Abstract Flowering in petunias is hastened by long days, but little is known about when the plants are most sensitive to photoperiod, or how light integral or temperature affect such phases of sensitivity. The effects of these factors on time to flowering was investigated using reciprocal transfer experiments between long (16 h d -1) and short days (8 h d -1). The effect of light integral on the phases of photoperiod sensitivity was examined using two sowing dates and a shading treatment (53% transmission). The effects of temperature were investigated by conducting reciprocal transfer experiments in glasshouse compartments at five temperature regimes (means of 13.7, 19.2, 22.3, 25.0 and 28.7 °C). The length of the photoperiod-insensitive juvenile phase of development, when flowering cannot be induced by any environmental stimulus, was sensitive to light integral; low light integrals prolonged this phase, from 23 d at 2.6 MJ m -2d -1to 36 d at 1.6 MJ m -2d -1(total solar radiation). The length of this development phase was shortest (12.5 d) at 21 °C; it was longer under cooler (21 d at 13.5 °C) and warmer temperatures (17.6 d at 28.3 °C). After this phase, time to flowering was influenced greatly by photoperiod, with long days hastening flowering by between 28 and 137 d, compared with short days. Plants also showed some sensitivity to both temperature and light integral during this phase, but the duration of the final phase of flower development, during which plants were photoperiod-insensitive, was dependent primarily on the temperature at which the plants were grown; at 14.5 °C, 33.9 d were required to complete this phase compared with 11.4 d at 25.5 °C. The experimental approach gave valuable information on the phases of sensitivity to photothermal environment during the flowering process, and could provide the basis of a more physiologically-based quantitative model of flowering than has hitherto been attempted. The information is also useful in the scheduling of lighting and temperature treatments to give optimal flowering times of high quality plants.