Abstract Plants were grown at either 350 or 1000 μl l −1CO 2and in one of three photoperiod treatments: continuous short days (SD), continuous long days (LD), or short switched to long days at day 41 (SD–LD). All plants received 9 h of light at 450 μmol m −2s −1and LD plants received an additional 4 h of light at 8 μmol m −2s −1. Growth of SD plants responded more positively to elevated CO 2than did LD plants, due largely to differences in the effect of CO 2on unit leaf rate. High CO 2increased height and decreased branching under SD conditions, but had no effect under LD conditions. Elevated CO 2also increased the number of buds and open flowers, the effect for flower number being greater in short than in long days. The specific leaf area of plants grown at 1000 μl l −1CO 2was reduced regardless of daylength. High CO 2also decreased leaf and increased reproductive allocation, the magnitude of these effects being greater under SD conditions. Bud formation and flower opening was advanced under high CO 2conditions in SD plants but bud formation was delayed and there was no effect on flower opening under LD conditions. The effects of CO 2on plants switched from SD to LD conditions were largely intermediate between the two continuous treatments, but for some parameters, more closely resembled one or the other. The results illustrate that daylength is an important factor controlling response of plants to elevated CO 2.