HIGH NIGHTTIME TEMPERATURES DECREASE FRUITSET AND YIELD OF GREENHOUSE TOMATOES

Abstract

In the southern U.S. and other mild winter areas, the length of the harvest season for greenhouse tomatoes is limited by high night temperatures. The purpose of this study was to determine the extent of this limitation by installing mechanical refrigeration to provide nighttime cooling in two of four computer-controlled greenhouses. For three crops of greenhouse tomatoes, nighttime temperatures in cooled houses were not allowed to rise above 20°C. Sixteen-week old transplants were placed in greenhouse treatments starting Mid-April ('91), mid-July ('90) and mid-August ('89). Fruit weights were significantly increased by nighttime cooling on all three planting dates, with weights increasing 11%, 28% and 53%, respectively. For the mid-July and mid-August plantings, fruitset, fruit size and % uncracked fruit were also increased significantly by nighttime cooling. Data collected in '90 showed that plants in the cooled houses required only an additional 2.4 days to mature and were only 10-15% taller, suggesting there were no significant plant-related disadvantages to nighttime cooling. Lack of stored heat and nighttime heat load in the greenhouses resulted in low cooling costs and refrigeration requirements, so nighttime cooling may also be commercially feasible.