Determination of photoperiodic response group and effect of supplemental irradiance on flowering of several bedding plant species

Flower induction of many plant species is synchronized temporally during the year by utilizing changes in day or night length. Commercial potted plant growers have used photoperiod manipulation to induce flowering of short-day plants on a year round basis. A lack of application of photoperiod to manipulate growth of current spring annuals has, in part, been due to the lack of information identifying the photoperiodic classifications of each species. This paper outlines a series of experiments that identified the photoperiodic group classifications and responses to supplemental irradiance of 28 spring annual species. No species studied were identified as obligate short-day plants. Most species were either obligate or facultative long-day plants. Species in which growers have traditionally had difficulty in producing marketable flowering plants in spring tended to be obligate long-day plants. In contrast, a number of species that tend to flower later in the season than desirable were identified as facultative short-day plants. In addition, species varied in their flowering response to supplemental lighting treatments. Leaf number below the first flower was affected by the addition of supplemental lighting under inductive conditions with approximately one half of the species studied.