeared to be the increase in reducing sugars that affected osmotic potential in the leaves. Considering the retardation of root growth, these data suggest that the pollutant gases may have interfered with partitioning of dry matter from the leaves to the roots.

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Hiroshi Shimizu, Erik S. Runkle, and Royal D. Heins

A model was constructed to predict shoot-tip temperature of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) according to an energy-balance equation by using five greenhouse environmental factors: dry-bulb, wet-bulb, and sky (glazing or shade screen) temperature; transmitted shortwave radiation; and air velocity. An experiment was conducted to collect the five environmental variables that were used as model inputs, and shoot-tip temperature data were used to validate the predicted shoot-tip temperature in a commercial greenhouse. The standard deviation of the difference between predicted and measured shoot-tip temperature was 0.798 and was calculated by using 8547 data points, and >84% of the actual and predicted data points were within 1 °C. A sensitivity analysis performed with the model indicated that, among the three temperatures measured, plant shoot-tip temperature was primarily influenced by the dry-bulb temperature. For example, shoot-tip temperature increased an average of 0.74 °C for every 1 °C increase in dry-bulb temperature when dry-bulb temperature varied from 28 to 42 °C, wet-bulb temperature was 27.8 °C, sky temperature was 39.8 °C, shortwave radiation (285 to 2800 nm) was 760 W·m-2, and air velocity was 0.44 m·s-1. Under these conditions and a dry-bulb temperature of 32.6 °C, an increase in shortwave radiation of 500 W·m-2 increased the shoot-tip temperature by an average of 3.3 °C. This developed model may be a useful tool to predict shoot-tip temperature and evaluate the effect of greenhouse environmental factors on shoot-tip temperature.