The Influence of Day and Night Temperature Fluctuations on Growth and Flowering of Annual Bedding Plants and Greenhouse Heating Cost Predictions

Volatile energy costs and lower profit margins have motivated many greenhouse growers in temperate climates to improve the energy efficiency of crop production. We performed experiments with dahlia (Dahlia xhybrida Cav. 'Figaro Mix'), French marigold (Tagetes panda L. 'Janie Flame'), and zinnia (Zinnia elegans Jacq. 'Magellan Pink') to quantify the effects of constant and fluctuating temperatures on growth and flowering during the finish stage. Plants were grown in glass-glazed greenhouses with a day/night (16 h/8 h) temperature of 20/14, 18/18, 16/22 (means of 18 degrees C), 24/18, 22/22, or 20/26 degrees C (means of 22 degrees C) with a 16-h photoperiod and under a photosynthetic daily light integral of 11 to 19 mol.m(-2).d(-1). Flowering times of dahlia, French marigold, and zinnia (Year 2 only) were similar among treatments with the same mean daily air temperature (MDT). All species grown at 20/14 degrees C were 10% to 41% taller than those grown at 16/22 degrees C. Crop timing data and computer software that estimates energy consumption for heating (Virtual Grower) were then used to estimate energy consumption for greenhouse heating on a per-crop basis. Energy costs to produce these crops in Charlotte, NC, Grand Rapids, MI, and Minneapolis, MN, for a finish date of 15 Apr. or 15 May and grown at the same MDT were estimated to be 3% to 42% lower at a +6 degrees C day/night temperature difference (DIF) compared with a 0 degrees C DIF and 2% to 90% higher at a -6 degrees C DIF versus a 0 degrees C DIF. This information could be used by greenhouse growers to reduce energy inputs for heating on a per-crop basis.