The 2011 ASHS Annual Conference

Bedding plants are grown in greenhouses during some of the coldest months of the year. Rising energy costs have prompted some growers to lower their greenhouse temperatures in an attempt to save on fuel costs for heating. However, plants develop slower at a lower temperature and thus, the production period is lengthened. Energy-efficient production requires species-specific information on how temperature influences flowering time and plant quality. Calendula (Calendula officinalis L.) ‘Bon Bon Orange’, geranium (Pelargonium × hortorum L.H. Bailey) ‘Pinto Red’ and torenia (Torenia fournieri Linden ex E. Fourn) ‘Clown Blue’ were grown in five glass greenhouse compartments maintained at constant air temperature setpoints of 14, 17, 20, 23 or 26 °C and under a 16-h photoperiod to quantify the effect of average daily temperature on flowering time and plant quality. The photosynthetic daily light integral during this experiment varied from 8 to 26 mol·m^-2·d^-1. Days to flower from transplant (DTF), number of nodes below first open flower (NN), and plant quality parameters such as flower or inflorescence number (FN), branch number (BN), and plant height or length of longest lateral branch (PH) were measured when the first flower opened on each plant. As temperature increased from 14 to 26 °C, DTF decreased for calendula, geranium and torenia by 15, 27, and 22 d, respectively. Flower number increased for all species as temperature decreased. A similar trend was observed for PH and BN, except for torenia, where PH was unaffected and BN decreased as temperature decreased.  Linear regression analysis was performed on the flowering rate (reciprocal of days to flower) data to estimate the base temperature (T_min) for each species, which is the temperature at which the flowering rate is zero. T_min was estimated as -2.8, 6.5 and 9.4 °C for calendula, geranium and torenia, respectively. This information can be used by growers to group crops with a similar temperature response together for energy-efficient greenhouse production.