Experimental investigation and modelling of condensation of a rose crop in a greenhouse in Zimbabwe

The main aim of this study was to experimentally determine the conditions when dew forms on leaf surfaces from measured ambient conditions, and to adapt an appropriate vegetative condensation model to describe and predict the condensation of a rose crop in a greenhouse. A vegetation sub-model within the Gembloux Dynamic Greenhouse Climate Model (GDGCM), that offers the possibility to predict whether condensation is present on the leaves of plants in a greenhouse was adapted and validated. Measurements of the leaf wetness were made with different orientations of leaf wetness sensors located above and within the canopy to determine the optimum placement of the sensor for accurate measurements. Measurements were also taken at several places within the greenhouse in order to investigate spatial variability. All measurements took place in a controlled greenhouse at the Biological Sciences Department, University of Zimbabwe Campus and in a commercial greenhouse at Floraline (Pvt.) Ltd. in Harare. Outside weather data was collected from established automatic weather stations at the sites. Relevant greenhouse meteorological and plant physiological parameters, such as air temperature and humidity, solar radiation, wind speed, leaf temperature, leaf wetness and leaf wetness duration, etc., were measured and together with outside weather measurements were used to calibrate and validate the vegetative condensation sub-model of the greenhouse climate model. Condensation on the leaf surfaces was observed whenever the relative humidity was high and the canopy temperature of plants in the greenhouse was close to that of the surrounding air. Because of its incorporation of both temperature and humidity in its calculation, vapour pressure deficit (∆e) was found to be a convenient indicator of the condensation potential. Condensation on plant surfaces inside the greenhouse was observed to occur at different vapour pressure deficit thresholds (∆eth), depending on the season and the night temperatures. During summer nights (November-March), the greenhouse air temperature was observed to range from 15°C – 18 °C, and the ∆eth was observed to range from 0.1 kPa to 0.27 kPa, corresponding to relative humidity threshold range of 83 % to 87 %. During winter nights, the air temperatures were observed to range from 9°C – 12 °C and the ∆eth was observed to range from 0.15 kPa to 0.3 kPa, and was corresponding to relative humidity threshold of 75 % to 80 %. When the greenhouse air temperature was maintained within the range of 22 °C - 25 °C, the ∆eth range from 0.2 kPa to 0.45 kPa, corresponding to relative humidity threshold of 84 % - 90 % was observed. Plant temperature was mostly below air temperature during the day and mostly close to air temperature just before sunrise. Comparison of measured and calculated temperatures and saturation deficits in the plant canopy showed a good agreement. In general, night-time air and plant temperatures, calculated by the model, compare quite satisfactorily with the measured temperatures. The comparison with measurements shows that the model allows prediction of condensation to within 82 % of the occasions when dew occurred. The fact that the mean temperature of the plant differs from that of the air and thus produces a quite different saturation deficit has important implications for disease control. The model gives ∆e values, which can be used to predict disease-causing climate conditions, while taking into account different temperature levels