Influence of changes in cover and height on the climate of Canary-screenhouses for tomato growth: Preliminary results

The Canary Islands have over 7000 hectares (15% of the national total) of protected cultivation crops, half of which are tomatoes. Over the last decade, tomato growers have been replacing the traditional 15 mesh (6 x 6 filaments per cm in each of the two perpendicular directions - warp and weft) net covers with denser nets or plastic films capable of providing improved pest and disease control (particularly exclusion of virus vectors) at the expense of greenhouse ventilation. This drawback was supposed to be compensated by increasing structure height from the traditional 2.5 - 3 m to 4 - 6 meters. Consequences of these simultaneous changes in cover and height on temperature, humidity and plant growth have not been studied. As a first step towards evaluating the performance of these greenhouses, data recording was set up in the two main tomato growing areas of Gran Canaria island, Santa Lucía (SE) and San Nicolas (W). Between December 2004 and May 2005, temperature and relative humidity at the plant-canopy level were recorded every 10 minutes in the 3.5 m and 5 m screenhouses and also outside. Temperatures in both areas showed small comparative differences between the two greenhouse heights except for the extremes of the maximums and minimums that persisted longer in the 3.5 m structures. The taller houses had fewer hours below 12°C (minimum required for tomato growth): 375 h versus 420 h in the SE zone, and 291 h versus 408 h in the W. These colder intervals are notably longer than those occurring outside (90 and 166 h in the SE and W zones respectively), denoting temperature inversion in the greenhouse on clear nights. Regarding the periods of time with temperatures higher than 26deg;C (considered as the maximum for optimal production and quality) greater differences were found in the West zone: the higher screenhouse had only 55 h compared to 132 h in the lower-one. Similar results were found for relative humidity; the difference between the mean values in the greenhouses with different heights was less than 12%. These results proved the better climatic performance of high structures.