Also, the effect of photoperiod can not be practically separated from the effect of irradiance over the year under greenhouse environments to confirm whether S. cardinalis is a long-day or daylength neutral plant. It is not practical to design experiments to provide different photoperiods under several levels of irradiance at the same daily light integrals even for short periods at different times of the year. Flowering was accelerated and the number of flowers was increased in Achimenes under 16-h days, depending on the interaction between daylength and cultivars (Vlahos, 1990a; Vlahos et al.,1991), although it has been reported as a day-neutral plant (Rohde,1974). These differences could be cultivar dependent or due to plant developmental stage at the inception of photoperiod treatments.