Categorizing Nineteen Bedding Plant Varieties Into Temperature Response Groups Based On Their Estimated Base Temperatures

The effect of average daily air temperature on the flowering time and plant quality was quantified for 19 varieties of 16 common bedding plant species. Antirrhinum, Calendula, Diascia, Gomphrena, Gerbera, Impatiens, Heliotropium, Matthiola, Nicotiana, Nemesia, Nierembergia, Osteospermum, Pelargonium, Petunia, Tagetes, and Torenia were grown in five glass greenhouse compartments maintained at constant temperature setpoints of 14, 17, 20, 23 or 26 °C.  The 16-h photoperiod consisted of the natural photoperiod with supplemental high-pressure sodium lighting from 0600 to 2200 HR. The photosynthetic daily light integral during this experiment varied from 8 to 26 mol·m^-2·d^-1. Days to flower from transplant (DTF), flower or inflorescence number (FN), branch number (BN), number of nodes below the first open flower (NN), and plant height or length of the 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 all crops except Impatiens walleriana Hook. Plant quality parameters (FN, PH, and BN) increased as temperature decreased for 13, 7 and 8 species, respectively. 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 ranged from -2.8 °C in Calendula officinalis L. to 10.9 °C in Gomphrena globosa L. Varieties were placed into three temperature response categories based on their estimated T_min.  Cold-tolerant, cold-intermediate, and cold-sensitive crops were those with estimated base temperatures of <4 °C, 4 to 7 °C, and >7 °C, respectively.  This temperature response categorization helps growers identify the relative tolerance of crops to low growing temperatures for energy-efficient greenhouse production.